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A  MANUAL 


PHYSIOLOGY. 


A  TEXT-BOOK  POE  STUDENTS  OF  MEDICINE, 


GERALD  F.  YEO,  M.D.DUBL.,  F.R.C.S., 

PROFESSOR   OF   PHYSIOLOGY   IN   KING'S   COLLEGE,  LOXPON,  ETC. 

THIRD  AMERICAN 

FKOM    THE 

SECOND  ENGLISH  EDITION. 

WITH  THREE  HUNDRED  AND  TWENTY-ONE  ILLUSTRATIONS 
AXD  A  GLOSSARY. 


PHILADELPHIA: 
P.  BLAKISTON,  SON  &  CO., 

1012  WALNUT  STREET. 
1888. 


BIOLOGY 
LIBRARY 

G 


PRESS  OF  WM.  F.  FELL  &  Co  - 

1220-24  SANSOM  STREET, 
PHILADELPHIA. 


PREFACE  TO  THE  SECOND  EDITION. 


IN  preparing  this  edition,  I  have  done  my  utmost  to  cor- 
rect inaccuracies  and  remove  obscurities.  The  changes  rendered 
necessary  by  recent  research  have  also  been  made. 

Some  parts  have  been  rewritten  ;  notably  the  chapters  on  the 
Central  Nervous  Systern,  to  which  additional  illustrations  have 
been  added. 

The  general  arrangement  remains  the  same  as.  that  of  the 
First  Edition. 

I  am  again  deeply  indebted  to  my  friend,  Mr.  E.  F.  Herrouu, 
for  much  valuable  assistance. 

KING'S  COLLEGE,  LONDON'. 


PREFACE  TO  THE  FIRST  EDITION. 


THE  present  volume  has  been  written  at  the  desire  on  the  part 
of  the  Publishers  that  a  new  elementary  treatise  on  Physiology 
should  be  added  to  the  series  of  admirable  students'  manuals 
which  they  had  previously  issued. 

In  carrying  this  desire  into  execution,  I  have  endeavored  to 
avoid  theories  which  have  not  borne  the  test  of  time,  and  such 
details  of  methods  as  are  unnecessary  for  junior  students.  I  do 
not  give  any  history  of  how  our  knowledge  has  grown  to  its 
present  standpoint ;  nor  do  I  mention  the  names  of  the  authori- 
ties upon  whose  writings  my  statements  depend.  I  have  also 
omitted  the  mention  of  exceptional  points,  because  I  find  that 
exceptions  are  more  easily  remembered  than  the  main  facts  from 
which  they  differ;  and,  since  we  must  often  be  content  with  the 
retention  of  the  one  or  the  other,  I  have  tried  to  insure  that  it 
shall  be  the  more  important. 

While  endeavoring  to  save  the  student  from  doubtful  and 
erroneous  doctrines,  I  have  taken  great  care  not  to  omit  any  im- 
portant facts  that  are  necessary  to  his  acquirement  of  as  clear  an 
idea  as  possible  of  the  principles  of  Physiology. 

I  have  not  hesitated  to  lay  unwonted  stress  upon  those  points 
which  many  years'  practical  experience  as  a  teacher  and  an  ex- 
aminer has  shown  me  are  difficult  to  grasp  and  are  commonly 
misunderstood  ;  and  I  have  treated  such  subjects  as  are  useful  in 
the  practice  of  medicine  or  surgery  more  fully  than  those  which 
are  essential  only  to  abstract  physiological  knowledge. 

As  medical  students  are  generally  obliged  to  commence  the 
study  of  Physiology  without  any  anatomical  knowledge,  I  believe 
it  to  be  absolutely  necessary  that  their  first  physiological  book 
should  contain  some  account  of  the  structure  and  relationships 

vii 


Vlll  PREFACE    TO    THE    FIRST    EDITION. 

of  the  organs,  the  functions  of  which  they  are  about  to  study. 
I  have,  therefore,  added  a  short  account  of  the  construction  of 
the  various  parts  discussed  in  each  chapter  ;  it  has,  however,  been 
found  necessary  to  curtail  this  anatomical  portion  to  a  mere 
introductory  sketch.  Numerous  illustrations,  with  full  descrip- 
tions attached  to  each,  are  introduced  to  supplement  the  explana- 
tion given  in  the  text. 

So  far  as  is  consistent  with  an  accurate  treatment  of  the  sub- 
ject, I  have  avoided  technical  terms  and  scientific  modes  of 
expression.  I  know  that  in  attempting  to  explain  physiological 
truths  in  every-day  language  and  in  a  plain  common-sense  way, 
I  run  the  risk  of  appearing  to  lack  the  precision  that  such  a 
subject  demands ;  but  after  mature  consideration  I  have  come 
to  the  conclusion  that  great  scientific  nicety  and  a  scholastic 
style  of  expression  have  a  deterrent  effect  upon  the  beginner's 
industry ;  and  I  think  it  better  that  he  should  acquire  the  first 
principles  of  the  science  in  homely  language,  than  pick  up  tech- 
nical odds  and  ends  in  learned  terms,  the  meaning  of  which  he 
does  not  comprehend. 

As  many  words,  strange  to  the  first  year's  student,  have  to  be 
used,  and  must  be  learned,  it  has  been  thought  advisable  to  add 
a  short  glossary,  containing  an  explanation  of  the  most  ordinary 
physiological  expressions. 

Great  difficulty  is  always  found  in  fixing  upon  a  starting 
point  at  which  to  begin  the  study  of  Physiology.  To  begin  with 
the  circulation  of  the  blood,  which  is  so  essential  for  the  life  of 
every  tissue,  one  should  have  some  knowledge  of  nerve  and  mus- 
cle. To  begin  with  nerves  and  muscles,  the  mechanisms  and  the 
uses  of  the  blood  current  should  be  understood  ;  and  so  on, 
throughout  the  various  systems,  which  are  so  inter-dependent 
that,  for  the  thorough  comprehension  of  any  one,  a  knowledge  of 
all  is  required. 

I  have,  therefore,  adopted  the  time-honored  plan  of  commenc- 
ing with  the  vegetative  systems  and  following  the  course  of  the 
aliments  to  their  destination  and  final  application,  as  I  believe 
this  arrangement  is  open  to  as  few  objections  as  any  other  knbwn 
to  me. 


PREFACE    TO    THE    FIRST    EDITION.  IX 

I  wish  here  to  express  my  most  cordial  thanks  to  many  friends 
who  have  aided  me  with  kind  assistance  and  advice.  I  am 
deeply  indebted  to  Mr.  Tyrrell  Brooks  for  the  great  help  he 
afforded  me  by  compiling  the  chapters  on  Development ;  and  I 
feel  I  cannot  sufficiently  thank  Mr.  E.  F.  Herroun  for  his  untir- 
ing and  valuable  assistance  in  the  revision  of  the  proof  sheets. 

To  Mr.  G.  Hanlon  I  am  indebted  for  the  careful  and  skillful- 
manner  in  which  he  has  executed  the  new  woodcuts,  most  of 
which  he  had  to  copy  from  rough  drawings. 

KING'S  COLLEGE,  LONDON. 


CONTENTS. 


CHAPTER  I. 
THE  OBJECTS  OF  PHYSIOLOGY. 

PAGE 

Introductory  Definitions 25 

Structural  and  Physical  Properties  of  Organisms 29 

Chemical  Composition 29 

Vital  Phenomena 32 

CHAPTER  II. 

GENERAL  VIEW  OF  THE  STRUCTURE  OF  ANIMAL  ORGANISMS. 

Cells 33 

Protoplasm,  Nucleus 35 

Cell  Wall 36 

Cell  Contents • 36 

Varieties  of  Cells 38 

Modifications  of  Original  Cell  Tissues 38 

I.  Epithelial  Tissues 43 

II.  Nerve  Tissues 46 

III.  Muscle  or  Contractile  Tissues 60 

IV.  Connective  Tissues 52 

CHAPTER  III. 
CHEMICAL  BASIS  OF  THE  BODY. 

Elements  in  the  Body 62 

Classification  of  Ingredients  found  in  the  Tissues 64 

Plasmata 64 

Albuminous  Bodies 66 

Classification  of  Albumins 68 

Albuminoids 71 

Products  of  Tissue  Change 73 

Carbohydrates 77 

Fats 78 

Inorganic  Bodies 79 

xi 


Xll  CONTENTS. 

CHAPTER  IV. 

THE  VITAL  CHARACTERS  OF  ORGANISMS. 

PAGE 

Protoplasmic  Movements 82 

Reproduction 85 

Bacteria 88 

Amoeba    .  • 91 

Paramcecium 93 

CHAPTER   V. 
NUTRITION  AND  FOOD  STUFFS.. 

Classification  of  Foods 98 

Composition  of  Special  Forms  of  Food 102 

Milk 102 

Cheese,  Meat,  Eggs,  etc 105 

Vegetables 107 

CHAPTER   VI. 

THE  MECHANISM  OF  DIGESTION. 

Mastication      112 

Deglutition      113 

Nervous  Mech'anism  of  Deglutition 118 

Vomiting 121 

Movements  of  the  Intestines 123 

Defecation 125 

Nervous  Mechanism  of  the  Intestinal  Motion 128 

CHAPTER  VII. 
MOUTH    DIGESTION. 

Salivary  and  Mucous  Glands 131 

Characters  of  Mixed  Saliva 134 

Nervous  Mechanism  of  Secretion  of  Saliva 136 

Changes  in  the  Gland  Cells 144 

Functions  of  the  Saliva 146 

CHAPTER  VIII. 

THE  STOMACH  DIGESTION. 

The  Gastric  Glands 148 

The  Characters  of  Gastric  Juice    . 150 

Mode  of  Secretion  of  Gastric  Juice 152 

Action  of  the  Gastric  Juice    .  154 


CONTENTS.  Xlll 

CHAPTER  IX. 

PANCREATIC  JUICE. 

PAGB 

Structure  of  the  Pancreas 160 

Characters  and  Mode  of  Secretion  of  Pancreatic  Juice    ......  161 

Changes  in  the  Gland  Cells 162 

Action  of  Pancreatic  Juice  on  Proteids 165 

Action  on  Fats .t. 166 

Action  on  Starch .    .  167 

CHAPTER   X. 

BILE. 

Functions  of  the  Liver 168 

Structure  of  the  Liver 169 

Method  of  Obtaining  Bile 174 

Composition  of  Bile 175 

Method  of  Secretion  of  Bile 178 

Functions  of  the  Bile 180 

CHAPTER  XL 
FUNCTIONS  OF  THE  INTESTINAL  Mucous  MEMIWANE. 

Structure  of  the  Small  Intestine 182 

Method  of  obtaining  Intestinal  Secretion 184 

Characters  and  Functions  of  the  Intestinal  Juice 185 

Functions  of  the  Large  Intestine 187 

Putrefactive  Fermentations  in  the  Intestine 188 

CHAPTER  XII. 

ABSORPTION. 

Interstitial  Absorption 190 

The  Lymphatic  System 191 

Structure  of  Lymphatic  Glamls 194 

Intestinal  Absorption 199 

Mechanism  of  Absorption 203 

Materials  Absorbed 205 

Lymph  and  Chyle 208 

Movement  of  the  Lymph 211 

CHAPTER  XIII. 
THE  CONSTITUTION  OF  THE  BLOOD  AND  THE  BLOOD  PLASMA. 

General  Characteristics  of  the  Blood 214 

Amount  of  Blood  in  the  Body 215 


XIV  CONTENTS. 

PAGE 

Physical  Construction  of  the  Blood 217 

Plasma 218 

Preparation  and  Properties  of  Fibrin 219 

Chemical  Composition  of  Plasma 220 

Serum 223 

CHAPTER  XIV. 

BLOOD  CORPUSCLES. 

Proportion  of  Red  to  White 224 

White  Blood  Cells 225 

Origin  of  the  Colorless  Blood  Cells 227 

The  Red  Corpuscles,  Sizes  and  Shapes 228 

Action  of  Reagents  on  Red  Corpuscles 230 

Method  of  counting  Corpuscles 233 

Chemistry  of  the  Coloring  Matter  of  the  Blood 235 

Spectra  of  the  Haemoglobin 237 

Haematin,  Haemin,  etc 240 

Development  of  the  Red  Discs 241 

The  Gases  of  the  Blood 243 

CHAPTER  XV. 
COAGULATION  OF  THE  BLOOD. 

Formation  of  the  Blood  Clot 245 

Circumstances  influencing  Coagulation 247 

The  Cause  of  Coagulation 248 

Coagulation  in  the  Vessels 249 

Formation  of  Fibrin j .    .  252 

CHAPTER  XVI. 
THE  HEART. 

Pulmonary  and  Systemic  Circulations 255 

Method  of  the  Circulation  of  the  Blood 256 

The  Heart '.....  258 

Arrangement  of  Muscle  Fibre 259 

Minute  Structure  of  the  Heart 261 

Action  of  the  Valves 263 

Cycle  of  the  Heart  Beat 265 

Movements  of  the  Heart 268 

The  Heart's  Impulse 270 

Heart  Sounds    .  .  272 


CONTENTS.  XV 

PAGE 

Innervation  of  the  Heart •  275 

Local  Centres 275 

Inhibitory  Nerves 280 

Accelerator  Nerves 281 

Afferent  Cardiac  Nerves 282 

CHAPTER   XVII. 

THE  BLOOD  VESSELS. 

Structure  of  the  Vessels 283 

The  Capillaries 285 

Relative  Capacity  of  the  Vessels 288 

Physical  Forces  of  the  Circulation 289 

The  Blood  Pressure 291 

Measurement  of  Blood  Pressure 297 

Variations  in  the  Blood  Pressure 300 

Influence  of  Respiration  on  the  Blood  Pressure 301 

The  Arterial  Pulse 307 

Methods  of  obtaining  Pulse  Tracings 309 

Variations  in  the  Pulse 312 

Velocity  of  the  Blood  Current   . 313 

Controlling  Mechanisms  of  the  Blood  Vessels 317 

CHAPTER  XVIII. 

THE  MECHANISM  OF  RESPIRATION. 

Gas  Interchange 323 

Structure  of  the  Lungs  and  Air  Passages 326 

The  Thorax 329 

Thoracic  Movements •    •  330 

Inspiratory  Muscles 333 

Expiration 337 

Function  of  the  Pleura 338 

Pressure  Differences  in  the  Air 340 

The  Volume  of  Air 341 

Nervous  Mechanism  of  Respiration 343 

Modified  Respiratory  Movements 349 

CHAPTER  XIX. 

THE  CHEMISTRY  OF  RESPIRATION. 

Composition  of  the  Atmosphere 351 

Expired  Air 352 

Changes  the  Blood  undergoes  in  the  Lungs    .           354 

2 


XVI  CONTENTS. 

PAGE 

Gases  in  the  Blood 355 

Internal  Respiration 359 

Respiration  of  Poisonous  Gases 360 

Ventilation      361 

Asphyxia 362 

CHAPTER  XX. 

BLOOD-ELABORATING  GLANDS. 

Ductless  Glands 366 

Supra-renal  Capsule  and  Thyroid  Body 367 

Thymus 368 

Spleen 369 

Functions  of  the  Spleen 372 

Glycogenic  Function  of  the  Liver 373 

Glycogen 375 

CHAPTER  XXI. 

SECRETIONS. 

Lachrymal  Glands 378 

Mucous  Glands 379 

Sebaceous  Glands 381 

Mammary  Glands 382 

Composition  of  Milk    .. 384 

Sudoriferous  Glands 387 

Cutaneous  Desquamation 389 

CHAPTER  XXII. 
URINARY  EXCRETION. 

Structure  of  the  Kidneys 391 

Blood  Vessels  of  the  Kidneys .    .  393 

Urine 395 

Method  of  Secretion  of  the  Urine 397 

Chemical  Composition  of  Urine 400 

Urea 400 

Uric  Acid 403 

Kreatinin,  Xanthin,  Hippuric  Acid,  Oxalic  Acid,  etc.    ......  403 

Coloring  Matters 404 

Inorganic  Salts 405 

Abnormal  Constituents 406 

Urinary  Calculi 407 

Source  of  Urea,  etc 408 


CONTENTS.  Xvii 

PAGE 

Nervous  Mechanism  of  the  Urinary  Secretion 410 

Outflow  of  Urine 413 

Nervous  Mechanism  of  Micturition 414 

CHAPTER  XXIII. 
NUTRITION. 

Tissue  Changes  during  Starvation 417 

Food  Requirements 420 

Ultimate  Uses  of  Food  Stuffs .. 424 

CHAPTER  XXIV. 
ANIMAL  HEAT. 

Warm  and  Cold-blooded  Animals 428 

Variations  in  the  Body  Temperature 429 

Mode  of  Production  of  Animal  Heat 431 

Income  and  Expenditure  of  Heat 432 

Maintenance  of  Uniform  Temperature 435 

CHAPTER  XXV. 

CONTRACTILE  TISSUES. 

Histology  of  Muscle 442 

Properties  of  Muscle  in  the  Passive  State 445 

Electric  Phenomena  of  Muscle 448 

Active  State  of  Muscle 451 

Muscle  Stimuli 452 

Changes  occurring  in  Muscle  on  its  entering  the  Active  State    .    .    .  454 

Muscle  Contraction 4o6 

Graphic  Method  of  Recording  Contraction 460 

Tetanus,  Fatigue,  etc 468 

Rigor  Mortis 473 

Unstriated  Muscle 475 

CHAPTER  XXVI. 

THE  APPLICATION  OF  SKELETAL  MUSCLES. 

General  Arrangements 477 

Joints 478 

Standing .    , 481 

Walking  and  Running ,,,,,,..  484 


XV111  CONTESTS. 

CHAPTER  XXVII. 

VOICE  AND  SPEECH.  PAGE 

Anatomical  Sketch 486 

Mechanism  of  Vocalization 488 

Properties  of  the  Human  Voice 491 

Nervous  Mechanism  of  Voice 493 

Speech 494 

CHAPTER  XXVIII. 
GENERAL  PHYSIOLOGY  OF  THE  NERVOUS  SYSTEM. 

Anatomical  Sketch 496 

Functional  Classification 498 

Chemistry  and  Electric  Properties  of  Nerves 500 

The  Active  State  of  Nerve  Fibres 501 

Nerve  Stimuli 501 

Velocity  of  Nerve  Impulse 504 

The  Electric  Changes  in  Nerves 506 

Electrotonus 507 

Irritability  of  Nerve  Fibres 508 

Law  of  Contraction 511 

Nerve  Corpuscles  or  Terminals 513 

Functions  of  the  Nerve  Cells 515 

CHAPTER  XXIX. 

SPECIAL  PHYSIOLOGY  OF  NERVES. 

Spinal  Nerves 519 

The  Cranial  Nerves • 522 

The  Trochlear  Nerve,  Portio  Dura,  etc 523 

Efferent  and  Afferent  Fibres 526 

Ganglia  of  the  Fifth  Nerve 528 

The  Glosso-pharyngeal  Nerve 529 

The  Vagus  Nerve 530 

The  Hypoglofisal  Nerve 532 

CHAPTER  XXX. 

SPECIAL  SENSES. 

Skin  Sensations 537 

Nerve  Endings 538 

Sense  of  Locality 54] 

Sense  of  Pressure 543 

Temperature  Sense 545 

General  Sensations    .       .  547 


CONTENTS.  XIX 

CHAPTER  XXXI. 

TASTE  AND  SMELL. 

PAGE 

Sense  of  Taste 550 

Sense  of  Smell  .  .    553 


CHAPTER  XXXII. 
VISION. 

The  Construction  of  the  Eyeball 557 

Dioptric  Media  of  the  Eyeball 560 

Structure  of  the  Lens 562 

The  Dioptrics  of  the  Eye 564 

Accommodation 570 

Defects  of  Accommodation 572 

Defects  of  Dioptric  Apparatus 574 

The  Iris 575 

The  Ophthalmoscope 578 

Visual  Impressions 582 

The  Function  of  the  Retina 583 

Color  Perceptions 591 

Mental  Operations  in  Vision 694 

Movements  of  the  Eyeballs 595 

Binocular  Vision 596 

CHAPTER  XXXIII. 

HEARING. 

Sound 598 

Conduction  of  Sound  Vibrations  through  the  Outer  Ear 602 

Conduction  through  the  Tympanum 603 

Conduction  through  the  Labyrinth 607 

Stimulation  of  the  Auditory  Nerve 610 

CHAPTER  XXXIV. 

CENTRAL  NERVOUS  ORGANS. 

Nerve  Cells 614 

The  Spinal  Cord  as  a  Conductor 616 

The  Spinal  Cord  as  a  Collection  of  Nerve  Centres •.  625 

Special  Reflex  Centres .  632 

Automatism    .                                                                                           .  634 


XX  CONTENTS. 

CHAPTER  XXXV. 
THE  MEDULLA  OBLONGATA. 

The  Medulla  Oblongata  as  a  Conductor 639 

The  Respiratory  Centre 640 

The  Vasomotor  Centre 641 

The  Cardiac  Centre  . 643 

CHAPTER  XXXVI. 
THE  BRAIN. 

The  Mesencephalon  and  Cerebellum 648 

Crura  Cerebri 652 

Basal  Ganglia 653 

Cerebral  Hemispheres 656 

Localization  of  the  Cerebral  Functions 660 

CHAPTER  XXXVII. 
REPRODUCTION. 

Origin  of  Male  and  Female  Generative  Elements 666 

Menstruation  and  Ovulation 669 

Changes  in  the  Ovum  subsequent  to  Impregnation 671 

Formation  of  the  Membranes 675 

The  Placenta 681 

CHAPTER  XXXVIII. 
DEVELOPMENT. 

Development  of  the  Vertebral  Axis 686 

Development  of  the  Central  Nervous  System      691 

The  Alimentary  Canal  and  its  Appendages 697 

The  Genito-urinary  Apparatus 702 

The  Blood-vascular  System 708 

Development  of  the  Eye 721 

Development  of  the  Ear 726 

Development  of  the  Skull  and  Face 729 


GLOSSARY 733 

INDEX 743 


COMPARISON  OF   MEASURES. 


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XX11 


COMPARISON   OF   MEASURES. 


MEASURES  OF  WEIGHT. 

Tons 
=  20  Cwt. 
=  15,620,000  Grains. 

»-lOGO-*<M^SDOi 
O  *—  i^GC^^OlC 

CORRESPONDING  DEGREES  IN 
THE   FAHRENHEIT  AND 
CENTIGRADE  SCALES. 

00000000 

Fahr.            Cent. 
500°  260.0° 
450°  232.2° 
400°  204.4° 
350°  176.7° 
300°  1-48.9° 
212°  100.0° 
210°  98.9° 
205°   96.1° 
200°   93.3° 
195°  90.5° 
190°  87.8° 
185°  85.0° 
180°  82.2° 
175°  79.4° 
170°  76.7° 
165°  73.9° 
160°  71.1° 
155°  68.3° 
150°  65.5° 
145°  62.8° 
140°  60.0° 
135°  57.2° 
130°  54.4° 
125°  51.7° 
120°  48.9° 
115°  46.1° 
110°  43.3° 
105°  40.5° 
100°  37.8° 
95°  35.0° 
90°  32.2° 
85°  29.4° 
80°  26.7° 
75°  23.9° 
70°  21.1° 
65°  18.3° 
60°  15.5° 
55°  12.8° 
50°  10.0° 
45°  7.2° 
40°  4.4° 
35°  1.7° 
32°  0.0° 
30°  —  1.1° 
25°  —  3.9° 
20°  —  6.7° 
15°  —  9.4° 
10°  —12.2° 
5°  —15.0° 
0°  —17.8° 
—  5°  —20.5° 
—10°  —23.3° 
—15°  —26.1° 
—20°  —18.9° 
—25°  —31.7° 
-30°  —34.4° 
—35°  —37.2° 
—40°  —40.0° 
—45°  —42.8° 
—50°  —45.6° 

Cent.         Fahr. 
100°  212.0° 
98°  208.4° 
96°  204.8° 
91°  201.2° 
92°   197.6° 
90°  194.0° 
88°   190.4° 
86°  186.8° 
84°  183.2° 
82°  179.6° 
80°  176.0° 
78°  172.4° 
76°  168.8° 
74°  165.2° 
72°  161.6° 
70°  158.0° 
68°  154.4° 
66°  150.8° 
64°  147.2° 
62°  143.6° 
60°  140.0° 
58°  136.4° 
56°  132.8° 
54°  129.2° 
52°  125.6° 
50°  122.0° 
48°  118.4° 
46°  114.8° 
44°  111.2° 
42°  107.6° 
40°  104.0° 
38°  10(1.4° 
36°  96.8° 
34°  93.2° 
32°  89.6° 
30°  86.0° 
28°  82.4° 
26°  78.8° 
24°  75.2° 
22°  71.6° 
20°  68.0° 
18°  64.4° 
16°  60.8° 
14°  57.2° 
12°  53.6° 
10°  50.0° 
8°  46.4° 
6°  42.8° 
4°  39.2° 
2°  :!5.6° 
0°  32.<i° 
2°  28.4° 
_  4°  24.8° 
—  G°  21.2° 
—  8°  17.6° 
—10°  14.0° 
—12°  10.4° 
—14°  6.8° 
—16°  3.2° 
—18°  —0.4° 
—20°  —4.0° 

In  Cwts. 
=  112  Lbs.  =  784,000 
Grains. 

(MOt~-OOTj<--C<IOO 

o'  o  o  o  o  o  o  o 

In  Avoirdupois  Lbs. 
=  7000  Grains. 

<M  O  IO  O  <M  ^  CO  tO 

<M  IM  o  ^f  tr  *M  ^  CM 

OOOOOC-IC^IO 

odododcMgj 

In  Troy  Ounces 
=  480  Grains. 

llgSjggSS 

3«Jddo«§g 

ja 

.J2    03 

'SiS 

9 

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MANUAL  OF  PHYSIOLOGY 


CHAPTER  I.   *:      ,-';,  i  ;::•  ;          •' 

THE  OBJECTS  OF  PHYSIOLOGY. 

Biology,  the  science  which  deals  with  living  beings,  may  be 
divided  into  two  branches,  viz. : — 

1.  MORPHOLOGY,  which  treats  of  the  forms  and  structure  of 
living  creatures ;  and, 

2.  PHYSIOLOGY,  which  attempts  to  explain  the  modes  of  activity 
exhibited  by  them  during  their  lifetime,  and  may  therefore  be 
defined  as  the  science  which  investigates  the  phenomena  presented 
by  the  textures  and  organs  of  healthy  living  beings ;  or,  in  short, 
the  study  of  the  actions  of  organisms  in  contradistinction  to  that 
of  their  shape  and  structure. 

The  organic  or  living  world  is  naturally  divided  into  the  Animal 
and  Vegetable  kingdoms.  We  have,  therefore,  both  animal  and 
vegetable  morphology  and  physiology.  In  studying  the  vegetable 
kingdom,  the  form  and  structure  as  well  as  the  activity  of  plants 
are  associated  together  to  form  the  subject  known  as  Botany.  The 
physiology  of  plants  need  not,  therefore,  be  considered  here ; 
though,  indeed,  a  knowledge  of  it  proves  useful  in  considering 
many  of  the  processes  belonging  to  animal  life.  On  the  other 
hand,  the  morphology  and  the  physiology  of  animals  are  com- 
monly taught  separately,  and  in  the  medical  curriculum  are 
made  distinct  subjects. 

Morphology  includes  the  external  form,  the  general  construc- 
tion or  anatomy  of  organisms,  and  the  minute  structure  of  their 
textures  as  revealed  by  the  microscope.  This  latter  branch  of 
3  25 


26  MANUAL   OF   PHYSIOLOGY. 

study,  under  the  name  Histology,  has  now  developed  into  a  very 
extensive  subject,  which  is  inseparable  from  either  physiology  or 
anatomy.  In  this  country  histology  is  commonly  taught  in  the 
medical  schools  with  physiology,  because  the  time  of  the  teachers 
of  morphology  is  occupied  in  expounding  the  nomenclature  of 
descriptive  anatomy,  while  the  microscope  is  in  every-day  use  in 
the  physiological  laboratory.  Moreover,  an  adequate  knowledge 
of  microscopic,  mfethods,  and  of  the  various  form  elements  of  the 
different 'textures,  of  the  body,  is  one  of  the  first  essentials  for 
physiological  study. 

As  the  different  actions  of  the  body  are  performed  by  different 
tissues,  which  in  the  higher  animals  are  grouped  together  as  dis- 
tinct organs,  a  general  idea  of  the  position  and  construction  of 
these  different  parts  of  the  body  must  be  acquired  before  the 
study  of  physiology  can  be  commenced.  Anatomy  and  general 
morphology  are  the  frameworks  upon  which  physiological  knowl- 
edge is  built  up.  Some  knowledge  of  these  subjects  must  there- 
fore precede  the  study  of  physiology,  in  order  that  the  student 
may  be  in  a  position  to  grasp  even  the  simplest  facts  connected 
with  any  physiological  question. 

We  shall  soon  find  that  the  assistance  of  other  sciences  is 
also  indispensable  to  physiology.  Thus  every  action  of  a  living 
texture  or  tissue  is  accompanied  by  some  chemical  change,  the 
chemical  process,  in  fact,  being  the  common  essential  part  of  the 
phenomena  of  life.  The  student  of  physiology  must,  then,  know 
something  of  the  science  of  chemistry  ;  indeed,  the  mode  of  action 
of  chemical  elements  forms  quite  as  important  a  groundwork  for 
the  study  of  the  activity  of  the  living  tissues  as  their  general  form 
or  minute  structure. 

Further,  the  laws  which  govern  the  motions  of  inanimate 
bodies  also  control  the  actions  of  living  tissues,  for  we  cannot 
claim  to  understand  or  recognize  the  existence  of  any  laws  affect- 
ing living  organisms  other  than  those  known  to  be  applicable  to 
dead  matter.  There  are  a  great  number  of  activities  shown  by 
living  textures  which  we  cannot  explain  by  the  recognized  laws 
of  chemistry  or  physics.  We  therefore  use,  for  convenience'  sake, 
the  term  "  vital  phenomena,"  to  indicate  processes  which  are 


THE   OBJECTS   OF    PHYSIOLOGY.  27 

beyond  our  present  chemical  and  physical  knowledge.  In  using 
this  term  we  must  not  think  it  implies  a  separate  set  of  natural 
laws  belonging  to  life.  We  cannot  discover  or  formulate  any 
special  laws  affecting  living  beings  only,  and  therefore  we  must  not 
assume  that  any  such  exist.  We  must  rather  endeavor  to  ex- 
plain all  the  so-called  "  vital  phenomena  "  by  means  of  the  laws 
known  to  chemists  and  physicists.  By  this  means  we  shall 
certainly  get  a  closer  insight  into  the  processes  of  life,  and  if 
there  be  laws  governing  the  living  beings  we  may  learn  to  know 
them.  This  method  of  working  has  already  given  good  results, 
for  within  comparatively  recent  times  many  of  the  processes 
which  were  regarded  as  specially  vital  in  character  have  been 
shoAvn  to  be  within  the  power  of  the  experimenter  and  to  depend 
on  purely  physico-chemical  processes. 

It  is  therefore  necessary  for  the  physiologist,  before  he  attempts 
to  explain  the  activities  of  any  organism,  to  be  familiar  not  only 
with  the  structure  of  its  body,  but  also  with  the  various  laws 
which  chemists  and  physicists  teach  us  control  the  operations  of 
inanimate  matter. 

The  sciences  of  chemistry  and  physics  may,  in  fact,  be  re- 
garded as  the  physiology  of  inorganic  matter,  just  as,  when 
chemistry  and  physics  are  applied  to  the  elucidations  of  the 
functions  of  living  creatures  by  the  biologist,  the  study  is  called 
physiology.  When  we  consider  how  far  from  thoroughly  grasp- 
ing and  interpreting  all  the  phenomena  presented  by  the  various 
kinds  and  conditions  of  matter  the  chemist  and  the  physicist 
still  are,  we  cannot  be  surprised  that  those  who  attempt  to  explain 
the  actions  of  living  beings  find  many  processes  that  they  are 
unable  to  comprehend.  So  that  when  physiologists  make  use  of 
the  convenient  term  "  vital  phenomena,"  it  must  be  remembered 
that  they  do  not  thereby  imply  the  existence  of  a  special  living 
force  or  any  kind  of  energy  peculiar  to  living  creatures. 

The  ultimate  object  of  physiology  is  not  yet  within  the  reach 
of  our  modern  methods  of  research.  To  explain  the  mode  of 
activity  of  living  beings,  and  grasp  the  exact  relation  borne  by 
their  living  phenomena  to  the  laws  which  govern  them,  is  a  task 
of  enormous  difficulty.  Indeed,  the  manifestations  of  certain  ener- 


28  MANUAL   OP   PHYSIOLOGY. 

gies  in  living  organisms  are  so  complicated  that  it  is  often,  if  not 
generally,  impossible  to  say  exactly  how  they  are  brought  about, 
and  we  are  therefore  obliged,  for  the  present  at  least,  to  be  satisfied 
with  the  mere  recognition  and  description  of  the  phenomena. 

Since  the  human  organism  is  the  special  study  of  students 
of  medicine,  the  contents  of  this  volume  should  properly  be 
restricted  to  the  physiology  of  man.  But  human  physiology 
cannot  be  studied  alone;  because  in  man  we  cannot  watch 
sufficiently  closely,  or  question  fully,  by  experiment,  the  phenom- 
ena of  life.  Further,  no  sharp  line  of  separation  can  be  drawn 
between  the  actions  of  the  various  organs  of  man  and  those  of 
the  lower  animals.  The  consideration  of  the  physiology  of  those 
animals  which  are  akin  to  man  must  therefore  go  hand  in  hand 
with  the  study  of  the  physiology  of  man  himself.  Much  light 
has  been  thrown  on  the  actions  of  the  complex  textures  of  the 
highest  animals,  by  the  observation  of  the  activities  of  the  lowest 
organisms,  where  the  manifestations  of  life  may  be  carefully 
watched  with  the  microscope  in  the  living  animal  under  its 
normal  conditions. 

GENERAL  CHARACTERS  OF  ORGANISMS. 

The  term  organism,  which  is  commonly  used  as  having  the 
same  meaning  as  living  being,  owes  its  derivation  to  the  com- 
plexity of  structure  common  among  the  higher  forms  of  life, 
which  are  made  up  of  several  distinct  organs.  This  organic 
construction  does  not  hold  good  as  a  distinguishing  mark  between 
living  beings  and  inanimate  matter,  because  we  are  acquainted 
with  a  vast  number  of  living  organisms,  both  plants  and  animals, 
which  are  not  made  up  of  organs,  but  are  composed  of  a  minute 
piece  of  a  soft,  jelly-like  material,  which  is  simply  granular 
throughout,  and  devoid  of  structural  differentiation  during  the 
life  of  the  creature. 

We  may  classify  the  general  characters  of  living  beings  as 
follows : — 

1.  Structural  and  physical  properties. 

2.  Chemical  composition. 

3.  Activities  during  life  (vital  phenomena). 


CHARACTERS   OF   ORGANISMS.  29 

1.  Structural  Characters  of  Organisms. — The  minute  structure 
of  living  beings  as  shown  by  the  microscope  no  doubt  helps  to 
distinguish  the  textures  of  organisms  from  inorganic  structures. 
Although  organic  textures  are  found  to  differ  very  widely  in 
their  characters,  they  are  all  related  in  one  respect,  namely,  that 
at  the  earliest  period  of  their  existence  they  consist  of  a  minute 
mass  of  a  substance  called  Protoplasm,  known  as  a  cell.    In  plants 
a  cellular  structure  remains  obvious  in  all  parts  of  the  adult,  no 
matter  how  much  the  texture  may  be  modified  by  adaptation  to 
the  requirements  of  any  given  duty  or  function.     If  we  examine 
with  the  microscope  the  leaves,  bark,  wood,  or  pith  of  a  plant,  in 
all  of  them  a  cellular  structure  can  be  recognized.     In  the  less 
developed   members   of  the   animal   kingdom,  and  during  the 
initial  stages  in  the  existence  of  the  highest  animals,  the  textures 
are  composed  exclusively  of  aggregations  of  living  cell  elements. 
We  shall  shortly  see  that  in  the  more  fully  developed  condition 
of  the  higher  animals,  the  cells  become  variously  modified  in  form 
and  function,  and  the  protoplasm  manufactures  various  structures 
adapted  to  the  performance  of  the  diverse  functions  of  the  dif- 
ferent parts.     In  all  organic  textures  which  can  be  said  to  be 
living,  cells  are  dispersed  in  greater  or  less  number,  and  regulate 
their  nutrition  and  repair. 

2.  Chemical  Composition. — There   are  no  characters  in   the 
chemical  composition  of  the  textures  of  organic  beings  which  can 
be  said  to  be  absolutely  distinctive  or  to  separate  them  from  in- 
organic matter.    No  doubt  their  chemical  construction  frequently 
exhibits  certain  peculiarities,  not  seen  in  dead  matter,  which  may 
be  taken  as  characteristic,  but  living  textures  only  differ  in  the 
general  plan  of  arrangement  and  composition  from  that  most 
commonly  met  with  in  the  construction  of  inorganic  materials. 

In  the  first  place,  the  great  majority  of  the  chemical  elements 
which  we  know  of,  take  no  share  in  the  formation  of  living 
creatures,  and  are  never  found  to  enter  into  their  composition. 
Practically,  only  fifteen  of  some  seventy  elements  known  to  chem- 
ists take  part  in  making  up  the  tissues  of  animals.  The  majority 
of  these  are  only  present  in  very  small  quantity  and  with  no 


30  MANUAL   OF   PHYSIOLOGY. 

great  constancy.  On  the  other  hand,  there  are  four  elements, 
namely,  carbon,  oxygen,  hydrogen  and  nitrogen,  which  are  found 
with  such  great  regularity,  and  in  so  great  quantity,  that  they 
may  be  said  to  make  up  the  great  bulk  (97  per  cent.)  of  the 
animal  frame.  The  great  constancy  with  which  the  first  three 
of  these  elements  occur  must  be  regarded  as  a  most  important 
character  of  organic  tissues. 

Secondly,  in  organic  substances  the  chemical  elements  are  asso- 
ciated in  much  more  complex  and  irregular  proportions.  Gen- 
erally, a  large  number  of  atoms,  of  each  element,  are  grouped 
together  to  form  the  molecule,  and  often  the  compound  is  so  com- 
plex that  its  chemical  formula  remains  a  matter  of  doubt.  As 
an  example  of  the  complexity  of  bodies  found  in  organic  analysis, 
a  remarkable  substance,  called  lecithin,  which  appears  in  the 
analysis  of  protoplasm  and  many  tissues,  may  be  mentioned. 
Its  formula  may  be  expressed  thus : — 

f  C18  H35  02 

n     TT       I     ^18  ^35  ^2 

35          O          PO|OHN(CH3)3 

0  \  0  -  C2H4  —  OH. 

It  is  peculiar  in  containing  nitrogen  and  phosphorus,  and  in  con- 
struction is  said  to  be  like  a  fat. 

In  inorganic  substances,  on  the  other  hand,  the  elements  are 
found  to  be  combined,  as  a  general  rule,  in  simple  and  regular 
proportions.  The  molecules  are  made  up  of  but  few  elements 
arranged  in  a  definite  manner  and  firmly  bound  together,  so 
that  they  are  not  prone  to  undergo  decomposition.  As  an 
example,  we  may  take  water,  which  has  the  well-known  formula, 

H,0. 

Though  these  bodies  may  be  taken  as  types  of  organic  and 
inorganic  substances  respectively,  it  must  not  be  imagined  that 
all  organic  bodies  are  as  complex,  irregular  and  unstable  as 
lecithin,  or  that  inorganic  compounds,  as  a  rule,  are  invariably 
simple  and  stable  like  water. 

It  is  further  remarkable  that  Carbon — an  element  which  is 
exceptional  in  forming  but  few  associations  in  the  mineral  world, 
where  it  chiefly  combines  with  oxygen  to  form  CO2 — is  almost 


CHARACTERS   OF   ORGANISMS.  31 

invariably  present  in  living  textures,  in  which  it  is  combined 
with  hydrogen  and  nitrogen  as  well  as  oxygen  in  various  propor- 
tions. The  constancy  of  carbon  as  an  ingredient  of  organic 
bodies  is  so  great  that  what  formerly  was  called  organic  chemis- 
try is  now  often  called  the  chemistry  of  the  carbon  compounds. 

These  complex  associations  of  many  atoms  of  carbon  with- 
many  atoms  of  other  elements,  are  readily  dissociated  when 
exposed  to  the  air  under  even  slightly  disturbing  influences.  When 
heated  to  a  certain  degree  they  burn,  i.  e.,  unite  rapidly  with  the 
oxygen  of  the  air;  and  in  the  presence  of  minute  organisms  they 
putrefy.  Thus  instability  is  a  general  feature  commonly  met  with 
in  most  substances  of  organic  origin. 

Chemical  instability  reaches  the  highest  pitch  in  tissues  which 
are  actually  alive  and  engaged  in  vital  processes.  So  long  as 
any  texture  lives,  i.  e.,  is  capable  of  performing  its  functions,  it 
must  constantly  undergo  ceitain  chemical  changes,  a  kind  of 
decomposition,  tending  to  produce  disintegration,  and  a  reinte- 
gration  by  means  of  new  chemical  associations  with  fresh  mate- 
rials. A  tissue  may  then  be  said  to  deserve  the  term  living,  only 
as  long  as  it  undergoes  these  antagonistic  chemical  changes.  The 
tendency  to  destructive  oxidation  or  disintegration  is  intimately 
connected  with  the  functional  activity  of  the  living  texture  and 
increases  with  this  activity.  The  reintegration  or  constructive 
process  requires  the  presence  of  suitable  materials  with  which  the 
texture  may  combine,  in  order  to  make  up  for  the  loss.  Thus  living 
tissues  are  ever  on  the  point  of  destruction,  which  can  only 
be  warded  off  by  the  timely  reconstruction  of  their  chemical 
ingredients  by  suitable  fresh  materials.  This  reconstruction  by 
means  of  fresh  matter  from  without  is  called  assimilation,  and 
forms  the  most,  if  not  the  only,  satisfactory  criterion  by  which 
adequately  to  distinguish  living  beings  from  inorganic  matters. 

The  object  of  assimilation  is  to  supply  suitable  fresh  materials 
to  the  various  textures  for  the  chemical  processes  required  for 
their  function  while  living.  This  will  be  found  to  form  a  great 
part  of  physiological  study.  Further,  the  energy  manifested  in 
the  living  activity  of  the  textures  depends  upon  the  various 
oxidizing  processes,  and  the  exact  laws  which  govern  these  com- 


32  MANUAL   OF    PHYSIOLOGY. 

bustions,  and  the  results  they  produce  in  the  various  tissues,  prac- 
tically make  up  the  other  part  of  physiology. 

3.  Vital  Phenomena. — The  so-called  vital  phenomena  which 
take  place  in  the  textures  of  organisms  are,  for  the  most  part, 
performed  by  the  agency  of  the  living  cell  elements,  in  which 
'we  can  recognize  independent  manifestations  of  life,  such  as  the 
response  to  stimuli,  motion,  nutrition,  growth,  etc.  The  living 
activity  of  organisms  requires  for  its  perfect  development  certain 
external  conditions,  namely,  a  certain  degree  of  warmth  and 
moisture.  Without  a  certain  degree  of  warmth  and  moisture  the 
chemical  interchanges  just  mentioned  cannot  go  on,  and  the 
organism  is  either  destroyed  or  remains  in  a  state  of  inactivity. 

The  nutrition  of  the  animal  body  which  is  accomplished  by 
means  of  the  processes  of  assimilation  already  mentioned  enables 
it  to  grow,  and,  up  to  a  certain  point,  increase  in  size,  and  further 
undergo  many  changes  in  form  and  texture.  There  is,  however, 
a  limit  to  this  assimilative  power:  nutritive  activity  diminishes, 
growth  gradually  stops,  and  after  a  time  decay  appears  and  is 
followed  by  death. 

Thus  organisms  exist  only  for  a  limited  period  of  time,  during 
which  their  size,  form  and  functional  activity  are  constantly 
undergoing  some  general  alteration  dependent  on  or  concurrent 
with  the  incessant  changes  in  their  molecular  construction. 

This  cycle  of  changes  through  which  organisms  pass  we  speak 
of  as  their  lifetime.  'During  this  lifetime,  at  the  period  when 
their  functional  activity  is  at  its  height,  they  possess  the  remark- 
able faculty  of  producing  individuals  like  themselves. 

This  is  accomplished  by  setting  apart  a  cell  which,  under 
favorable  circumstances,  assumes  special  powers  of  growth, 
increases  in  size  by  the  rapid  formation  of  new  cells,  and  develops 
into  an  independent  living  unit.  In  time  it  arrives  at  maturity, 
and  becomes  like  its  parent,  and  then  passes  through  the  same 
cycle — by  its  power  of  assimilation  it  grows  to  maturity,  repro- 
duces its  like,  decays  and  dies. 


STRUCTURAL   CHARACTERS   OF   ANIMALS. 


33 


FIG.  1. 


CHAPTER  II. 

GENERAL    VIEW    OF    THE    STRUCTURAL    CHARACTERS    OF 
ANIMAL   ORGANISMS. 

The  parts  played  by  Cells  in  the  functions  of  living  beings  are 
so  many  and  so  important  that  it  is  necessary  at  the  very  outset 
to  consider  the  properties  of  the 
individual  elements  somewhat  in 
detail. 

The  demonstration  of  the  cel- 
lular structure  of  plants  was  first 
made  in  1832  by  a  distinguished 
German  botanist  named  Schlieden, 
who  considered  the  cells  to  be 
characteristic  of  plant  tissue.  A 
few  years  later  Schwann  showed 
that  the  animal  tissues,  though  not 
so  obviously,  were  also  made  up  of 
cells,  and  that  they  owed  their 
beginning  and  development  to  the 
activity  of  cell  elements.  Thus 
originated  the  "cellular  theory," 
which,  with  some  modifications,  is 
now  the  basis  of  all  physiological 
inquiry. 

The  first  idea  which  was  conveyed 
by  the  term  cell  varied  much  from 
that  which  we  now  accept  as  a 
proper  definition  of  such  an  organic 
unit. 

Fully  developed  vegetable  cells 
being  the  first  discovered  were  taken 
as  the  type  of  all.  The  main  charac- 
teristics of  these  may  be  briefly 


Cells  from  the  root  of  a  plant.  (X550.) 

1.  Showing  youngest  cells  with  thin 
walls  (w),  filled  with  protoplasm 
and   containing   nucleus  (N),  and 
nucleolus  (N'). 

2.  Older  cells  with  thicker  walls  with 
vacuoles  and  cell  sap  (s). 

3.  Shows  further  diminution  of  pro- 
toplasm and  increase  in  cavity  (s) 
in  proportion  to  the  growth  of  the 
cell  wall  (w). 


34 


MANUAL   OF   PHYSIOLOGY. 


summed  up.  First,  a  membranous  sac  called  the  cell  wall,  gen- 
erally very  well  defined,  and,  secondly,  within  the  cell  wall  vari- 
ous cell  contents.  Among  the  more  conspicuous  of  the  latter  may 
be  mentioned  (1)  a  soft,  clear,  jelly-like  substance  called  pro- 
toplasm, in  which  lies  a  nucleus,  and  (2)  certain  cavities  called 
vacuoles,  which  are  filled  with  a  clear  fluid  or  cell  sap. 

Further  investigation  of  the  life  history  of  cells,  particularly 
in  the  early  stages  of  their  development,  showed  that  the  cell  wall, 
which  played  so  important  a  part  in  the  original  conception  of  a 
cell,  was  not  always  present,  but  was  formed  by  the  protoplasm 
in  a  later  stage  of  growth.  The  cell  sap  and  other  matters  were 
found  to  occur  less  commonly,  and  appeared  still  later  than  the 
cell  wall  in  the  lifetime  of  the  vegetable  cell ;  hence  it  was  con- 

FIG.  2. 


Diagram  of  animal  cell 
(ovum).  (Gegenbauer.) 
a.  Granular  protoplasm. 
6.  Nucleus. 
c.  Nucleolus. 


Liver  cell  of  man,  containing  fat  globules  (b)  and 
biliary  matters.    (Cadiat.) 


eluded  that  they  were  the  outcome  of  changes  due  to  the  activity 
of  the  protoplasm,  and  that  this  latter  was  the  only  essential  and 
formative  part  of  the  cell. 

Subsequently,  from  the  facts  that  some  vegetable  cells  in  the 
youngest  and  most  active  stage  of  their  growth  have  no  limiting 
wall,  and  that  most  animal  cells  have  none  during  any  part  of 
their  life,  it  was  proposed  to  define  a  cell  as  a  mass  of  protoplasm 
containing  a  nucleus.  But  further  research  showed  that  the 
nucleus  was  not  always  present.  In  many  cryptogamic  plants 
no  nucleus  can  be  found,  and  in  some  animal  cells,  which  must 
be  regarded  as  independent  individuals  (Protamoeba),  there  is  no 
nucleus  at  any  part  of  their  lifetime.  This  would  lead  us  to 
suppose  that  a  mass  of  protoplasm  capable  of  manifesting  all  the 


STRUCTURAL   CHARACTERS    OF   ANIMALS.  35 

phenomena  of  life  would  be  a  sufficient  definition.  Though  this 
is  probably  correct  in  a  few  cases,  the  vast  majority  of  cells  do 
contain  nuclei.  As  it  is  difficult  to  divest  our  minds  of  the  con- 
nection between  the  two,  it  has  been  proposed  to  give  the  name 
cytode  to  the  non-nucleated  forms,  which  certainly  are  very 
exceptional,  reserving  the  term  cell  for  the  common  nucleated  unit. 
Each  part  of  the  cell  may  now  be  considered  in  the  order  of  its 
importance,  viz.,  protoplasm,  nucleus,  cell  wall,  and  cell  contents. 

1.  Protoplasm   is   commonly    seen   to   be   a   colorless,   pale, 
milky,   semi-translucent    substance,    more    or    less    altered    in 
appearance  by  various  foreign  matters  lying  in  it.     These  latter 
also  give  it  a  granular  appearance,  and  when  dead  it  commonly 
exhibits  a  linear  marking  or  fine  network.     During  life  its  con- 
sistence is  nearly  fluid,  varying  with  the  circumstances  in  which 
it  is  placed,  from  that  of  a  gum  solution  to  a  soft  jelly.     When 
living  unmolested  in  its  normal  medium  it  seems  to  flow  into 
various  shapes,  but  this  is  a  living  action  which  does  not  prove 
it  to  be  diffluent,  for  any  attempt  to  investigate  it  by  experiment 
causes  a  change  in  its  consistence  approaching  to  rigidity. 

As  the  full  comprehension  of  the  function  of  this  substance 
lies  at  the  root  of  the  greater  part  of  Physiology,  the  reader  is 
referred  for  a  detailed  account  of  its  properties  to  Chapter  in,  on  p. 
Vital  Phenomena,  where  it  will  be  discussed  at  greater  length. 

2.  The   Nucleus. — The   majority   of   independent   masses   of 
protoplasm,  and  all  highly  organized  cells,  contain  one  or  more 
nuclei  in  their  substance.     The  nucleus  is  sharply  marked  off 
from  the  protoplasm,  and  is  supposed  ta  be  surrounded  by  a  spe- 
cial limiting  membrane.     Its  presence  can  generally  be   made 
much  more  conspicuous  by  treating  the  cell  with  certain  chemical 
reagents,  notably  dilute  acids  and  various  dyes.      The  nucleus  is 
able  to  resist  the  action  of  dilute   acetic   acid  better  than  the 
remainder  of  the  cell,  so  that  it  stands  out  clearly,  when  the  rest 
becomes  transparent.      Many  staining  agents,  such  as  magenta 
(one  of  the  aniline  dyes),  color  the  nucleus  more  quickly  and 
deeply  than  the  protoplasm.  Although  it  is  accredited  with  spe- 
cial independent  movements  that  occur  under  certain  circum- 


36  MANUAL   OF   PHYSIOLOGY. 

stances,  compared  with  the  protoplasm  it  is  not  very  contractile. 
It  appears  to  be  intimately  associated  with  the  vital  phenomena 
of  the  cell,  and  may  be  said  to  control  or  initiate  its  most 
important  activity,  namely,  its  division.  In  the  nuclear  matrix, 
which  is  clear  and  homogeneous,  may  often  be  seen  an  irregular 
network,  one  point  of  which  stands  out  more  clearly,  and  is  called 
the  nucleolus.  Remarkable  changes  in  the  arrangement  of  this 
network  are  seen  in  some  cells  to  precede  the  division  of  the 
protoplasm.  This  is  called  Jcaryokinesis. 

3.  The  Cell  Wall.— It  has  already  been  stated   that  the  most 
active  cells,  such  as  are  found  in  the  earliest  stages  in  the  life  of 
an  organism  (embryonic  cells),  have  no  enclosing  membrane  or 
cell  wall.     But  in  the  more  advanced  stages  of  cell  life  we  find 
this  second  form  of  protoplasmic  differentiation  to  be  common 
enough.     In  animal  cells  the  limiting  membrane  has  never  the 
same  importance  as  the  cell  wall  in  vegetable  tissues,  where  some 
of  the  principal  textures  may  be  traced  to  a  direct  modification 
of  the  cell  wall,  still  recognizable  as  such.     Whenever  such  a 
limiting  membrane  exists,  it  is  formed  by  the  outer  layers  of 
protoplasm  undergoing  changes  so  as  to  become  of  greater  con- 
sistence.    In  the  animal  tissues  the  cells  form  various  structures, 
which  are  not  limiting  membranes  or  cell  walls,  but  rather  give 
the  idea  of  lying  between  the  cells.     Hence,  in  one  large  group 
of  tissues,  they  have  been  called  intercellular  substance,  while 
in  others  they  appear  as  materials   specially  modified  for  the 
furtherance  of  the  functions  of  the  special  tissues. 

4.  Cell    Contents. — Regarding   protoplasm    as    the   essential 
living  part  of  the  cell,  under  this  heading  will  come  only  those 
extraneous   matters  which   are    the   outcome   of   protoplasmic 
activity. 

The  cell  contents  which  are  present  with  such  constancy  and 
in  such  variety  in  vegetable  cells,  form  in  them  an  all-important 
part ;  but  in  most  animal  cells  the  contents  do  not  occupy  such 
a  striking  position. 

No  doubt  animal  protoplasm  is  quite  as  capable  as  that  of 
vegetables  of  making  out  of  its  own  substance,  or  the  nutriment 


STRUCTURAL   CHARACTERS   OF   ANIMALS.  37 

supplied  to  it,  a  great  variety  of  materials,  but  these  are  seldom 
stored  in  such  large  quantities  in  animal  cells  as   in  those  of 

PlantS"  FIG.  4. 

In  the  cells  of  some  kinds  of 
animal  textures,  particularly  that 
called  Connective  Tissue,  we  com- 
monly find  large  quantities  of  fat 
formed  and  accumulated  to  such 
a  degree  in  the  cell  that  the  proto- 
plasm can  be  no  longer  recognized 
as  such.  Its  remnant  is  devoted  to 
forming  a  limiting  membrane  for 

fVio    -fat+TT-    nrmfpntc    er»   tVint    tV»p  ppll     Cell  from  connective  lissue  cuiitaiuhig 

tne  tatty  contents,  so  tnat  tne  cei  large  fat  globule  (a)i  and  showing 
is  converted  into  an  oil  vesicle,  and  £™£bp^  ($^4?"cleu8  ^  (m) 
here  what  may  be  termed  the  con- 
tents become  the  most  important  part  of  the  cell.  In  various 
glandular  cells,  as  will  be  seen  hereafter,  different  substances  are 
made  and  stored  up  temporarily  in  the  protoplasm.  These  may 
be  seen  as  bright  refracting  granules,  which  are  subsequently 
discharged  in  the  secretion  of  the  gland. 

In  other  cells  (liver)  nutrient  material  allied 'to  starch  may 
be  deposited  in  considerable  quantity,  just  as  starch  is  stored  in 
certain  cells  of  plants,  but  owing  to  the  greater  and  more  con- 
stant activity  of  animals,  the  amount  laid  by  never  attains  any- 
thing like  that  found  in  the  store  textures  of  vegetables,  where 
the  result  of  an  entire  summer's  active  work  is  put  by  as  a  pro- 
vision for  the  next  winter  and  the  fresh  burst  of  energy  which 
follows  it  in  the  spring. 

But  while  the  above  are  all  more  or  less  temporary  contents 
of  cells,  we  have  an  example  of  a  permanent  deposit  in  them, 
viz.,  pigment ;  this  substance  is  formed  by  the  protoplasm  in 
various  parts,  and  has  a  special  physiological  use.  Thus  in  the 
tissue  behind  the  retina — or  nerve  layer  of  the  eyeball — the 
cells  are  filled  with  granules  of  a  pigmented  substance,  which 
absorbs  the  light  falling  upon  it,  and  thus  prevents  the  reflec- 
tions which  would  interfere  with  the  clearness  of  sight. 

It  also  occurs  in  the  skin  of  the  negro  and  other  races,  and  in 


38  MANUAL   OF   PHYSIOLOGY. 

that  of  the  frog  and  other  animals,  but  in  these  its  function  is 
not  fully  known. 

Varieties  of  Cells. — Great  varieties  of  cells  are  found  in  the 
various  mature  tissues  of  the  higher  animals,  all  of  which  have 
passed  through  the  stage  of  being  a  simple  nucleated  mass  of 
protoplasm  in  the  earlier  periods  of  their  development.  All 
cells  may  then  be  divided  into  two  chief  types,  the  indifferent 
and  the  differentiated. 

Under  the  category  of  indifferent  cells  may  be  placed  all  such 
as  retain  the  characters  of  the  first  embryonic  cells,  and  have 
not  acquired  any  special  structure  or  property  by  which  they 
can  be  distinguished  from  the  simplest  form.  Such  cells  are  the 
only  ones  in  the  early  stages  of  the  embryo.  In  the  adult 
tissues  they  also  occur,  having  various  duties  to  perform.  They 

FIG.  5. 


Transverse  section  of  Blastoderm,  showing  the  elements  in  the  earlier  stage  of  the 
development.    A,  epiblast;  B,  mesoblast ;  C,  hypoblast. 

are  found  in  the  blood  and  lymph,  and  scattered  throughout  the 
tissues.  They  are  without  a  cell  wall,  and  have  no  special  con- 
tents to  mark  their  function. 

Among  the  differentiated  cells  we  find  many  special  characters, 
adapting  them  to  certain  special  duties,  for  all  these  cells  are 
modified  from  the  original  type  and  applied  to  the  performance 
of  some  special  function. 

Space  prevents  even  a  short  enumeration  of  the  varieties  of 
cells  met  with  in  the  tissues  of  plants,  where  they  not  only  carry 
on  the  active  functions  of  the  organism,  but  also  form  the  sup- 
porting structures. 

The  differentiation  of  a  cell  is  accomplished  by  its  protoplasm, 
which  forms  new  structural  parts,  and  itself  sometimes  seems  to 


TISSUE   DIFFERENTIATION.  39 

diminish  in  quantity  until  an  element  is  produced  in  which  there 
may  be  no  protoplasm  recognizable. 

We  find,  then,  matured  and  differentiated  cells  which  vary — 

1.  In  shape,  being  spherical,  flattened,  fusiform,  stellate,  etc. 

2.  In  size. 

3.  In  their  mode  of  connection. 

Cells  may  also  be  classified  according  to  their  function,  e.  'g., 
Glandular,  Nervous,  etc.,  and  the  greater  portion  of  the  follow- 
ing pages  will  be  devoted  to  the  functions  of  these  various  forms 
of  cells. 

So  long  as  a  cell  remains  in  its  indifferent  stage  it  possesses 
the  properties  of  ordinary  protoplasm  only ;  but  by  its  further 
development  it  acquires  special  properties  not  common  to  all  pro- 
toplasm. These  properties  may  or  may  not  be  accompanied  by 
structural  change.  Thus  the  protoplasm  of  a  gland  cell  differs  in 
little  from  that  of  any  other  cell  except  in  the  capabilities  of  its 
nutritive  changes  and  its  chemical  products;  while  on  the  other 
hand,  those  epithelial  cells  which  form  the  outer  layer  of  the  skin 
lose  their  protoplasmic  characters  and  are  completely  modified  in 
structure. 

TISSUE  DIFFEEENTIATION. 

The  first  stage  in  the  existence  of  any  organism,  from  the 
simplest  form  of  plant  to  man,  consists  of  a  single  ceil  (in  ani- 
mals called  the  ovum  or  egg),  which  differs  in  no  essential  points 
of  structure  from  an  ordinary  cell. 

There  is  moreover  a  class  of  organisms  in  which  the  individ- 
uals never  go  beyond  this  stage,  but  pass  their  entire  lifetime 
in  the  state  of  a  simple  unicellular 

rru          •      T     •  i        i  FlG-  6- 

organism.  The  individuals  com- 
posing this  group  (Protista),  though 
insignificant  in  point  of  size,  may 
vie  with  the  higher  plants  and 
animals  in  number,  species  and 
variety  of  form,  so  that  they  might 

Well    be     placed    in    a    kingdom    bv     Unicellular  organism.    Small  amoiba. 
^  f       ,         ,  ICadial.) 

themselves  (as  has  been  proposed), 

apart  from  the  vegetable  and  animal  kingdoms. 


40 


MANUAL   OF   PHYSIOLOGY. 


The  group  of  these  organisms  which  most  resembles  animals, 
is  called  Protozoa,  and  is  divided  from  other  animal  forms  by  the 


FIG.  7. 


Stages  in  the  division  of  the  egg  cell  (ovum),  showing  the  production  of  a  multiple  mass 
by  division.    (Gegenbauer.) 

manner  of  development  of  the  ovum  of  the  latter,  which  divides 
into  cells  that  subsequently  become  differentiated  into  tissue. 
This  group  is  called  the  Metazoa. 

In  the  Protozoa  the  ovum  never  divides,  the  animal  always 
remaining  a  single  cell.  On  the  contrary,  the  ovum  of  the 
Metazoa- changes  its  characters  during  its  development.  At  first 
possessing  a  stage  common  to  both  divisions,  viz.,  a  single  cell,  it 
soon  passes  through  rapid  stages  of  cell  proliferation,  and  is  con- 
verted into  a  multiple  mass,  the  mulberry  stage  or  Morula. 

The  cells  forming  this  Morula  stage  approach  the  periphery  of 
the  mass,  where  they  arrange  themselves  in  two  layers,  and  form 
a  cavity  in  the  centre.  This  is  known  as  the  Oastrula  stage. 
Following,  then,  this  cell  multiplication,  we  find  a  qualitative 
differentiation  of  the  cells,  by  which  certain  groups  of  cells  assume 
special  peculiarities,  fitting  them  for  some  specific  duty. 

Thus  we  arrive  at  the   production   of  special   textures  and 


TISSUE   DIFFERENTIATION. 


41 


Diagram  showing  the  first 
differentiation  of  the  organ- 
ism into  an  external 
and  internal  layer,  (a) 
Mouth,  (ft)  alimentary  cav- 
ity, (d)  ectoderm,  (c)  endo- 
derm.  (Gegenbauer.) 


organs  such  as  are  met  with  in  the  higher  animals,  and  which 
are  necessary  for  the  efficient  discharge  of  the  various  functions 
carried  on  during  their  lives.  The  divi- 
sion of  the  original  mass  of  indifferent 
cells  into  two  layers  of  special  cells  is  the 
first  step  toward  tissue  differentiation, 
and  in  some  animals  is  the  only  one 
arrived  at  in  their  life  history,  throughout 
which  they  remain  a  simple  sac  made  up 
of  an  external  layer,  Ectoderm,  and  an 
internal  layer,  Endoderm. 

The  groups  of  cells  forming  the  outer 
and  inner  layers  of  this  stage  of  develop- 
ment, not  only  form  the  primitive  tissues, 
but  also  represent  the  first  appearance 
of  organs  or  parts  with  a  specific  func- 
tion. The  external  or  ectodermic  layer 
is  the  supporting,  protecting,  motor  and  respiratory  organ,  while 
the  inner  or  endodermic  layer  is  devoted  to  a  primitive  form 
of  digestion,  preparing  the  food  for  assimilation,  and  generally 
presiding  over  the  nutrition  of  the  body. 

Although  this  sac-like  (Gastrula)  stage  is  supposed  to  have 
formed  a  step  in  the  life  history  of  nearly  all  animals,  yet  it 
forms  a  less  striking  part  in  the  development  of  the  individuals 
as  we  ascend  the  scale,  and  in  the  higher  animals  no  such  stage 
has  been  recognized.  In  the  Vertebrates,  the  germ  cells  derived 
from  the  ovum  are  from  an  early  period  divided  into  three 
distinct  layers,  as  those  which  correspond  to  the  Ectoderm  and 
Endoderm  of  the  lower  organisms  form  between  them  a  third 
layer  or  Mesoblast. 

From  these  germinal  layers  all  the  organs  and  tissues  of  the 
body  are  subsequently  evolved.  In  embryological  language  the 
three  primitive  layers  are  called  JEpir,  Meso~,  and  Hypo-blast. 

Thus  it  can  be  seen  that,  as  we  can  compare  the  primitive  uni- 
cellular state  of  the  lowest  animals  with  the  first  egg-cell  stage  of 
existence  of  the  highest  animals,  so  we  can  compare  all  the  steps 
of  tissue  and   organ   differentiation    as  we  trace  them  in  the 
4 


42 


MANUAL   OF   PHYSIOLOGY. 


embryo  of  a  mammal,  with  the  steps  of  elaboration  in  organic 
and  textural  parts  that  we  find  in  ascending  the  scale  of  animal 
life. 

The  history,  then,  of  the  development  of  any  mammal  from  a 
single  cell  or  egg  to  the  complex  adult  individual,  is  analogous 
with  the  more  protracted  history  of  the  evolution  of  the  animal 
kingdom  from  the  Protista  upward. 

It  is  impossible  to  separate  the  differentiation  of  tissues  and 
organs,  or  to  say  which  is  of  older  date  in  the  history  of  animal 
evolution.  Even  in  unicellular  animals,  where  we  have  no  trace 
of  tissue  difference  (Paramsecium,  Vorticella),  there  being  only 
one  cell,  we  have  a  distinct  foreshadowing  of  organ  and  func- 


FIG.  9. 


A.  Epiblast. 


Transverse  section  of  blastoderm  of  chick. 

B.  Mesoblast.        C.  Hypoblast.       pr.  Primitive  groove. 


tional  differentiation  (vide  Chapter  in).  And  in  creatures 
made  of  many  parts,  the  same  cells  have  several  duties  to  per- 
form. But  when  an  aggregation  of  specialized  cell  units  exists, 
it  may  be  said  to  be  a  tissue.  If  these  cells  have  no  very  special 
characteristic,  then  the  tissue  may  be  called  primitive  or 
embryonic.  But,  as  has  just  been  stated,  the  aggregation 
of  embryonic  cells — in  the  higher  forms  of  life — have  special 
characters  from  the  very  first,  which  mark  them  off  from  one 
another  as  destined  for  different  functions. 

The  middle  germ  layer  (mesoblast)  is  derived  from  the  upper 
(epiblast)  and  lower  (hypoblast),  the  relative  amount  contributed 
by  each  being  doubtful.  From  the  earliest  period  the  middle 


TISSUE   DIFFERENTIATION.  43 

layer  has  distinctive  characteristics,  and  ultimately  gives  rise  to  a 
set  of  tissues  which  can  always  be  distinguished  from  those  which 
originate  from  the  upper  and  lower  layers. 

From  the  inner  and  outer  germ  layers  are  formed  several  con- 
nective tissues,  which,  in  a  more  or  less  perfect  degree,  retain  the 
activity  of  the  original  protoplasm,  and  hence  may  be  called 
active  tissues.  From  the  middle  germinal  layer  is  developed 
a  set  of  textures,  in  the  majority  of  which  the  protoplasmic 
elements  are  reduced  to  a  minimum,  and  are  therefore  grouped 
together  as  supporting  tissues. 

The  tissues  formed  in  the  adult  may  be  classified  into  four 
groups : — 

1.  Epithelial  Tissues.     The  primitive  surface  tissue  of  the 

epiblast  and  the  hypoblast,  which  are  variously  modi- 
fied for  several  distinct  functions. 

2.  Nerve  Tissues.     Springing  from  the  former,  are  modified 

for  receiving,  conducting,  controlling  and  distributing 
impressions. 

3.  Muscle,  or  Contractile  Tissues.     In  close  relation  to  both 

the  previous  and  the  next  groups. 

4.  Connective  Tissues  formed  only  from  the  middle  germ 

layer.  They  are  much  modified  in  different  parts,  so  as 
to  give  shape  to  the  body,  and  to  support  and  hold  the 
various  organs  and  parts  firmly  together.  They  are, 
in  fact,  the  materials  used  in  the  general  body  archi- 
tecture. 

Epithelial  Tissue,  although  the  oldest  kind  of  tissue  both  in 
the  animal  series  and  in  the  germinal  layers,  retains  the  embry- 
onic character  of  being  entirely  composed  of  cells  placed  in 
close  relationship  on  the  internal  and  external  surfaces  of  the 
body.  The  individual  cells  retain  the  embryonic  character  in 
form  and  function,  being  soft,  rounded  masses  of  protoplasm, 
only  altered  in  shape  by  the  pressure  of  their  neighbors.  The 
cells  which  lie  next  the  nutrient  vessels  of  the  mesoblast  are 
endowed  with  energetic  powers  of  growth  and  reproduction. 
As  the  young  cells  are  produced  they  take  the  place  of  the  parent 


44 


MANUAL   OF   PHYSIOLOGY. 


cell,  whose  future  life  history  determines  the  special  characters 
of  the  different  kinds  of  tissues. 

Sometimes  the  cells  are  retained,  as  in  the  skin,  and  are 
arranged  in  several  layers,  one  over  the  other.  As  the  cells  are  con- 
veyed from  the  deeper  layer,  where  they  take  their  origin,  toward 
the  surface,  the  efforts  of  the  waning  nutritive  power  of  the 
protoplasm  are  devoted  to  the  manufacture  of  a  tough,  insoluble 

FIG.  10. 


Section  of  the  epiderm  of  the  prepuce  showing  the  superimposed  layers  of  cells  of  a 
stratified  epithelium.    (Cadiat.) 

a.  Young  proliferating  cells.    6 — d.  Cells  advancing  toward  surface,    e.  Flattened  cell 
of  horny  layer.   /.  Basement  membrane,    g.  Connective  tissue. 

substance.  The  cells  thus  gradually  lose  their  vital  activities, 
and  are  converted  into  horny  scales,  which  form  the  external 
protecting  skin,  and  its  many  modifications  that  give  rise  to 
the  different  dermal  appendages,  such  as  hair,  feathers,  etc. 
Instead  of  a  horny  substance,  the  protoplasm  may  manufacture 
fat  in  the  bodies  of  the  cells,  as  seen  in  the  mammary  and  the 


EPITHELIAL   TISSUE. 


45 


sebaceous  glands  of  the  skin.  In  other  cases  the  reproductive 
activity  of  the  cell  is  in  abeyance,  and  its  nutritive  energy  is 
devoted  to  the  manufacture  of  a  material  which  is  poured  out  of 


FIG.  11. 


FIG.  12. 


Two  cells  of  scaly  epithelium  from  the  inside 
of  the  cheek.    (Ranvier.) 


Section  of  milk  gland  of  cat,  show- 
ing secreting  cells  containing  fat 
globules,  and  some  secretion  in 
alveoli. 


the  cell  at  certain  periods.     Thus  we  have  another  function  per- 
formed by  the  epithelial  tissues,  namely,  that  of  manufacturing 


FIG.  13. 


FIG.  14. 


Ciliated  epithelial  cells  from  the       Stratified  ciliated  epithelial  cells  from  the  trachea 
gills  of  mussel.    (Cadial.)  of  man.    (Cadiat.) 

a.  Large  surface  cells,  with  cilia  on  surface. 
6.  Lower  cells  in  earlier  stage  of  development. 
e.  Cell  charged  with  mucus. 

certain  materials  which,  being  collected  by  suitable   channels, 
appear  as  secretions. 


46  MANUAL   OP   PHYSIOLOGY. 

The  active  elements  of  glandular  tissue  are  epithelial  cells 
whose  nutrition  leads  to  the  formation  of  specific  chemical  pro- 
ducts within  their  protoplasm.  These  products  pass  out  com- 
monly as  fluids,  and  form  various  substances  of  great  importance 
in  the  economy.  A  gland  is  simply  a  special  arrangement  of 
epithelial  cells  lining  the  sacs  or  tubes  into  which  the  secretion 
is  poured.  Some  tracts  are  covered  with  fine,  moving,  hair-like 
processes,  called  cilia,  which  give  rise  to  a  slight  motion  of  the 
fluids  in  contact  with  them. 

The  epithelium  in  various  places  is  thus  seen  to  be  modified  in 
different  ways,  so  as  to  make  it  suitable  for  the  special  function 
of  the  part  in  which  it  is  placed. 

Other  differences  will  be  given  in  detail  with  the  description 
of  the  uses  of  the  many  mucous  surfaces.  The  most  interesting 
modifications  are  those  in  the  special  sense  organs,  where  the 
cells  are  in  immediate  connection  with  nerves,  and  aid  in  form- 
ing the  special  nerve  terminals.* 

Nerve  Tissue. — The  great  nervous  centres  are  formed  from 
the  cells  of  the  epiblast,  which,  in  the  earliest  days  of  the  embryo, 
form  a  longitudinal  furrow,  which  sinks  into  the  cells  of  the 
mesoblast.  By  the  rapid  growth  of  the  latter  the  depressed 
part  is  cut  off  from  the  rest  of  the  epiblast,  and  forms  the  rudi- 
ment of  the  spinal  cord,  and  brain.  In  looking  for  special  con- 
ducting tissue  in  animals  possessing  the  most  simple  structure, 
we  find  cells  which  would  seem  to  possess  certainly  a  twofold, 
and  possibly  a  threefold  function, — one  of  which  is  conduction. 
In  the  so-called  "  neuro-muscular  "  cells  of  the  hydra,  processes 
are  described  as  passing  off1  from  them,  and  uniting  beneath 
the  ectoderm  with  other  fibre-like  processes,  which  are  evidently 
contractile.  Here  we  find  for  the  first  time  a  portion  of  proto- 
plasm specially  devoted  to  acting  as  a  conductor  of  impulses,  and 
attached  by  the  one  end  to  a  contractile  fibre,  and  by  the  other 
to  a  surface  (sensory)  cell.  The  intimate  relation  between  the 
development  of  nerve  and  muscle  fibres  is  thus  established,  and 

*  A  further  account  of  the  Histology  of  these  tissues  will  be  found  in  the  chapters 
specially  devoted  to  these  subjects. 


NERVE   TISSUE. 


47 


we  have  the  first  indication  of  a  nerve  mechanism,  viz.,  a  cell 
capable  of  receiving  stimulations,  and  a  fibre  capable  of  trans- 
mitting the  resulting  impulses.  As  further  differentiation  pro- 


FIG.  16. 


Epithelial  cells,  some  of  which  are 
filled  with  mucus  (d),  forming  goblet- 
like  cells.  (Cadiat.) 


Neuro-inuscular    cells  of    hydra,     m. 
Contractile  fibres.    (Kleinenberg.) 


ceeds,  each  of  these  parts  becomes  more  distinct  from  the  other, 
and  ultimately  the  adult  nerve  tissue  is  found  to  be  made  up  of 
nerve  fibres,  and  special  cells,  forming  nerve  endings. 


FIG.  17. 


FIG.  18. 


S.  Sensory  receiving  organ 
with  attached  afferent 
nerve  fibre. 

(jr.  Central  organs— ganglion 
cells. 

M.  Peripheral  organ  and  effe- 
rent nerve. 


Three  medullated  nerve  fibres,  the 
medullary  sheath  of  which  is 
stained  dark  with  osrnic  acid. 
N,  Nodes  of  Ranvier. 

Two  non-medullated  nerve  fibres, 
with  nuclei  in  the  primitive 
sheath. 


The  fibres  act  as  lines  of  communication  between  ganglion 
cells  :  they  connect  together  the  numerous  cells  in  the  various 


48 


MANUAL   OF   PHYSIOLOGY. 


parts  of  the  brain  and  spinal  cord,  or  pass  between  those  of 
the  central  nervous  organs  and  ganglia  distributed  throughout 
the  body,  which  might  be  called  the  peripheral  nerve  organs. 

The  simplest  idea,  then,  of  a  special  nerve  apparatus  is  a  fibre 
connecting  two  cells.  The  peripheral  cell  may  be  a  receiving 
organ  (Fig.  17,  s),  from  which,  when  stimulated,  impulses  are 
transmitted  along  the  fibre  to  the  central  nerve  cell,  where  they 
give  rise  to  certain  impressions,  and  so  we  have  a  sensory  nerve 
apparatus.  Or  the  central  nerve  cell  may  be  the  receiving 
agent,  getting  stimuli  from  its  central  neighbors,  and  transmitting 


FIG.  19. 


Multipolar  cells  from  the  anterior  gray  column  of  the  spinal  cord  of 
the  dogfish  (a)  lying  in  a  texture  of  fibrils;  (6)  prolongation 
from  cells;  (c)  nerve  fibres  cut  across.  (Cadiat.) 

impulses  to  a  peripheral  nerve  terminal,  by  which  they  are 
handed  over  to  a  muscle  (M)  or  gland,  and  thus  we  have  a  simple 
motor  or  secretory  apparatus.  Where  the  effect  of  a  stimulus 
can  be  definitely  traced  from  one  nerve  cell  to  another,  and  from 
thence  by  a  second  fibre  to  a  third  cell,  the  impulse  is  said  to  be 
reflected  by  the  second  cell  to  the  third.  And  there  we  have 
what  is  called  a  reflex  act. 

The  essential  part  of  a  nerve  fibre  is  a  kind  of  protoplasmic 
band,  in  which  the  finest  fibrilla  or  thread-like  marking  can  be 


NERVE   TISSUE. 


'49 


made  out  with  the  aid  of  reagents  and  a  powerful  microscope. 
This  is  called  the  axis  cylinder.  In  some  nerve  fibres  (mostly  in 
the  brain  and  spinal  cord)  the  axis  cylinder  is  naked,  and  even 
a  single  fibril  may  so  pass  from  one  cell  to  another  in  the  brain 
matter.  In  other  parts  the  axis  cylinder  is  generally  covered  by 
a  thin  membrane,  called  the  primitive  sheath,  or  with  a  soft,  oil- 
like  substance,  called  the  medullary  sheath,  or,  as  is  commonly 
the  case  in  most  peripheral  nerves,  by  both.  The  primitive 
sheath  encloses  the  medullary  sheath,  which  surrounds  the  axis 
cylinder. 

These  fibres  are  made  of  peculiarly  modified  cells,  which  are, 


FIG.  20. 


FIG.  21. 


Ganglion  cells  of  frog,  showing 
straight  and  spiral  fibres. 
( After  Beale  and  Arnold.) 


Cells  from  the  sympathetic  ganglion  of 
a  cat.  The  protoplasm  is  retracted 
here  and  there  from  the  cell  wall. 


however,  so  elongated  as  not  to  be  very  easily  recognized  as  such 
in  adult  tissue. 

The  nerve  or  ganglion  cells  vary  extremely  in  general  form  and 
size.  The  commonest  in  the  nerve  centres  are  large  bodies  with 
a  clear,  well-defined,  vesicular,  single  nucleus,  and  distinct  nucle- 
olus ;  they  have  two  or  more  processes,  which  are  connected  by 
nerve  fibres  to  other  cells,  and  to  the  axis  cylinder  of  nerves. 

The  peripheral  nerve  cells  are  generally  much  modified,  and 
often  small  compared  with  those  in  the  centres.  Besides  the  cells 
in  the  sporadic  ganglia,  which  are  large  rounded  corpuscles  with 


50  MANUAL   OF   PHYSIOLOGY. 

but  few  processes,  there  are  many  other  bodies  connected  with 
the  peripheral  nerves  which  cannot  be  called  ganglion  corpuscles. 
They  are  nevertheless  nerve  cells. 

Muscles  or  Contractile  Tissues. — When  changes  take  place  in 
protoplasm  adapting  it  specially  for  contraction,  it  is  termed 
muscle  tissue.  The  large  masses  of  this  tissue  attached  to  the 
skeleton  so  as  to  move  its  various  parts,  form  the  flesh  of  the 
higher  animals.  Muscle  tissue  is,  almost  invariably,  connected 
with  nerve  tissue,  and  acts  in  response  to  stimuli  communicated 
from  the  nerves.  In  some  of  the  lower  animals  the  two  tissues 
are  so  intimately  related  that  it  is  not  easy  to  distinguish  them, 
and  the  development  of  both  progresses  equally  as  we  ascend  the 
scale  of  animal  life.  They  are  nearly  related  in  their  origin,  or 
even  spring  from  the  same  primitive  tissue.  In  fact,  as  has 
already  been  mentioned  (vide  p.  46),  they  form  but  one  structure 
in  some  of  the  more  simple  and  less  differentiated  animals.  The 
neuro-muscular  tissue,  which  is  formed  from  the  outer  layer  of 
the  embryo,  is  the  forerunner  of  the  muscles  as  well  as  of  the 
nerves  of  the  embryo  of  the  higher  animals. 

In  the  higher  animals  and  man  muscle  tissue  consists  of  two 
distinct  kinds  of  textures,  known  as — 

(a)  Smooth,  or  non-striated  muscle. 

(6)  Striated  muscle. 

In  the  smooth  muscle  the  individual  elements  present  the 
characters  of  an  elongated  and  flattened  cell,  and  contain  a  single 
long  nucleus.  They  contract  very  slowly,  and  require  a  com- 
paratively long  time  for  the  nerve  influence  to  affect  them,  so 
that  an  obvious  interval  exists  between  the  moment  of  their 
stimulation  and  their  contraction.  They  are  found  in  the  inter- 
nal organs  and  in  situations  where  gradual  and  lasting  contrac- 
tions are  required.  They  receive  their  nervous  supply  generally 
from  the  sympathetic  system,  and  perform  their  duty  without 
our  being  conscious  of  their  activity  or  being  able  to  control  it 
by  our  will. 

Striated  muscle  tissue  is  made  up  of  cylindrical  fibres  of  such 
length  that  both  extremities  cannot  be  brought  into  the  field  of 


MUSCLE   TISSUE. 


51 


the  microscope  at  the  same  time.  Their  exact  relation  to  cells  is 
not  so  easily  made  out  as  in  smooth  muscle,  and  doubtless  varies 
in  different  muscles.  Sometimes  the  fibres  are  made  up  of  single 
cells,  and  in  other  cases  they  are  formed  by  the  permanent  fusion 


FIG.  22. 


FIG.  23. 


Cells  of  smooth  muscle  tissue  ironi  the  in- 
testinal tract  of  rabbit.  (Ranvier.) 

A  and  B.— Muscle  cells  in  which  differen- 
tiation of  the  protoplasm  can  be  well  seen. 
(Sch&fer.) 


Two  fibres  of  striated  muscle,  in 
which  the  contractile  substance  (TO) 
has  been  ruptured  and  separated  from 
the  sarcolemma (a)  and  (s) ;  (p)  space 
under  sarcolemma.  (Ranvier.) 


52  MANUAL    OF"  PHYSIOLOGY. 

of  several  cell  elements  which  never  differentiate  into  separate 
elements,  owing  to  the  imperfect  division  of  the  cells,  but  make 
up  one  mass,  the  multiple  nuclei  of  which  alone  make  its  mode 
of  origin  apparent.  The  contractile  substance  is  made  up  of  two 
kinds  of  material,  one  of  which  refracts  light  singly,  while  the 
other  is  doubly  refracting.  These  are  ranged  alternately  across 
the  fibre,  making  the  transverse  markings  or  striae  from  which 
it  gets  its  name.  This  striated  material  is  quite  soft  and  is 
encased  in  a  thin  homogeneous  elastic  sheath  called  sarcolemma, 
which  fits  closely  around  the  soft  contractile  substance. 

This  form  of  muscle  is  the  widest  departure  from  the  primitive 
protoplasmic  type,  being  specially  modified  so  as  to  perform 
strong  and  quick  contractions.  It  moves  with  wonderful  rapidity, 
contracting  almost  the  instant  its  nerve  is  stimulated.  It  forms  the 
great  mass  of  the  quick-acting  skeletal  muscles,  being  attached  to 
the  bones  by  bands  composed  of  a  form  of  fibrous  tissue,  which 
form  the  tendons  and  fasciae.  Muscles  made  of  striated  tissue 
are  commonly  under  the  control  of  the  will,  and  hence  are 
frequently  spoken  of  as  voluntary  muscles,  but  this  term  is 
misleading,  for  many  striated  muscles  are  not  governed  by 
voluntary  control. 

The  Connective  Tissue  group,  coming  exclusively  from  the 
mesoblast,  exhibits  very  great  varieties  of  form.  Its  cells  differ 
much  from  the  epithelial  cells  both  in  their  character  and  their 
relations,  and  particularly  in  the  adult  tissues. 

Under  the  heading  Connective  Tissues  are  generally  classed 
all  those  which  support  the  frame  and  hold  together  the  various 
other  tissues  and  organs.  They  are — 

1.  Mucous  and  retiform  connective  tissues. 

2.  White  and  yellow  fibrous  tissue. 

3.  Cartilage. 

4.  Bone. 

5.  Endothelium. 

The  cells  of  all  these  tissues  have  the  property  of  manufactur- 
ing some  material  which  does  not  generally  enclose  them  as  a  cell 
wall,  but  remains  between  the  cells  and  forms  the  intercellular 


53 


Transverse  section  of  the  chorda  dorsalis  and  neighboring  substance,  a,  cartilage  cells ; 
b,  cell  of  the  middle  layer  of  embryo;  c,  mucous  tissue;  d,  boundary  of  chorda. 
(Cadiat.) 


FIG.  25. 


Cells  of  mucous  tissue  with  branching  processes  (B)  and  a  couple  of  elastic  fibres  (F). 

(Itanvier.) 


54 


MANUAL    OF    PHYSIOLOGY. 


substance.  The  younger  the  tissue  the  greater  is  the  proportion 
of  its  cellular  constituents,  and  the  older  the  tissue  the  greater 
will  be  found  the  preponderance  of  the  intercellular  substance. 

Mucous  Tissue, — In  certain  parts  of  the  embryo  and  in  some 
of  the  lower  animals  a  kind  of  connective  tissue  is  found  in 
which  there  is  but  little  intercellular  substance,  the  mass  of  the 
tissue  being  thus  made  up  of  cells.  The  cellular  connective 
tissue  never  forms  an  important  texture  in  the  adult,  but  is  inter- 
esting as  the  probable  tissue  from  which  the  connective  tissues 
are  formed  in  the  embryo,  and  as  occurring  in  abnormal  growths 
or  tumors. 

The  first  step  in  its  differentiation  is  the  secretion  of  a  large 


FIG.  26. 


Portion  of  tendon  from  the  tail  of  a  young  rat,  stained  with  gold  chloride,  showing 
arrangement  of  flattened  cells  on  bundles  of  fibrils.    (After  Klein.) 

quantity  of  soft,  homogeneous,  semi-gelatinous  or  fluid  material 
like  the  mucus  secreted  by  epithelium.  In  this  the  cells  lie, 
either  free  or  united  by  long  protoplasmic  processes.  The  pro- 
cesses uniting  the  cells  may  not  be  present,  and  the  cells  may  be 
reduced  to  a  minimum,  as  occurs  in  the  vitreous  humor  of  the 
eye.  But  more  commonly  the  soft  gelatinous  substance  is 
reduced  in  amount,  and  the  processes  connecting  the  cells  are 
converted  into  a  dense  network  of  delicate  threads  to  form  the 
retiform  tissue  of  lymphoid  structures. 

White  Fibrous  Tissue. — The  cells  of  the  last  described  variety 
may  become  differentiated   by  a   process   of  fibrillation.     The 


CONNECTIVE   TISSUES. 


55 


growth  of  the  cells  leads  to  the  formation  of  a  fibrillated  sub- 
stance which  ultimately  forms  the  great  bulk  of  the  tissue,  while 
the  cells  become  gradually  and  proportionately  fewer  in  number. 
In  this  case  only  sufficient  of  the  mucous  substance  generally 
remains  to  cement  the  fibrils  together  into  bundles.  A  few  of 
the  cells,  however,  remain  between  the  bundles  of  fibrils  to 


FIG.  27. 


FIG.  28. 


Coarse  (a)  and  fine  (b)  yellow  elastic  fibres 
after  treatment  with  strong  acetic  acid. 
(Cadiat.) 


Elastic  membrane  from  inner  coat  of 
aorta,  and,  below,  meshwork  of  elas- 
tic fibres  from  a  yellow  ligament. 
(Cadiat.) 


preside  over  the  nutrition  of  the  tissue.     Thus  is  formed  the 
non-elastic  or  white  fibrous  tissue  of  tendon. 

These  fibrils  of  white  fibrous  tissue  are  easily  affected  by 
chemical  reagents.  Weak  acids  cause  them  to  swell  up  and 
become  indistinct.  Baryta  water  affects  the  cement  and  renders 
them  easily  separable.  They  swell  and  dissolve  in  boiling  water, 
yielding  gelatine,  which  forms  a  jelly  on  cooling. 


56 


MANUAL   OF    PHYSIOLOGY. 


FIG.  29. 


Yellow  Elastic  Tissue. — In  some  parts  of  the  body  a  kind  of 

intercellular  substance 
is  formed,  which  dif- 
fers in  many  respects 
from  the  foregoing.  It 
is  highly  elastic,  does 
not  give  gelatine  on 
boiling,  and  is  not  af- 
fected by  weak  acids 
or  alkalies.  In  bulk  it 
has  a  pale  yellow  color, 
and  is  spoken  of  as  yel- 
low elastic  tissue.  It  is 


A  teased  preparation  of  connective  tissue  showing  fine  Sometimes  found  alone, 
and  coarse  elastic  fibres  mingled  with  bundles  of  fnrrmnrr  «n  p  1  n  e  1 1  r> 
fibrillar  tissue  and  connective  tissue  corpuscles.  1U&  ' 

band  or  ligament,  but 


more  commonly  mingled  with  fibrillar  tissue  to  form  the  connect- 
ing medium  which  lies  under  the  skin  and  between  the  various 
other  textures. 


FIG.  31. 


Section  of  hyaline  cartilage  from  the 
end  of  a  growing  bone,  showing  a 
decrease  in  the  intercellular  sub- 
stance compared  with  the  number 
of  cell  elements,  which  are  ar- 
ranged in  rows. 


Elastic  fibro-cartilage,  showing  cells  in  capsules 
and  elastic  fibres  in  matrix.    (Cadiat.) 


CARTILAGE. 


57 


Cartilage. — In  this  tissue  the  intercellular  substance  secreted 
by  the  cells  is  hard,  and  forms  in  the  earlier  stages  of  its  develop- 
ment cases  or  cell  walls  for  the  cells.  These  cases  subsequently 


FIG.  32. 


White  fibro-cartilage,  showing  cells  (cri  in  capsules  and  fibrillar 
matrix  (b).    (Cadiat.) 


Transverse  section  of  a  system  of  Havers,  showing 
Haversian  canal  in  centre,  with  bone  cells  arranged 
around  it  in  lacuna,  which  are  connected  by  the  deli- 
cate canaliculi.  (Cadiat.) 

increase  in  thickness,  and  become  fused  together  into  a  homo- 
geneous   intercellular   substance,   where   ultimately    the    areas 


58  MANUAL   OF    PHYSIOLOGY. 

belonging  to  the  different  cells  can  no  longer  be  distinguished 
from  one  another,  so  that  in  the  adult  tissue  there  is  a  tough 
matrix  of  intercellular  substance,  in  which  the  cells  are  scattered, 
apparently  occupying  small  cavities.  These  cells  preside  over 
the  nutrition  of  the  tissue.  The  intercellular  substance,  which 
is  quite  homogeneous  in  common  hyaline  cartilage,  is  sometimes 
modified  so  as  to  resemble  fibrous  tissue,  sometimes  the  fibrillar, 
and  sometimes  the  elastic  form  being  produced.  (Figs.  31  and 
32.) 

Bone. — This  is  the  most  marked  differentiation  of  the  connec- 
tive tissue  group.  The  intercellular  substance  is  characterized 
by  containing  a  great  quantity  of  earthy  or  inorganic  matter 
(65  %),  which  gives  the  bone  its  enormous  strength.  The  cells 
of  the  tissue  are  enclosed  in  little  cavities  called  lacuna,  which 
are  related  by  minute  canaliculi  to  each  other.  The  intercel- 
lular substance  is  everywhere  traversed  by  the  processes  of  the 
cells  lying  in  the  little  canals  which  connect  the  lacunae,  and 
thus  the  adequate  nutrition  of  the  tissue  is  secured.  Chemically, 
bone  tissue  consists  of  about — 

Parts. 

Calcium  phosphate 53 

Calcium  carbonate ..,..'. 11 

Magnesium  phosphate,  calcium  fluoride  and  soda  salts 1 

Gelatine  yielding  animal  matter 33 

In  the  formation  of  bone  from  fibrous  or  cartilaginous  tissue 
the  original  intercellular  substance  disappears,  and  a  set  of  cells 
with  new  formative  powers  come  upon  the  field  (Fig.  34).  These 
new  cells  (osteoblasts)  cover  the  growing  surface  of  the  bone 
and  secrete  and  lay  down  in  layers  a  new  kind  of  intercellular 
substance,  which  is  the  bone  matrix.  Here  and  there,  at  won- 
derfully regular  intervals,  an  osteoblast  ceases  to  secrete  the 
calcareous  intercellular  substance,  while  its  neighbors  continue 
formative  activity.  Consequently,  this  osteoblast,  or  as  it  may 
now  be  called  young  bone  cell,  becomes  surrounded  by  calcare- 
ous intercellular  substance,  and  is  permanently  lodged  in  the 
bone  tissue. 

Endothelium. — Wherever  a  surface  occurs  in  the  connective 
tissues  it  is  generally  covered  by  a  single  layer  of  thin  cells  with 


STRUCTURAL   CHARACTERS   OF   ANIMAL    ORGANISMS. 
FIG.  34. 


59 


. 


:;•  v.-J:  .;:::>-  .  -.::::     ::::.::^.---—-*~  \ 

L«Rst^l^s]™K-S 


Section  through  ossifying  cartilage  and  young  bone.    (Cadiat.) 


a.  Cartilage  cells. 

6.  Degenerating  cartilage  cells. 

c.  Cell  space,  empty. 

d.  Spiculse  of  calcareous  deposit. 


e.  Blood  corpuscles. 
/.  Osteoblasts. 
a.  Ditto  of  periosteum. 
A.  Bone  cells. 


60  MANUAL   OF    PHYSIOLOGY. 

a  characteristic  outline,  which  can  only  be  made  visible  by  stain- 
ing the  intervening  cement  substance  with  silver  nitrate.  This 
tissue,  which  forms  the  immediate  lining  of  all  vessels  and  spaces 
developed  in  the  tissues  arising  from  the  mesoblast,  is  called 
endothelium,  in  contradistinction  to  the  epithelium  developed  from 
the  epi-  and  hypo-blast. 

The  Vascular  System  is  developed  in  the  mesoblast  with  the 
earliest  stages  of  the  connective  tissue.  The  blood  vessels,  which 
are  chiefly  made  up  of  connective  tissues,  soon  traverse  all  parts 
of  the  body,  and  distribute  the  nutrient  fluid  or  blood.  The 
blood  may  be  considered  as  an  outcome  of  the  connective  tissues, 
since  the  corpuscles  of  the  blood  are  at  first  formed  from  the 
cells  of  the  mesoblast,  and  later  from  the  connective  tissue  cor- 
puscles. 

An  arrangement  of  special  cells,  such  as  epithelial  or  muscle 
cells,  with  a  special  function,  constitutes  an  organ.  However, 
in  the  higher  animals  and  man  an  organ  is  almost  invariably  a 
complex  structure,  having  various  tissues  entering  into  its  con- 
struction. Thus  a  skeletal  muscle  is  made  up  of  a  quantity  of 
muscle  fibres  held  together  by  sheets  of  connective  tissue,  and 
attached  to  bones  by  connecting  bands.  It  is  further  traversed 
by  many  blood  vessels,  and  the  fibres  are  in  immediate  relation 
to  certain  nerves  which  terminate  in  them.  The  various  secret- 
ing organs  are  made  up  of  epithelial  cells,  held  together  by  con- 
nective tissue  in  close  relation  to  blood  vessels  and  nerves,  and 
are  so  arranged  that  they  pour  their  secretion  into  a  duct.  The 
bones,  which  are  the  organs  which  give  the  body  support,  con- 
tain, in  addition  to  the  bone  tissue  of  which  they  are  composed,  a 
great  quantity  of  indifferent  cells,  fat  cells,  nerves  and  blood  ves- 
sels. They  are  covered  on  the  outside  with  a  tough  vascular 
coat,  which  gives  them  strength,  assists  their  nutritive  repair  and 
reproduction,  and  acts  as  a  point  of  attachment  for  the  muscles 
and  ligaments.  Where  the  bones  are  in  contact  at  the  joints, 
they  are  tipped  with  hyaline  cartilage. 

If,  then,  we   analyze   anatomically   the   architecture   of  the 


STRUCTURAL   CHARACTERS    OF    ANIMAL    ORGANISMS. 


61 


human  body,  we  find  that  it  is  made  up  of  a  number  of  complex 
parts,  each  adapted  to  some  special  function,  and  composed  of 
an  association  of  simple  tissues  such  as  the  requirements  of  the 
special  part  demand. 

The  general  arrangements  of  these  organs  and  their  modes  of 
action  will  be  discussed  in  future  chapters. 


62  MANUAL   OF    PHYSIOLOGY. 


CHAPTER  III. 
CHEMICAL  BASIS  OF  THE  BODY. 

It  seems  natural  to  commence  the  description  of  the  molecular 
changes  that  take  place  in  the  various  tissues  and  organs  of 
the  body,  with  a  brief  account  of  the  chemical  composition  of 
the  most  characteristic  substances  found  in  animal  textures, 
because  none  of  the  processes  of  cell  life,  or  tissue  activity,  can 
be  satisfactorily  studied  without  familiarity  with  the  more  com- 
mon terms  occurring  in  physiological  chemistry. 

The  chapter  on  this  subject  here  introduced,  is  intended  rather 
to  give  the  medical  student  a  general  view  of  the  chemical  com- 
position and  characters  of  the  substances  most  frequently  met 
with  in  the  chemical  changes  specially  connected  with  animal 
life,  than  to  supply  a  complete  or  systematic  account  of  the  rela- 
tionships of  the  chemical  bases  of  the  body,  for  which  reference 
must  be  made  to  more  advanced  text-books,  or  treatises  on  the 
special  subject  of  physiological  chemistry.  This  review  must, 
for  the  sake  of  brevity,  be  inadequate  in  the  case  of  many  sub- 
stances, but  these  will  be  again  referred  to  when  speaking  of  the 
function  with  which  they  are  associated. 

It  has  already  been  stated  that  of  the  seventy  elements  known 
to  chemists,  a  comparatively  small  number  form  the  great  bulk 
of  the  animal  body,  although  traces  of  many  are  constantly 
present.  Thus,  we  shall  see  that  four  elements,  namely,  (1) 
oxygen,  (2)  carbon,  (3)  hydrogen,  (4)  nitrogen,  are  present  in 
large  proportions  in  every  tissue,  and  together  make  up  about 
97  per  cent,  of  the  body ;  and  sulphur,  phosphorus,  chlorine, 
fluorine,  silicon,  potassium,  sodium,  magnesium,  calcium,  iron, 
and  in  certain  animals  copper,  are  indispensable  to  the  economy, 
and  are  widely  distributed,  but  are  found  in  comparatively  minute 
quantities.  Occasionally  traces  of  zinc,  lead,  lithium,  and  other 
minerals  may  be  detected,  but  these  must  be  regarded  rather  as 
accidental  than  indispensable  ingredients. 


CHEMICAL    BASIS    OF   THE    BODY.  63 

The  attempt  to  investigate  the  composition  of  a  living  tissue 
by  chemical  analysis,  must  cause  its  death,  and  thus  alter  the 
arrangements  of  its  constituents,  so  that  its  true  molecular  con- 
stitution during  life  cannot  be  determined. 

We  know  that  the  composition  of  all  living  textures  is 
extremely  complicated,  having  a  great  number  of  components, 
most  of  which  contain  many  chemical  elements  associated 
together  in  very  complex  proportions. 

But  as  has  already  been  pointed  out,  the  complexity  of  their 
chemical  constitution  is  not  so  wonderful  as  the  fact,  which 
indeed  sounds  paradoxical,  that  in  order  to  preserve  their  elab- 
orate composition,  they  must  constantly  undergo  a  change  or 
renewal,  which  is  necessary  for,  and  forms  the  one  essential  char- 
acteristic of,  their  life.  In  fact,  their  complexity  and  instability 
is  such,  that  they  require  constant  reconstruction  to  make  up  for 
the  changes  inseparable  from  their  functional  activity. 

Their  chemical  constituents  are  easily  permanently  dissociated, 
and  the  various  components  are  themselves  readily  decomposed, 
generally  uniting  with  oxygen  to  form  more  stable  compounds. 

The  investigation  of  the  chemical  changes  known  as  assimila- 
tion forms  a  great  part  of  physiological  study,  and-  therefore 
will  occupy  many  chapters  of  this  book.  Here  we  can  only  call 
attention  to  the  chief  characteristic  substances  to  be  found  in  the 
animal  body,  as  the  result  of  the  primary  dissociation  or  death 
of  the  textures,  and  briefly  enumerate  the  products  of  their  fur- 
ther decomposition  as  obtained  by  the  analysis  of  the  different 
substances. 

The  tissues  of  the  higher  animals  present  a  great  variety  of 
substances,  materially  differing  in  chemical  composition;  they 
have  all  been  made  from  protoplasm,  and  contain  a  proportion 
of  some  substance  forming  a  leading  chemical  constituent  of 
protoplasm.  Every  living  tissue  contains  either  protoplasm  or 
a  derivative  of  it,  and  the  special  characters  of  each  tissue 
depend  upon  the  greater  development  of  some  one  of  these  sub- 
stances. 

It  is  of  little  use  to  classify  the  numerous  chemical  constitu- 
ents found  in  the  animal  body  in  such  a  systematic  manner  as  to 


64  MANUAL   OF    PHYSIOLOGY. 

satisfy  the  rules  of  modern  chemistry,  because  their  classification, 
from  a  strictly  chemical  point  of  view,  does  not  set  forth  their 
physiological  importance  or  express  adequately  the  relation  they 
bear  to  the  vital  phenomena  of  organisms. 

The  following  enumeration  of  the  chief  chemical  ingredients 
found  in  the  tissues  has  regard  to  their  physiological  dignity  as 
well  as  to  their  chemical  construction,  and  will  thus,  it  is  hoped, 
assist  the  student  to  distinguish  the  different  groups,  and  give 
him  a  better  idea  of  their  vital  relationships,  than  a  more  strictly 
systematic  classification. 

(A)     NITROGENOUS. 
I.  Complex   bodies   forming   the   active  portion  of    many 

tissues — Plasmata,  e.  g.,  protoplasm,  blood  plasma. 
II.  Bodies  entering  into  the  formation  of  and  easily  obtained 
by    analysis    from    Group    I,  Albumins,   e.  g.,  serum 
albumin. 

III.  Bodies  the  outcome  of  differentiation,  manufactured  in 

the   tissues  by  Group  I,  Albuminoids,   e.  g.,  gelatin, 
etc. 

IV.  Bodies  containing  nitrogen,  being  intermediate,  bye,  or 

effete  products  of   tissue  manufacture,   e.  g.,  lecithin, 
urea,  etc. 

(B)     NON-NITROGENOUS. 

V.  Carbohydrates  in  which  the  hydrogen  and  oxygen  exist 
in   the    proportion  found  in    water,  e.  g.,  starch    and 
sugar. 
VI.  Substances  containing  oxygen  in  less  proportions  than 

the  above,  t.  g.,  fats 
VII.  Salts. 
VIII.  Water 

CLASS  A.— NITROGENOUS. 

GROUP  I.— PLASMATA. 

Under  this  group  may  be  placed  a  variety  of  substances  which 
must  be  acknowledged  to  exist  in  the  living  tissues  as  complex 
chemical  compounds,  of  whose  constitution  we  are  ignorant, 
since  it  is  altered  by  the  death  of  the  tissue. 


PL ASM  AT A.  65 

There  are  some  exceedingly  unstable  associations  of  albumin- 
ous bodies  with  other  substances,  and  they  at  once  break  up  into 
their  more  stable  constituents,  albumins,  fats,  salts,  etc.,  when 
they  are  deprived  of  the  opportunities  of  chemical  interchange 
and  assimilation  which  are  necessary  for  their  life. 

Although  we  can  only  theorize  as  to  the  real  chemical  consti- 
tution of  such  substances,  we  must  believe  that  they  really  exist 
in  the  living  tissues  as  chemical  compounds,  and  as  chemical 
compounds  endowed  with  special  properties  which  impart  the 
specific  activity  of  their  textures,  whose  molecular  motions,  in  fact, 
are  the  essence  of  the  life  of  the  tissues. 

Protoplasm. — By  far  the  most  widely  spread  and  important  of 
these  is  the  soft,  jelly-like  substance,  Protoplasm.  This  is  the 
really  active  part  of  growing  textures  of  all  organisms,  whether 
animal  or  vegetable,  and  forms  the  entire  mass  of  those  inter- 
mediate forms  of  life,  the  protista,  now  generally  regarded  as 
the  original  fountain  head  of  life  on  the  globe. 

This  material  commonly  exists  in  small  independent  masses 
(cells),  in  which  we  can  watch  all  the  manifestations  of  life, 
assimilation,  growth,  motion,  etc.,  taking  place.  We  must 
assume  that  this  substance  is  a  definite  chemical  compound  ;  and, 
further,  since  the  living  phenomena  are  exhibited  only  so  long 
as  it  preserves  its  chemical  integrity,  we  may  conclude  that  its 
manifestations  of  life  depend  upon  the  sustentation  of  a  special 
chemical  equilibrium.  Not  only  is  this  equilibrium  destroyed 
by  any  attempt  to  ascertain  the  chemical  composition  of  proto- 
plasm by  analysis,  but  even  for  its  preservation  the  proto- 
plasm must  be  surrounded  by  those  circumstances  which  are 
known  to  be  necessary  for  life,  viz.,  moisture,  warmth,  and  suit- 
able nutritive  material,  or  its  destruction  must  be  warded  off  by 
a  degree  of  cold  that  checks  its  chemical  activity. 

If  the  chemical  integrity  of  protoplasm  be  destroyed  and  its 
death  produced,  many  new  substances  appear,  among  which 
are  representatives  of  each  of  the  great  chemical  groups  found 
in  the  animal  tissues.  Thus,  besides  water  and  inorganic  salts, 
we  find  in  protoplasm  carbohydrates  represented  by  glycogen, 
lecithin  and  other  fats,  and  several  albuminous  bodies,  which  will 
6 


66  MANUAL   OF    PHYSIOLOGY. 

be  described  in  the  groups  to  which  they  belong.  In  addition 
to  these,  protoplasm  often  contains  some  foreign  bodies  which 
have  come  from  without,  and  special  ingredients  of  its  own 
manufacture,  such  as  oil,  pigment,  starch  and  chlorophyll. 

Blood  Plasma. — There  is  in  living  blood  also  a  body  which  must 
be  included  in  this  group,  as  it  undoubtedly  has  a  much  more 
complex  constitution  than  any  of  the  individual  albuminous  bodies, 
presently  to  be  described,  which  can  be  obtained  from  it.  This 
is  proved  by  the  following  facts :  first,  its  death  is  accompanied 
by  a  series  of  chemical  changes,  viz.,  disappearance  of  free 
oxygen,  diminution  of  alkalinity,  and  a  rise  in  temperature,  and 
secondly,  certain  albuminous  bodies  appear  which  were  not  present 
in  the  living  plasma. 

The  spontaneous  decomposition  of  separated  blood  plasma  may 
be  delayed  by  cold  ;  at  freezing  point  the  chemical  processes  are 
held  in  check.  During  life  the  exalted  constitution  of  the  plasma 
is  sustained  by  certain  chemical  interchanges  which  go  on 
between  it  and  its  surroundings.  This  question  will  be  more 
fully  discussed  when  the  coagulation  of  the  blood  is  described. 

Muscle  Plasma. — Likewise,  as  will  be  found  in  the  chapter  on 
Muscles,  there  exists  in  the  soft,  contractile  part  of  striated  muscle 
a  plasmas  which  at  its  death  spontaneously  breaks  up  into  other 
distinct  albuminous  bodies  and  forms  a  coagulum.  These  changes 
are  accompanied  by  acidity  of  reaction,  the  disappearance  of 
oxygen  and  an  elevation  of  temperature,  showing  that  distinct 
chemical  change  is  taking  place. 

Oxyhcemoalobin,  the  coloring  matter  of  the  blood,  should  be 
included  here  among  the  important  chemical  bodies  more 
complex  than  the  albumins.  This  singular  body  can  be  broken 
up  into  a  globulin  and  a  coloring  matter,  hcematin,  containing  iron. 
It  differs  from  all  other  bodies  of  a  similarly  complex  nature 
from  the  fact  that  it  readily  crystallizes,  and  also  in  the  very 
remarkable  manner  in  which  it  combines  with  oxygen,  and  again 
yields  it  up. 

GROUP  II.— ALBUMINOUS  BODIES, 

It  is  difficult  to  say  how  far  these  bodies  exist  as  such  in  the 
living  organism,  but  they  can  be  obtained  from  nearly  all  parts, 


ALBUMINOUS    BODIES.  67 

particularly  those  which  contain  active  protoplasm,  and  after  its 
death  they  can  be  detected  in  abundance.  As  may  be  seen,  by 
testing  for  their  presence  in  living  protoplasm,  the  addition  of 
any  chemical  reagent  or  treatment  causes  its  death,  so  that, 
although  albumins  appear  in  the  test  tube,  this  cannot  be  accepted 
as  proof  that  they  would  have  answered  to  the  tests  before  the 
protoplasm  was  changed  by  its  death. 

They  do  not  occur  normally  in  any  secretion  except  those  sub- 
stances which  tend  to  nourish  the  adult  body,  and  to  form  and 
nourish  the  offspring,  viz.,  the  ovum,  semen  and  milk.  No 
satisfactory  formula  has  been  suggested  to  express  their  chemical 
composition,  but  the  average  percentage  of  the  elements  they 
contain  is  remarkably  alike  in  all  members  of  the  group.  This 
may  be  said  to  be  in  round  numbers  as  follows: — 

Oxygen 22  per  cent. 

Hydrogen  7       " 

Nitrogen 16       " 

Carbon 53       " 

Sulphur 2       " 

They  are  amorphous,  of  varying  solubility,  and,  with  one 
exception,  indiffusible  in  distilled  water. 

As  far  as  we  know  at  present,  albumins  cannot  be  constructed 
de  novo  in  the  animal  body,  but  must  be  supplied  in  one  form  or 
another  as  part  of  the  food.  Albumins  are  therefore  always  the 
outcome  of  the  activity  of  vegetable  life. 

They  can  be  recognized  by  the  following  tests  : — 

1.  Strong  nitric  acid  gives  a  pale  yellow  color  to  solutions  or 

solid  albumin,  especially  on  heating,  which  turns  to  deep 
orange  when  ammonia  is  added  (Xanthoproteic  test}. 

2.  Millon's  Reagent  (acid  solution  of  proto-nitrate  of  mer- 

cury) gives  a  white  precipitate  which  soon  turns  yellow, 
changing  to  rosy-red  on  boiling,  or  standing  for  some 
days. 

3.  Solution  of  caustic  soda  and  a  drop  of  copper  sulphate 

solution  give  a  violet  color  to  the  liquid. 

4.  Acetic  acid  and  boiling  give  a  white  precipitate,  except 

with  derived  albumins  and  peptones. 


68  MANUAL   OF   PHYSIOLOGY. 

5.  Acetic  acid  and  potassium  ferrocyanide  give  a  flocculent 

white  precipitate,  except  with  peptones. 

6.  Acetic  acid  and  equal  volumes  of  sodium  sulphate  solu- 

tion give  a  precipitate  on  boiling. 

7.  With  sugar  and  sulphuric  acid  they  become  violet. 

8.  Crystals  of  picric  acid  added  to  solutions  dissolve  and 

cause  bead-like  local  coagulations,  except  with  peptones. 

CLASSIFICATION  OF  ALBUMINS. 

Under  the  head  of  the  albuminous  bodies  we  find  several  classes 
which  differ  from  each  other  in  slight  but  very  important  points. 
The  first  class  may  be  called — 

(A)  ALBUMINS  PROPEK,  OR  NATIVE  ALBUMINS. 

They  consist  of — 

1.  Egg  Albumin,  which  does  not  occur  in  the  ordinary  tissues 
of  the  animal,  can  be  procured  by  filtration  from  the  white  of 
an  egg.     It   makes   a  clear  or  slightly  opalescent  solution  in 
water,  from  which  it  is  precipitated  by  mercuric  chloride,  silver 
nitrate,  lead  acetate,  and  alcohol.     It  is  coagulated  by  heat, 
strong  nitric  and  hydrochloric  acids,  or  prolonged  exposure  to 
alcohol  or  ether. 

2.  Serum  Albumin,  on  the  other  hand,  is  one  of  the  chief  forms 
of  albumin  found  in  the  nutrient  fluids. 

It  differs  from  egg  albumin  in — 

(a)  Not  coagulating  with  ether. 

(b)  The  precipitate  obtained   by  strong  hydrochloric  acid 

being  readily  redissolved  by  excess  of  the  acid. 

(c)  Coagulum  being  more  readily  soluble  in  nitric  acid. 

(d)  Its  specific  rotary  power  being  56°,  while  that  of  egg 
albumin  is  35.5°. 

(e)  If  introduced  into  the  circulation,  it  is  not  eliminated 

with  urine,  as  is  egg  albumin. 

(B)  GLOBULINS. 

Associated  with  the  last  during  the  life  of  the  tissues  we  find 
another  class  of  albumins,  namely,  the  globulins,  which  do  not 


GLOBULINS    AND   ALBUMINATES.  69 

dissolve  in  pure  water,  but  are  more  or  less  soluble  in  a  solution 
of  common  salt.     These  may  be  divided  as  follows  : — 

1.  Globulin  (crystallin)  occurs  in  many  tissues,  but  is  usually 
obtained  from  an  extract  of  the  crystalline  lens  made  by  tritu- 
rating it  with  fine  sand  in  a  weak  solution  of  common  salt,  and 
then  passing  a  current  of  carbon  dioxide  through  the  solution. 
The  globulin   falls,   being   easily   precipitable   from   its  saline 
solution  by  very  weak  acid.     This  form  of  globulin  does  not 
cause  coagulation  when  added  to  serous  fluids,  and  in  this  respect 
differs  from  the  next  members  of  this  division. 

2.  Paraglobulin  (serum  globulin)  can  be  obtained  by  passing 
through  diluted  serum  a  brisk  stream  of  carbon  dioxide.     It  is 
also  precipitated  by  adding  sodic  chloride  to  saturation.     When 
a  fluid  containing  paraglobulin  is  added  to  a  serous  transudation, 
it  causes  coagulation  of  the  fluid,  giving  rise  to  fibrin. 

3.  fKbrinogen,  a  viscous  precipitate  got  from  serous  fluids  or 
blood  plasma   in  the   same  way  as  the  last,  but  with  greater 
dilution  and  more  prolonged  use  of  carbon  dioxide.  It  is  similar 
in  its  characters  to  the  last,  but  coagulates  at  a  lower  tempera- 
ture (55°  C.)   (paraglobulin   coagulating  at  60°-70°  C.).     On 
its  addition  to  defibrinated  blood,  or  a  fluid  containing  para- 
globulin, it  forms  a  coagulum. 

4.  Myosin  is  obtained  from  dead  muscle,  being  the  soft,  jelly- 
like  clot   formed  during  rigor  mortis  from   the   dying   muscle 
plasma.  It  is  not  so  soluble  as  globulin,  for  it  requires  a  stronger 
solution  of  salt  (10  %)  to  dissolve  it,  and  is  precipitated  from  its 
saline  solution  by  solid  salt  or  by  dilution.     It  is  coagulated  at 
60°  C. 

5.  Vitellin,  a  white  granular  proteid  obtained  from  the  yelk 
of  egg.     It  is  very  soluble  in  10  percent,  saline  solution,  from 
which  it  can  be  precipitated  by  extreme  dilution,  but  not  by 
saturation  with  salt.    It  coagulates  between  70°  and  80°  C. 

(C)   DERIVED  ALBUMINS   (ALBUMINATES). 

1.  Add  Albumin  (syntonin)  can  be  made  from  any  of  the 
preceding  by  the  slow  action  of  a  weak  acid;  or  by  adding 
strong  acetic  or  hydrochloric  acids  to  native  albumin,  such  as 


70  •       MANUAL    OF    1'HYSIOLOGY. 

exists  in  white  of  egg,  and  dissolving  the  jelly  thus  formed  in 
water.  It  is  only  soluble  in  weak  acids — exact  neutralization 
precipitating  it.  With  the  least  excess  of  alkali  the  precipitate 
redissolves,  changing  into  alkali  albumin. 

If  it  be  dissolved  in  weak  acid  it  will  not  coagulate  on  boiling, 
but  it  coagulates  and  becomes  incapable  of  re-solution  if  heated 
while  precipitated  by  neutralization. 

2.  Alkali  Albumin. — Similar  to  the  last,  but  produced  by  the 
action  of  either  weak  alkalies  on  dilute  solutions,  or  strong  solu- 
tion of  potash  on  white  of  egg.     Its  general  behavior  is  the  same 
as  the  above,  but  if  prepared  by  strong  solution  of  potash  and 
allowed  to  stand   some  time   it   differs   in   composition,   being 
deprived  of  its  sulphur.     It  can  then  be  distinguished  by  the 
absence  of  the  brown  coloration  which  appears  on  heating  acid 
albumin  with  caustic  potash  and  lead  acetate. 

3.  Casein  is  the  proteid  existing  in  milk,  and  resembles  alkali 
albumin  in  its  reactions.     It  can  be  precipitated  from  milk  by 
rennet,  or  acetic  acid  in  excess,  but  not  by  exact  neutralization, 
owing  to  the  presence  of  neutral  potassium  phosphate,  which  must 
be  converted  into  the  acid  salt  before  precipitation  begins. 

(D)  FIBRIN. 

A  solid  filamentous  body,  the  result  of  chemical  changes 
accompanying  the  death  of  the  blood  plasma,  during  which  the 
so-called  fibrin  generators  are  set  free.  It  swells  in  weak  hydro- 
chloric acid,  but  does  not  dissolve  while  cold.  If  heated  to  60° 
C.  in  acid,  it  changes  to  acid  albumin  and  dissolves.  By  10  per 
cent,  neutral  saline  solutions,  a  substance  like  a  globulin  may  be 
extracted  from  it.  If  heated,  it  assumes  the  characters  of  a 
coagulated  proteid. 

(E)  COAGULATED  ALBUMIN. 

If  any  of  the  above  be  heated  over  70°  C.  (except  acid  and 
alkali  albumin,  which  must  first  be  precipitated  by  neutraliza- 
tion), they  coagulate,  and  become  extremely  insoluble  and  lose 
their  former  characters.  They  are  but  very  slightly  acted  on  by 
weak  acids,  even  when  warmed.  Strong  acids  dissolve  them, 


GLOBULINS  AND  ALBUMINATES.  71 

but  this  solution  is  associated  with  a  destructive  change.  They 
are,  however,  converted  by  the  digestive  ferments  and  juices  into 
peptones,  and  thus  dissolved. 

(F)  PEPTONE. 

This  substance  is  formed  by  the  action  of  the  digestive  fer- 
ments from  any  of  the  above  albumins,  in  the  stomach  by  pepsin 
in  the  presence  of  dilute  acid,  and  in  the  small  intestines  by  tryp- 
sin  in  the  presence  of  dilute  alkali.  This  change  renders  them 
more  soluble  and  diffusible,  and  thus  enables  them  to  pass  out  of 
the  alimentary  canal  into  the  system^  and  makes  them  more 
suited  to  take  part  in  the  nourishment  of  the  body. 

The  leading  characteristics  of  peptones  may  be  thus  enumer- 
ated : — 

1.  Very  ready  solubility  in  hot    or   cold  water,  acids    or 

alkalies. 

2.  Not  coagulable  by  heat. 

3.  They  are  precipitated  by  alcohol  but  not  changed  to  the 

coagulated  form. 

4.  They  diffuse  more  readily  through  animal  membrane  than 

any  other  albumins. 

5.  They  are  not  precipitated  by  copper  sulphate,  ferric  chlo- 

ride, or  potassium  ferrocyanide  and  acetic  acid. 

6.  They  are  precipitated  by  iodine,  chlorine,  tannin,  mer- 

curic chloride,  and  the  nitrates  of  silver  and  mercury. 

7.  Caustic  potash  and  a  trace  of  copper  sulphate  added  to 

their  solutions  give  a  red  color  which  deepens  to  violet 

if  too_much  of  the  copper  salt  be  used. 

The  formation  of  peptones  is  a  gradual  process  having  many 
intermediate  steps,  in  the  earlier  stages  of  which  precipitates  are 
formed  by  potassium  ferrocyanide  and  acetic  acid.  (  Vide  Chaps, 
vni  and  ix  on  Chemistry  of  Digestion.) 

GROUP  III.— ALBUMINOIDS. 

These  are  the  outcome  of  nutritive  modification  of  protoplasm, 
and  may  be  said  to  be  directly  manufactured  by  that  substance, 
and  to  be  specially  adapted  to  meet  the  requirements  of  certain 
textures  differing  widely  in  function. 


72 


MANUAL    OF    PHYSIOLOGY. 


They  are  allied  to  one  another  and  to  the  last  group  by — (a) 
their  percentage  composition ;  *  (6)  containing  nitrogen ;  (c) 
being  amorphous  colloids. 

They  differ  from  albuminous  bodies  in — (a)  their  solubility ; 
(6)  their  behavior  to  heat,  acids,  alkalies  and  the  digestive 
fluids ;  and  (c)  their  value  as  food  stuffs. 

1.  Mucin  is  the  characteristic  ingredient  of  the  mucus  manu- 
factured by  epithelial  cells,  and  is  also  found  in  connective  tissue 
(abundantly  in  that  of  the  foetus)  and  in   some   pathological 
growths.     It  gives  a  peculiar  thick  ropy  consistence  to  the  fluid 
containing  it,  enabling  it  to  be  drawn  into  threads.     It  is  precipi- 
tated by  mineral  acids,  alum  and  alcohol,  and  the  precipitate 
swells  in  water  and  is  redissolved  in  excess  of  the  acid.     With 
acetic  acid  a  precipitate  is  formed  which  does  not  redissolve  in 
excess  of  the  acid.     When  boiled  with  sulphuric  acid  it  yields 
leucin  and  tyrosin. 

2.  Chondrin  is  obtained  by  the  prolonged  boiling  in  water  of 
slices  of  cartilage  cleared  of  the  perichondrium.     On  cooling, 
this  solution  forms  a  jelly.     The  jelly  dissolves  easily  in  hot 
water  or  alkalies,  and  can  be  precipitated  by  acetic  or  weak  min- 
eral acids,  alum  or  acetate  of  lead.     It  gives  only  leucin  on  boil- 
ing with  sulphuric  acid. 

3.  Gelatin  is  produced  by  boiling  fibrous  connective  tissues, 
such  as  ligaments,  tendons,  the  true  skin  and  bones  in  water.    On 
cooling,  the  fluid  forms  a  jelly,  which  can  be  dried  to  a  colorless 
brittle  body  which  swells  in  cold  water  and  dissolves  on  being 
heated.     It  is  not  precipitated  by  acetic  acid,  but  yields  precipi- 
tates with  mercuric  chloride  or  with  tannin,  as  seen  in  making 


*The  following  Table  gives  the  composition  of  the  principal  albuminoids  and  albu- 
min:— 


Gelatin. 

Elastin. 

Chondrin. 

Mucin. 

Keratin. 

Albumin. 

C 

50$ 

55* 

47$ 

50$ 

51* 

51-54  $ 

H 

7 

7 

6 

7 

6 

6-7 

N 

18 

17 

14 

10 

17 

15-17 

0 

23 

20 

31 

33 

21 

20-23 

s 

0.5 

0.6 

3 

2-2.3 

PRODUCTS    OF   TISSUE   CHANGE.  73 

leather.     On  boiling  with  sulphuric  acid  it   yields  glycin  and 
leucin  but  no  tyrosin. 

4.  Elastin  is  obtained  from  yellow  elastic  tissue  by  boiling  with 
caustic  alkalies.     It  is  little  affected  by  boiling  water,  strong 
acetic  acid,  or  weak  alkalies,  but  dissolves  in  concentrated  sul- 
phuric acid.  It  is  precipitated  by  tannin,  and  yields  leucin  when 
boiled  with  sulphuric  acid. 

5.  Keratin  exists  in  the  epidermic  appendages  (hair,  horn, 
nails,  etc.).     It  resembles  the  albuminous  bodies  in  containing  a 
considerable  quantity  of  sulphur,  but  differs  from  them  and  the 
other  albuminoids  in  general  properties.     It  is  soluble  in  alka- 
lies, swells  in  strong  acetic  acid,  gives  the  xanthoproteic  reac- 
tion, and  is  insoluble  in  the  digestive  juices. 

GROUP  IV.— PRODUCTS  OF  TISSUE  CHANGE. 
INTERMEDIATE  OR  BYE  PRODUCTS. 

These  are  protoplasmic  manufactures  destined  for  some  useful 
purpose,  but  they  do  not  long  exist  in  their  original  form  ;  being 
often  broken  up  into  other  compounds,  they  are  reabsorbed,  or 
pass  away  with  the  faeces.  These  bodies  are  found  in  the  various 
secretions.  Most  of  them  can  be  better  described  with  the  func- 
tion of  the  gland  which  forms  the  secretion  in  which  they  occur. 

Attention  must  here  be  drawn  to  certain  complex  bodies  exist- 
ing in  the  bile.  Some  complex  nitrogenous  substances  and  the 
monatomic  alcohol,  cholesterin,  will  now  also  be  mentioned.  But 
the  reader  must  remember  that  chemically  they  are  not  con- 
nected with  the  other  bodies,-  the  description  of  which  immedi- 
ately follows  theirs,  namely,  the  effete  products. 

Bile  Salts. — Two  acids  exist  in  the  bile  united  with  soda  to 
form  soluble  soap-like  salts.  They  may  be  recognized  by  the 
purple- violet  color  produced  by  cane  sugar  and  sulphuric  acid  at 
a  temperature  of  about  70°  C.  (Pettenkofer's  reaction). 

Taurocholic  Acid,  C26H45NSO7,  is  most  plentiful  in  the  bile  of 
camivora,  where  it  occurs  combined  with  soda.  It  is  decomposed 
by  prolonged  boiling  with  water  into  taurin  and  cholic  acid, 
thus : — 

Taurocholic  Acid.  Taurin.  Cholic  Acid. 

C26H45NS07  +  HaO  =  C2H7NS03  +  C24H40O5. 

7 


74  MANUAL   OF   PHYSIOLOGY. 


Glycocholic  Acid,  C26R^Oe,  found  in  the  bile  of  herbivora  and 
man.  It  crystallizes  in  fine  white,  glistening  needles.  It  exists 
as  the  glycocholate  of  soda  in  the  bile.  By  boiling  with  weak 
acid,  it  yields  glycin  and  cholic  acid. 

Glycocholic  Acid.  Glycin.  Cholic  Acid. 

C26H45N06  +  H20  =  C2H5N02  +  C24H40O5. 

In  the  bile  certain  matters  also  exist  to  which  the  color  is  due, 
the  principal  being  bilirubin  in  man  and  carnivora,  and  biliver- 
din  hi  herbivora.  They  are  probably  derived  from  the  coloring 
matter  of  the  blood.  They  can  be  recognized  by  treating  the 
solution  with  nitric  acid  which  is  colored  with  red  fumes,  when  a 
play  of  colors  is  seen  passing  through  stages  of  green,  blue, 
violet,  red  and  yellow. 

Lecithin,  C44H9oNPO9,  is  a  complex  nitrogenous  fat  found  in 
most  tissues  and  fluids  of  the  body,  particularly  in  the  nerve 
tissues  and  yelk  of  egg.  It  is  an  interesting  product  of  decom- 
position of  the  constituents  of  the  brain,  and  is  related  in  con- 
stitution to  the  neutral  fats  ;  it  may  be  regarded  as  an  acid 
glycerine  ether.  It  is  easily  decomposed  when  heated  with 
baryta  water,  splitting  into  glycerin  -phosphoric  acid,  neurin,  and 
barium  stearate. 

Another  body  called  Cerebrin,  not  containing  any  phosphorus 
and  of  doubtful  composition,  can  be  obtained  from  brain  sub- 
stance, and  is  also  found  in  nerve  fibres  and  pus  corpuscles.  It 
is  a  light  colorless  powder  which  swells  in  water. 

Protagon,  C16oH308N5PO35,  is  by  some  supposed  to  be  the  chief 
constituent  of  brain  substance,  and  by  others  a  mixture  of  the 
last  two  bodies. 

Neurin  (Choliri),  C5H15NO2,  is  an  oily  liquid  only  found  in  the 
body  as  a  product  of  the  decomposition  of  lecithin,  but  it  has 
been  obtained  synthetically. 

Cholesterin,  C26H41O  -f-  H2O,  exists  throughout  the  body  where 
active  tissue  change  is  going  on,  particularly  the  nervous  centres. 
It  is  a  monatomic  alcohol,  and  is  the  only  one  existing  free  in 
the  body.  It  may  be  obtained  from  gall  stones,  some  of  which 
consist  entirely  of  cholesterin.  It  may  occasionally  be  found  in 
a  crystallized  form  in  many  of  the  fluids  of  the  body  but  never 


WASTE   PRODUCTS.  75 

in  the  tears  or  urine,  and  only  seems  to  be  an  effete  product, 
nearly  all  that  produced  in  the  body  being  discharged  with  the 
effete  portions  of  the  bile.  It  may  be  recognized  by  the  shape 
of  the  crystals  formed  from  a  solution  in  alcohol,  which  are 
rhombic  plates,  in  which  one  corner  is  generally  deficient. 

EFFETE  PRODUCTS. 

These,  as  has  been  stated  before,  are  generally  the  outcome  of 
the  active  chemical  changes  necessary  for  the  growth  and  vitality 
of  the  living  protoplasm,  and  are  for  the  most  part  soon  elimi- 
nated by  the  excretory  glands,  so  that  but  small  quantities  of 
them  can  be  found  in  the  active  tissues  where  they  are  produced. 

Urea,  CO(NH2)2,  is  the  most  important  constituent  of  the 
urine  of  mammalia,  but  not  of  that  of  birds  or  reptiles.  Traces 
of  it  may  be  found  in  the  fluids  and  tissues  of  the  body.  It  is 
readily  soluble  in  water  and  alcohol,  and  forms  crystals  when  its 
solution  is  concentrated.  It  decomposes  when  treated  with  some 
strong  acids  or  alkalies,  taking  up  water  and  yielding  CO2  and 
NH3;  and  with  nitrous  acid  gives  CO2  -f  N2  -f  2(H2O).  It 
was  the  first  of  the  so-called  "  organic  "  compounds  to  be  made 
artificially,  being  obtained  by  Wohler  in  1828  by  mixing  watery 
solutions  of  potassium  cyanate  and  ammonium  sulphate,  evapo- 
rating to  dryness  and  extracting  with  alcohol,  or,  in  short,  by 
heating  ammonium  cyanate,  with  which  it  is  isomeric. 

Ammonium  Cyanate.  Urea. 

NH4.CNO      =      CO.NH2.NH2. 

It  can  now  be  produced  artificially  in  other  ways. 

It  has  also  been  considered  a  monamide  of  carbamie  acid 
(CO.OH.NH2),  a  molecule  of  hydroxyl  being  replaced  by  one 
of  amidogen,  NH2,  thus — CO.NH2.NH2.  In  the  presence  of 
septic  agencies,  in  a  watery  solution,  urea  takes  up  two  molecules 
of  water  and  is  converted  into  ammonium  carbonate — 

CO(NH2)2  -jr  2H20  ==  CO(ONH4)2. 

The  so-called  alkaline  fermentation  of  urine  depends  upon  this 
change.  The  reader  is  referred  to  the  Chapter  on  Excretions 
(xxn),  where  more  complete  information  is  given. 

Kreatin,  C4HaN3O2,  occurs  in  muscle  and  many  other  textures. 


76  MANUAL   OF   PHYSIOLOGY. 

It  may  be  converted  into  kreatinin  by  the  action  of  acids  by 
simple  dehydration.  It  can  also  be  split  up  into  sarcosin  and 
urea. 

Kreatinin,  C4H7N3O,  is  a  dehydrated  form  of  kreatin,  which  is 
a  normal  constituent  of  urine.  In  watery  solutions  it  is  slowly 
converted  into  kreatin. 

AUantoin,  C4H6N4O3,  found  in  the  allantoic  fluid  and  the 
urine  of  the  foetus  and  pregnant  women.  It  is  crystallizable, 
and  is  converted  into  urea  and  allantoic  acid  by  oxidation. 

Glydn  (Glycocoll  or  Glycocine),  C2H2(NH2)O.OH,  is  regarded 
as  amido-acetic  acid.  It  does  not  occur  free  in  the  body,  but 
enters  into  the  composition  of  the  bile  acids  and  hippuric  acid. 
It  is  soluble  in  water,  and  insoluble  in  cold  alcohol  and  in  ether. 

Leucin,  C6H1C(NH2)O.OH,  or  amido-caproic  acid,  is  found  in 
the  secretion  of  the  pancreas  and  some  other  glands.  It  is  one 
of  the  principal  products  of  the  decomposition  of  albuminous 
bodies,  from  which  it  can  be  obtained  by  boiling  with  sulphuric 
acid,  in  the  form  of  peculiar  rounded  crystals. 

Tyrosin,  C9HnNO3,  though  belonging  to  a  distinct  chemical 
series  (aromatic),  is  only  found  in  company  with  leucin  in  the 
decomposition  of  albuminous  bodies,  and  normally  in  the  pan- 
creatic secretion.  Its  constitution  is  said  to  give  warranty  for 
the  name  oxy-phenyl-amido-propionic  acid. 

Taurin,  C2H7NSO3,  is  a  constituent  of  one  of  the  bile  acids, 
and  is  also  found  in  muscle  juice.  It  may  be  regarded  asamido- 
ethyl-sulphonic  acid. 

Uric  Acid,  C5H4N4O3  (dibasic),  is  found  in  large  quantities  in 
the  excrement  of  birds  and  reptiles,  but  in  a  small  and  variable 
quantity  in  the  urine  of  man.  Traces  have  been  found  in  many 
tissues,  in  some  of  which  quantities  accumulate  as  the  result  of 
pathological  processes  (gout).  It  forms  salts  which  are  much 
less  soluble  in  cold  than  in  hot  water,  and  make  the  common 
sediment  in  urine.  The  acid  salts  are  less  soluble  than  the 
neutral.  The  common  test  for  uric  acid  consists  of  slowly 
evaporating  the  substance  to  dryness  with  a  little  nitric  acid, 
and  to  the  residue  adding  ammonia,  when  a  bright  purple  color 
is  produced  (murexide  test).  Uric  acid  is  supposed  to  be  a  step 


NON-NITROGENOUS. 

in  the  production  of  urea,  which  is  one  of  the  results  of  il 
tion  in  the  presence  of  acids,  thus : — 

Uric  Acid.  Alloxan.  Urea. 

C5H,NA  +  H20  +  0  =  C4H2N204  +  CO(NH2)2. 
Hippurie  Acid,  C9H9NO3,  occurs  in  considerable  quantities  in 
the  urine  of  the  horse  and  herbivora  generally.  It  is  found  but 
very  sparingly  in  man's  urine,  but  it  appears  in  large  quantities 
after  benzoic  acid  and  some  other  medicaments  have  been  taken. 
In  constitution  it  is  an  amido-acetic  acid  in  which  one  atom  of  the 
hydrogen  is  replaced  by  the  radical  benzoyl  (C7H5O).  In  the 
body  it  is  combined  with  bases,  and  is  formed  out  of  benzoic  acid 
and  glycin  (amido-acetic  acid),  thus : — 

Glycin.  Benzoic  Acid.  Hippurie  Acid.  Water. 

C2H2(NH2)O.OH  +  C7H602  =  C2H(C,H50)(NH2)O.OH  +  H20. 
By  heating  or  putrefaction  it  is  resolved  into  these  constituents. 

Indol,  C8H7N,  is  produced  in  the  intestinal  canal  by  the  putre- 
factive changes  brought  about  by  septic  agencies  during  pan- 
creatic digestion.  It  gives  an  odor  to  the  fteces  and  a  red  color 
with  nitrous  acid. 

Indican,  a  peculiar  substance  sometimes  found  in  the  urine 
and  sweat.  With  oxidizing  agents  it  yields  indigo  blue.  By  this 
fact  it  is  easily  recognized.  An  equal  volume  of  hydrochloric 
acid  and  a  very  small  quantity  of  calcium  hypochlorite  (bleach- 
ing lime)  is  added,  and  the  indigo  which  is  formed  can  then  be 
dissolved  and  separated  by  agitation  with  chloroform. 

CLASS  B.— NON-NITROGENOUS. 
GROUP  V.— CARBOHYDRATES. 

Car bohydrates ^ (general  formula,  CmH2nOn)  are  bodies  in  which 
the  hydrogen  and  oxygen  exist  in  the  same  proportion  as  in 
water,  the  carbon  being  variable.  The  following  examples  of 
this  group  are  met  with  in  the  textures  of  the  body : — 

Grape  /Sugar  (Dextrose),  C6H12O6,  occurs  in  minute  quantities 
in  the  blood,  chyle  and  lymph.  It  forms  crystals  which  readily 
dissolve  in  their  own  weight  of  water.  The  watery  solution  has 
a  dextro-rotatory  power  on  the  ray  of  polarized  light.  When 
mixed  with  yeast,  the  fungus  (Saccharomyces  cerevisice)  of  the 


78  MANUAL   OF   PHYSIOLOGY. 

yeast  causes  alcoholic  fermentation  of  the  sugar,  whereby  alcohol 
and  carbon  dioxide  are  formed. 

Dextrose.  Alcohol. 

C6H1206  -  2C2H60  +  2C02. 

Moderate  heat  (25°  C.)  aids  the  process,  and  cold  below  5°  C. 
checks  it ;  an  excess  of  either  sugar  or  alcohol  stops  it. 

The  presence  of  casein  or  other  proteid  material,  when  decom- 
posing, gives  rise  to  lactic  fermentation,  producing  first  lactic  acid, 
then  butyric  acid,  carbon  dioxide  and  hydrogen. 

Dextrose.        Lactic  Acid.  Butyric  Acid. 

C6H1206  -  2C3H603  =  C4H802  +  2C02  4-  H4. 

Milk  Sugar  (Lactose),  C12H22On  +  H2O,  metameric  with  cane 
sugar  (sucrose).  It  is  the  characteristic  sugar  found  in  milk.  It 
is  not  so  soluble  as  dextrose,  and  does  not  undergo  direct  alcoholic 
fermentation,  but  under  the  influence  of  certain  organisms  it 
readily  gives  rise  to  lactic  acid  by  lactic  fermentation  in  the  same 
way  as  dextrose.  (See  page  102.) 

Inosit,  C6H12O6  -j-  2H2O,  is  an  isomer  of  grape  sugar,  which 
is  incapable  of  undergoing  alcoholic  fermentation.  It  is  crystal- 
lizable,  and  easily  soluble  in  water.  It  has  no  effect  on  the 
polarized  ray.  It  is  found  in  the  muscles,  and  also  in  the  lungs, 
spleen,  liver  and  brain. 

Glycogen,  C6H10O5,  a  body  like  dextrin,  first  found  in  the  liver.. 
It  gives  an  opalescent  solution  in  water,  and  is  readily  converted 
into  dextrose  by  an  amylolytic  ferment,  or  weak  acids.     It  has  a 
strong  dextro-rotatory  power.     It  can  be  found  in  most  rapidly 
growing  tissues.     (See  Glycogenic  Function  of  the  Liver.) 

GROUP  VI.— FATS. 

These  bodies  have  the  same  elements  in  their  composition,  but 
the  hydrogen  and  oxygen  have  variable  proportions — not  that 
of  water.  Fats  are  found  in  large  masses  in  some  tissues,  and 
also  as  fine  particles  suspended  in  many  of  the  fluids.  The  fat 
of  adipose  tissue  in  man  is  a  mixture  of  olein,  palmitiii  and 
stearin,  which  are  spoken  of  as  the  neutral  fats. 

The  first  is  liquid,  and  the  last  two  solid  at  normal  tempera- 
tures, and  the  varying  consistence  of  the  fat  of  different  animals 


INORGANIC   BODIES.  79 

depends  upon  the  relative  proportions  of  the  solid  or  liquid 
fats. 

Fats  are  soluble  in  ether  and  chloroform,  but  quite  insoluble 
in  water.  When  agitated  in  water  containing  an  albuminous 
body,  and  an  alkaline  carbonate  in  solution,  fluid  fat  is  broken 
up  into  small  particles,  which  remain  suspended  in  the  liquid, 
forming  an  opaque  milky  emulsion. 

Chemically,  they  are  regarded  as  ethers  derived  from  the 
triatomic  alcohol  glycerine,  C3H5(OH)3,  by  replacing  the  OH 
group  with  the  radicals  of  the  fatty  acids,  thus  : — 

Glycerine.  Palmitic  Acid.  Tripalmitin  Water. 

C3H5(OH)3  +  3(C16H3202)  =  C3H5  (C16H3102)3  +  3H20. 

Under  the  influence  of  certain  ferments  they  separate  into 
glycerine  and  the  fatty  acid,  uniting  with  the  necessary  elements 
of  water. 

When  the  neutral  fats  are  boiled  with  alkaline  solutions  they 
are  similarly  decomposed,  and  uniting  with  the  elements  of  water, 
form  glycerine  and  fatty  acids.  The  glycerine  is  thus  set  free, 
but  the  fatty  acid  combines  with  the  alkaline  metal  to  form  a 
soluble  soap.  An  insoluble  soap  may  be  obtained  by  substituting 
lead  or  lime,  etc.,  for  the  alkali. 

This  splitting  up  of  the  neutral  fats,  stearin,  palmitin  and  olein 
into  sodium  stearate,  palmitate,  or  oleate  goes  on  during  digestion, 
and  is  said  to  be  useful  in  aiding  the  absorption  of  fatty  matters. 

INORGANIC  BODIES. 

Water  (H2O)  is  present  in  nearly  all  tissues  in  larger  propor- 
tion than  any  other  compound,  making  up  about  70  per  cent,  of 
the  entire  body  weight.  The  amount  in  each  texture  varies,  the 
different  tissues  having  widely  different  consistence. 

Water  is  introduced  into  the  body  not  only  as  drink,  but  a 
large  quantity  is  also  taken  with  our  solid  food.  It  is  highly 
probable  that  in  the  chemical  changes  which  take  place  in  the 
tissues,  some  water  is  formed  by  the  oxidization  of  the  hydrogen 
of  the  more  complex  substances. 

In  the  economv  it  acts  as  the  universal  solvent  in  the  fluids  of 


80  MANUAL   OF    PHYSIOLOGY. 

the  body,  and  as  the  agent  by  means  of  which  the  chemical 
changes  of  the  various  organs  can  be  accomplished. 

Water  leaves  the  body  by  the  lungs  as  vapor,  and  by  the 
skin,  kidney,  and  many  other  glands,  as  the  fluid  in  which  the 
solids  of  their  secretions  are  dissolved. 

Inorganic  acids  occur  in  the  body  either  combined,  forming 
salts,  in  which  condition  we  find  several  (sulphuric,  phosphoric, 
silicic),  or  uncombined.  In  the  latter  state  we  have  only  two, 
viz. : — 

Hydrochloric  Acid,  HC1,  which  is  formed  in  the  stomach,  and 
plays  an  important  part  in  gastric  digestion. 

Carbonic  Acid  Gas,  CO2,  exists  in  most  of  the  fluids  of  the 
body,  having  been  absorbed  by  them  from  the  tissues.  The 
venous  blood  contains  a  considerable  quantity,  some  of  which  is 
got  rid  of  during  the  passage  of  the  blood  through  the  lungs. 
It  is  a  waste  product,  which  must  be  constantly  eliminated  from 
the  body  (see  Respiration). 

Salts. — A  large  number  of  salts  occur  in  the  tissues,  generally 
in  small  quantity,  in  solution.  In  the  teeth  and  in  bone  tissue 
salts  exist  in  the  solid  form,  and  in  much  greater  proportion 
than  in  any  of  the  soft  parts.  Most  of  the  salts  are  introduced 
into  the  economy  with  the  food,  but  some,  doubtless,  are  formed 
in  the  body  itself.  Our  knowledge  of  the  exact  position  occupied 
by  the  salts  in  the  textures  is  very  incomplete,  as  their  amount 
is  usually  estimated  from  the  ash  of  the  tissue  which  remains 
after  ignition,  by  which  process  some  become  altered,  so  that  it 
is  impossible  to  say  what  are  the  exact  salts  that  are  present  in 
the  body.  They  form  chemical  combinations  with  the  complex 
organic  compounds,  which  we  do  not  understand,  and  probably 
have  important  functions  to  perform,  such  as  rendering  certain 
materials  (globulins)  soluble,  or  otherwise  facilitating  tissue 
change.  The  salts  pass  out  of  the  body  in  many  secretions, 
largely  in  the  urine,  where  they  influence  the  elimination  of 
urea,  and  therefore  form  an  important  constituent  of  that 
secretion. 

Common  Salt  (Sodium  Chloride),  NaCl,  is  the  most  widely  dis- 
tributed, and  is  present  in  greater  quantity  than  any  other  salt 


INORGANIC   BODIES.  81 

in  all  animal  fluids  and  most  tissues,  except  bones,  teeth,  red 
blood  corpuscles  and  red  muscle. 

Potassium  Chloride  commonly  accompanies  sodium  chloride  in 
small  quantity.  In  the  red  blood  corpuscles  and  in  muscle  it 
occurs  in  greater  amount  than  the  sodium  salt,  while  in  the  blood 
plasma  but  little  is  found  in  comparison  with  the  sodium  salts, 
and  any  excess  seems  to  act  as  a  poison  to  the  heart. 

Carbonates  and  phosphates  of  calcium,  sodium,  potassium  and 
magnesium  occur  in  small  quantities  in  most  tissues.  The  earthy 
part  of  bone  is  chiefly  composed  of  calcium  and  magnesium  phos- 
phate and  calcium  carbonate,  together  with  some  calcium  fluoride. 

Sulphates  of  sodium  and  potassium,  probably  formed  in  the 
body  from  the  oxidization  of  the  sulphur  in  the  complex  proteid 
materials,  occur  in  most  tissues,  and  are  removed  from  the  body 
by  the  kidneys. 

Finally,  we  find  two  of  the  elements  free  in  the  textures.  Of 
these  Oxygen  plays  by  far  the  most  important  part.  It  is  widely 
distributed  among  the  fluids  of  the  body,  from  which  it  can  be 
removed  by  reducing  the  pressure  of  oxygen  of  the  atmosphere 
by  means  of  an  air  pump.  Oxygen  is  introduced  into  the  body 
by  the  lungs,  where  the  blood  takes  it  from  the  air.  In  the 
blood  only  a  small  quantity  of  that  which  can  be  removed  by 
the  air  pump  is  really  free ;  the  remainder  is  chemically  com- 
bined with  the  coloring  matter  of  the  blood.  It  is  absolutely 
necessary  for  life,  as  it  alone  can  enable  the  chemical  changes 
of  the  tissues,  which  are  mostly  oxidizations,  to  go  on.  It  is,  in 
fact,  the  element  necessary  for  the  slow  combustion  which  takes 
place  in  the  nutrient  material  after  its  assimilation. 

Nitrogen  also  occurs  in  the  blood,  but  in  insignificant  quantity. 
It  is  absorbed  from  the  atmosphere  as  the  blood  passes  through 
the  lungs.  So  far  as  we  know,  it  has  no  physiological  importance 
in  the  body. 


82  MANUAL   OF  PHYSIOLOGY. 


CHAPTER  IV. 

THE  VITAL  CHARACTERS  OF  ORGANISMS. 

The  manifestation  of  so-called  vital  phenomena  in  man  forms 
the  subject-matter  of  the  following  chapters,  and  some  explana- 
tory definition  of  the  vital  characters  of  the  simpler  organisms 
will  be  useful  in  preparing  the  beginner's  mind  for  the  more 
intricate  questions  in  human  physiology.  This,  with  the  fore- 
going short  account  of  the  chemical  and  structural  peculiarities 
of  animals,  will  complete  a  rough  outline  of  the  general  character 
of  organisms. 

Protoplasm  has  already  been  referred  to  as  the  material 
capable  of  showing  vital  phenomena,  the  most  obvious  and 
striking  of  which  are  its  movements. 

Besides  the  common  molecular  or  Brownian  movement  of  the 
granules  in  protoplasm — which  may  be  seen  in  most  cases  where 
fine  granules  are  suspended  in  a  less  dense  medium — protoplasm 
can  perform  motions  of  different  kinds  which  must  be  regarded 
as  distinctly  vital  in  character.  This  movement  may  be  said  to 
be  of  three  different  kinds,  according  to  the  results  produced, 
viz. :  (1)  The  production  of  internal  currents.  (2)  Changes  in 
form.  (3)  Locomotion.  In  reality,  the  two  latter  are  dependent 
on  the  first. 

The  existence  of  currents  moving  from  one  part  of  the  proto- 
plasm to  another  can  be  well  seen  in  vegetable  cells,  when  the 
cell  wall  restricts  the  more  obvious  change  in  form  or  place. 
Thus  in  the  cells  forming  the  hair  on  the  stamens  of  Tradeseantia 
Virginiea  the  various  currents  can  be  seen  in  the  layers  of  proto- 
plasm which  line  the  cell  wall. 

The  granular  particles  course  along  in  varying  but  definite 
directions,  passing  one  another  like  foot  passengers  in  a  crowded 
street.  The  first  and  most  obvious  result  of  this  is,  that  the 
different  parts  of  the  substance  are  frequently  brought  into  con- 
tact with  one  another,  and  thus  the  products  of  any  chemical 


PROTOPLASMIC   MOVEMENTS. 


83 


FIG.  35. 


changes  taking  place  at  a  given  part  of  the  cell  body  are  rapidly 
distributed  over  the  entire  mass  of  the  protoplasm. 

The  change  in  form  occurs  if  there  be  no  definite  cell  wall — as 
in  naked  vegetable  spores  and  amoeboid  forms  of  animal  life — to 
restrict  or  direct  the  current  of  protoplasm ;  it  flows  unto 
various  directions  in  bud-like  processes,  which  appear  at  various 
parts  of  the  protoplasmic  mass,  so  as  to  cause  a  constant  change 
in  the  form  of  the  cell.  These  outstretched  processes  sometimes 
flow  together  and  become  fused,  often  enclosing  some  of  the 
medium  in  which  the  creature  is  suspended,  or  catching  some 
foreign  particle  floating  near  them. 

The  flowing  out  of  these  pseudopodia  usually  takes  place  for 
some  time  persistently  from  one  side  of  the  cell ;  and  the  body 
of  the  cell  has  to  follow,  as  it  were, 
the  protrusion  of  the  processes  in 
such  a  manner  that  in  a  short  time 
definite  change  in  position  or 
movement  in  a  certain  direction 
occurs :  thus  the  protoplasmic  unit 
may  be  said  to  perform  definite 
progression  or  locomotion.  All 
these  movements  may  be  seen  in 
the  white  blood  corpuscle  of  a 

Cold-blooded  animal,  Such  as  a  frog,  An  amoeba  figured  at  two  different  mo- 

j      ,.n                          -i       .       ,1              .  merits  during  movement,  showing   a 

and    Still    more    easily    m    the   Uni-  clear  outer  layer  and  a  more  granular 

ppllnlar  nmrchfl  central  portion,    (n)    Nucleus  ;  (i)  In- 

Uiar  amceDa.  gestedfood.    (Gegenbauer.) 

Various  influences  may  be  seen 

to  affect  the  rate  of  movements  and  probably  influence  at  the 
same  time  the  other  activities  of  the  protoplasm.  Foremost 
among  these  must  be  named :  (1)  Temperature.  If  a  proto- 
plasmic unit,  which  is  observed  to  be  motile,  be  gently  warmed, 
the  movements  become  more  and  more  active  as  the  temperature 
is  raised,  up  to  a  certain  point,  about  35°-42°  C.,  when  a  spasm 
occurs,  resulting  in  the  withdrawal  of  the  pseudopodia;  soon 
after  this  the  cell  assumes  a  spherical  shape.  If  the  heat  be 
carefully  abstracted  before  it  has  attained  too  great  a  height,  the 
protoplasm  may  recover  and  again  commence  its  movements.  If, 


84  MANUAL   OF    PHYSIOLOGY. 

on  the  other  hand,  cold  be  applied  to  moving  protoplasm,  the 
motions  become  less  and  less  active,  and  commonly  cease  at  a 
temperature  about  or  a  little  above  0°  C.  (2)  Mechanical 
irritation  also  produces  a  marked  effect  on  the  movements  of  pro- 
toplasm. This  may  be  well  seen  in  the  behavior  of  a  living 
white  cell  of  frog's  blood  under  the  microscope.  It  is  spherical 
when  first  mounted,  owing  to  the  rough  treatment  it  goes  through 
while  being  placed  on  the  glass  slide  and  covered ;  shortly  its 
movements  become  obvious  by  its  change  in  form,  which  may 
again  be  checked  by  a  sudden  motion  of  the  cover  glass.  (3) 
Electric  shocks  given  by  means  of  a  rapidly-broken  induced 
current  cause  spasm  of  the  protoplasm,  the  cell  becoming 
spherical.  (4)  Chemical  stimuli  also  have  a  marked  effect ; 
carbonic  acid  causing  the  movements  to  cease,  and  a  supply  of 
oxygen  making  it  active.  The  movements  and  other  activities 
of  protoplasm  are,  during  life,  frequently  modified  and  controlled 
by  nerve  influence,  as  will  appear  in  the  following  pages.  This 
may  readily  be  seen  in  the  stellate  pigment  cells  of  the  frog's 
skin,  which  can  be  made  to  contract  into  spheres  by  the  stimula- 
tion of  the  nerves  leading  to  the  part. 

The  motions  of  protoplasm  are  thus  seen  to  be  affected  by 
external  influences,  but  the  most  careful  observer  cannot  find 
physical  explanations  of  the  various  movements  which  have  been 
described.  It  is  necessary,  therefore,  to  ascribe  this  power  of 
motion  to  some  property  inherent  in  the  protoplasm,  and  hence 
the  movements  are  called  automatic.  We  are  unable  to  follow 
the  chemical  processes  upon  which  the  activities  of  the  proto- 
plasm depend,  and  we  therefore  call  them  vital  actions;  but  we 
must  assume  that  these  so-called  vital  properties  depend  on  cer- 
tain decompositions  in  the  chemical  constitution  of  the  proto- 
plasm. We  know  that  some  chemical  changes  take  place,  as 
we  can  find  and  estimate  products  which  indicate  a  kind  of  com- 
bustion;  but  we  know  little  or  nothing  of  the  details  of  the 
jO  ,  chemical  process. 

From  the  foregoing  description  of  the  manner  in  which  proto- 
plasm responds  to  external  stimuli,  it  may  be  gathered  that  it  is 
capable  of  appreciating  impressions  from  without ;  indeed,  it  can 

•r- 


CELL    DEVELOPMENT. 


85 


be  said  to  feel.  We  can  only  judge  of  the  sensitiveness  of  any 
creature  by  the  manner  in  which  it  responds  to  stimuli,  and  we 
may  therefore  conclude  that  the  smallest  particle  of  living  proto- 
plasm is  endowed  with  definite  sensitiveness:  this  must  be  noted 
as  one  of  the  most  striking  properties  of  protoplasm. 

Every  particle  of  living  protoplasm  has  the  power  of  assimila- 
tion. Taking  into  its  structure  any  nutrient  matters  it  meets 
with,  by  flowing  around  them  in  the  way  mentioned,  it  brings 
them  into  direct  contact  with  difFerents  parts  of  its  protoplasmic 
substance.  This  nutrition  of  the  cells  gives  rise  to  their  growth, 
and  finally  leads  to  their  reproduction.  These  facts  will  be  more 
closely  examined  when  speaking  of  their  relation  to  cell  life. 


FIG.    36. 


FIG.  37. 


Cells  of  the  yeast  plant  in  pro- 
cess of  budding,  between  which 
are  some  bacteria. 


Cartilage  from  young  animal  showing  the 
division  of  the  cells  («,  b,  c,  d). 


When  a  certain  size  has  been  attained,  the  cell  does  not  further 
increase,  but  prepares  to  bring  forth  a  cell  unit  similar  to  itself. 
This  is  spoken  of  as  the  rej>roduction  of  cells. 

Different  kinds  of  cell  reproduction  have  been  observed,  which 
are  all,  however,  modifications  of  the  same  general  plan.  The 
first  is  that  by  the  formation  of  a  bud  from  the  side  of  the  parent 
cell ;  this  bud  increases  in  size,  and  finally  detaches  itself  from 
the  parent  and  becomes  a  separate  individual.  This  process, 
'which  is  called  gemmation,  can  readily  be  seen  in  all  its  stages 
in  growing  yeast,  where  the  torula  cells  have  various-sized  buds 


86  MANUAL   OF   PHYSIOLOGY. 

growing  from  them.  If  the  newly-formed  portion  be  large, 
nearly  equal  in  size  to  tLe  cell  itself,  the  process  receives  the 
name  of  fission,  or  division.  In  well-marked  typical  fission  the 

parent  cell  divides  into 
two  parts  of  equal  size, 
each  of  which  becomes 
a  perfect  individual. 
Various  gradations  may 
be  traced  between  the 
two  processes,  so  that 

Cells   of  a   fungus    (Glceoc.npsa)    showing   different      .,     •        }-ffi       •,,  •, 

stages    (1-4)    of    endogenous    division.       (After      it    IS    difficult      to  .  draw 

any  very  distinct  line 
between  budding  and 

fission.  The  budding  and  fission  may  be  multiple ;  many  buds 
and  several  units,  products  of  division,  may  remain  together, 
and  form  what  is  called  a  colony.  When  this  multiple  budding 
or  division  takes  place,  so  that  the  new  units  are  included  within 
the  body  of  the  parent  cell,  then  the  process  is  called  endogenous 
reproduction  or  spore  formation.  Just  as  there  are  gradations 
between  budding  and  fission,  so  it  is  difficult  to  draw  a  hard  and 
fast  line  between  what  may  be  called  multiple  fission  and  spore 
formation. 

In  tracing  the  stages  of  development  of  the  highly  differentiated 
cells  of  some  tissues,  we  have  to  pass  through  a  series  of  changes 
which  form  a  cycle  that  may  well  be  called  the  lifetime  of  the 
cell.  The  duration  of  this  cycle  varies  greatly  in  different  indi- 
vidual cells.  Some  cells  are  very  short-lived,  being  destroyed 
by  their  act  of  secretion ;  others  probably  endure  for  the  life- 
time of  the  animal.  The  life  history  of  all  cells  begins  with  the 
stage  when  they  are  composed  entirely  of  indifferent  protoplasm, 
in  which  various  modifications  are  subsequently  produced. 

Let  us  take,  as  an  example,  a  cell  of  the  outer  skin  or  cuticle,- 
and  examine  its  life  history.  The  cuticle  is  made  up  of  numer- 
ous layers  of  epithelial  cells  laid  one  on  the  other,  and  the  sur- 
face cells  are  constantly  being  rubbed  or  worn  off.  These  cells 
have  their  origin  from  the  cells  of  the  deepest  layer,  which  is  next 
to  the  supply  of  nutriment.  This  layer  is  made  up  of  soft  proto- 


REPRODUCTION. 


87 


plasmic  units,  which  have,  no  doubt,  certain  specific  inherited 
characteristics,  but  apparently  the  same  as  the  motile,  sentient, 
growing  protoplasm  of  an  indifferent  cell.  By  a  process  of 
fission  or  budding,  constantly  going  on  in  this  deepest  layer  of 
ceils,  new  protoplasmic  units  are  produced.  These  become 
distinct  individuals,  and  occupy  the  position  of  the  parent  cell, 
which,  having  produced  offspring,  is  moved  one  place  nearer  the 
surface,  away  from  the  supply  of  food.  The  new  cell  in  time 
gives  rise  to  offspring,  and  having  attained  reproductive  maturity, 
in  turn  is  moved  onward  to  the  surface.  The  result  of  this  is 


FIG.  39. 


Division  of  Egg  cell.    (Gegenbauer .) 

that  its  supply  of  nutrition  diminishes,  the  evidences  of  repro- 
ductive activity  disappear,  and  at  a  certain  point  all  signs 
of  protoplasmic  life  are  lost.  But  on  its  way  from  the  seat  of  its 
origin  .to  the  surface,  it  makes  use  of  its  limited  supply  of  nutri- 
tion for  the  purpose  of  manufacturing  a  special  kind  of  material 
which,  if  present  at  all,  only  occurs  in  the  minutest  traces 
in  ordinary  protoplasm.  As  the  cell  moves  toward  the  surface,  it 
loses  its  protoplasmic  characters,  becomes  tougher  and  drier,  and 
finally  nothing  but  the  special  horny  material  remains.  Thus, 
from  the  birth  of  the  cell,  its  energies  are  devoted,  first,  to  its 


88  MANUAL   OP    PHYSIOLOGY. 

own  growth,  then  to  the  reproduction  of  its  like,  and  finally  to 
the  formation  of  a  material  fitted  to  act  as  a  mechanical  protection 
to  the  surface  of  the  skin.  Having  manufactured  a  certain 
amount  of  this  material,  the  protoplasm  dwindles,  and  finally 
disappears,  so  that  the  cell  may  be  said  to  die.  Its  horny, 
insoluble  and  impermeable  skeleton  has,  however,  yet  to  do  service 
in  the  outer  layer  of  the  skin  while  it  is  passing  toward  the 
surface,  to  be  in  its  turn  rubbed  off. 

It  has  already  been  stated  that  the  material  protoplasm,  which 
forms  all  active  cells,  is  capable  of  carrying  on  the  many  func- 
tions required  for  the  independent  existence  of  simple  creatures. 
It  will  be  found  in  the  subsequent  pages  that  not  only  can  pro- 
toplasm perform  all  the  activities  necessary  for  the  life  history 
of  unicellular  organisms,  but  that  it  can  also  work  out  all  the 
functions  of  the  most  complex  animals.  Indeed,  the  cells  which 
accomplish  the  most  elaborate  functions  in  man,  are  but  pro- 
toplasm more  or  less  modified  for  the  special  purpose  to  be 
attained. 

The  different  living  operations  of  many  independent  unicel- 
lular organisms  can  be  more  completely  watched  than  the 
changes  which  take  place  in  the  cells  of  the  higher  animals, 
both  on  account  of  their  greater  size,  their  freedom,  and  the 
more  obvious  character  of  the  changes  taking  place  in  them 
The  student  is  therefore  advised  to  spend  some  time  in  contem- 
plating the  operations  which  go  on  in  those  simple  organisms 
whose  life  is  not  complicated  by  structural  or  functional  elabo- 
ration, before  attempting  to  solve  the  difficult  question  of  the 
mechanism  of  the  human  body. 

The  lowest  forms  of  living  creatures  that  we  are  acquainted 
with  (jnicroeoceus  and  bacterium),  are  placed  among  the  fungi  in 
the  vegetable  kingdom.  On  account  of  their  extremely  minute 
size — being  hardly  visible  as  spherical  or  elongated  specks  with 
a  powerful  microscope — we  can  say  but  little  about  their  struc- 
ture. They  appear  to  be  translucent  and  homogeneous. 

Since  we  use  the  term  protoplasm  to  denote  the  material  of 
which  the  active  part  of  the  simplest  forms  of  living  beings 
are  composed,  we  must  assume  that  bacteria  are  small  particles 


BACTERIA.  89 

of  that  material,  but  the  characters  attributed  to  protoplasm 
cannot  be  detected  in  the  minute  glistening  mass  which  makes 
up  their  body. 

They  are  so  certain  to  appear  in  a  couple  of  days  in  organic 
infusions,  or  in  any  fluid  prone  to  putrefaction,  and  multiply  with 
such  astounding  rapidity,  that  they  have  been  supposed  by 
some  to  develop  spontaneously.  But  this  is  now  known  not  to 
be  a  fact.  Bacteria  can  no  more  than  any  other  form  of  living 
thing  appear  without  progenitors.  They  float  inanimate  and  dry 
in  multitudes  through  our  atmosphere,  and  adhere  to  all  sub- 
stances to  which  the  air  has  free  access.  The  moment  they  alight 
upon  a  suitable  habitat,  they  burst  into  prodigious  activity,  at 
first  forming  masses  or  colonies,  which  may  be  seen  as  a  jelly- 
like  scum  on  the  fluid.  Such  a  habitat  is  supplied  by  any 
organic  substance  capable  of  ready  decomposition,  for  which 
process,  as  is  well  known,  the  great  requirements  for  life,  moisture 
and  warmth  are  to  a  certain  degree  necessary.  Vast  varieties  of 
these  organisms  are  now  known.  They  differ  slightly  in  shape, 
in  their  habitat,  and  in  their  properties.  Some  are  obviously 
composed  of  two  distinct  layers,  some  are  provided  with  a  fine 
hair- like  process,  by  the  lash-like  motions  of  which  they  move 
rapidly  in  a  definite  direction. 

They  are  known  to  be  inseparable  from  putrefactive  changes 
in  organic  materials ;  without  them  no  putrefaction  can  go  on, 
since  this  process  is  but  the  product  of  their  living  activity. 
Great  heat  kills  them,  too  great  cold  or  dryness  checks  their 
activity  and  stops  putrefaction.  When  an  organic  substance  is 
absolutely  protected  from  their  presence  by  exclusion  of  the  air, 
etc.,  no  putrefaction  occurs,  even  though  it  be  prone  to  spon- 
taneous decomposition,  and  be  placed  under  favorable  circum- 
stances as  to  warmth  and  moisture. 

Bacteria  would  not  deserve  so  much  notice  here  were  it  not 
for  the  pathogenic  influence  some  of  them  have  on  the  higher 
forms  of  life.  We  do  not  know  that  they  are  necessary  for  any 
of  the  more  important  processes  that  normally  go  on  in  the 
human  body,  though  they  are  constantly  present  in  the  intestinal 
tract,  and  are  inseparable  from  at  least  one  change  taking  place 
8 


90  MANUAL   OF   PHYSIOLOGY. 

there  that  may  be  regarded  as  physiological.  It  is  their  relation 
to  the  diseased  state  that  makes  a  knowledge  of  these  creatures 
imperative  to  medical  men. 

So  long  as  the  tissue  of  a  higher  animal  is  healthy  and  well 
nourished,  the  commoner  forms  of  septic  bacteria  cannot  thrive  in 
immediate  contact  with  it.  They  can  only  exist  in  the  intestine, 
etc.,  because  there  they  find  accumulations  of  lifeless  fluids 
which  offer  them  a  suitable  nidus.  Active  living  tissues  may  be 
said  to  have  antiseptic  power,  i.  e.  are  able  to  destroy  septic 
bacteria;  and  it  is  only  owing  to  this  bactericide  power  of  our 
textures,  that  we  can  with  immunity  breathe  into  our  lungs  the 
atmospheric  air  often  crowded  with  these  organisms,  and  swallow 
multitudes  of  them  with  our  food.  But  for  it  every  wound 
would  become  putrid,  every  breath  might  admit  deadly  germs  to 
our  blood. 

When  the  vitality  of  the  body  generally  is  lowered,  the  vital 
activity  of  the  tissue  may  fall  below  that  necessary  to  insure  the 
death  of  the  bacteria,  whose  victory  is  signaled  by  unwonted  and 
often  fatal  changes.  Morbid  fluids  allowed  to  accumulate  in  the 
textures  facilitate  the  growth  of  bacteria,  and  give  rise  to  various 
grades  of  "  wound  infection."  But  if  all  accumulations  be 
avoided,  the  bacteria  brought  into  relation  with  the  living  tissue 
only  irritate  it,  and  cause  general  fever  and  local  suffering  to 
the  patient.  They  cannot  propagate  in  live  tissue  as  in  lifeless 
fluids.  As  a  rule,  the  injurious  effect  of  bacteria  is  in  inverse 
proportion  to  the  vital  power  of  the  textures  which  they  invade. 
This  is  seen  in  many  cases  familiar  to  the  physician  and  the 
surgeon.  There  are,  however,  many  forms  of  pathogenic  bacteria 
which,  if  introduced  into  the  system  by  inoculation,  are  able  to 
overcome  the  vital  activity  of  the  tissues  of  certain  animals  even 
in  the  most  robust  health. 

We  next  come  to  forms  of  fungus,  which  set  up  a  process  very 
like  putrefaction,  such  as  the  yeast  plant,  Torula  cerevisia,  which 
causes  alcoholic  fermentation  in  sugar  solutions.  In  the  torula 
an  external  case  containing  protoplasm  may  readily  be  seen,  and 
multiplication  of  the  cells  goes  on  rapidly  by  a  process  of 
budding.  Torulse,  however,  like  bacteria,  though  called  vege- 


AMCEBA. 


91 


tables,  have  not  the  power  of  assimilating  as  ordinary  green 
plants  do,  but  require  nutriment  to  be  supplied  to  them  which 
already  contains  organic  or  complex  compounds.  Structurally 
but  little  different  from  torula  is  a  one-celled  plant,  the  green 
protococcus,  which,  like  a  higher  plant,  can  build  up  its  texture 
from  the  simplest  food  stuffs,  and  carry  on  its  functions.  It 
consists  of  a  case  made  of  cellulose,  within  which  lies  a  mass  of 
protoplasm  with  a  nucleus.  Their  protoplasm  is  colored  green  by 
a  peculiar  substance  called  chlorophyll.  We  shall  see  presently 
that  it  is  to  protoplasm  containing  chlorophyll  that  plants  owe 
all  their  most  characteristic  and  wonderful  properties ;  viz.,  the 
property  of  assimilating  so  as  to  construct  complex  carbon  com- 
pounds out  of  simple  inorganic  materials. 

FIG.  40. 


Two  different  forms  of  Amoebae  in  different  phases  of  movement.    Those  on  the  left  after 
Cadiat.    A  and  B  show  an  outer  clear  zone.    (Gegenbauer.) 

The  smallest  and  simplest  organisms  classed  as  animals  are 
generally  larger  than  the  vegetable  cells  just  alluded  to.  They 
consist  of  protoplasm  without  any  nucleus,  and  only  sometimes 
with  a  structural  difference  between  any  part  of  their  substance. 
As  an  example  we  may  take  Protamceba.  This  is  a  small  mass 
of  protoplasm  without  any  nucleus,  but  its  outer  layer  is  clearer 
and  less  granular  than  the  central  part.  It  can  move  by  sending 
out  protoplasmic  processes,  in  which  currents  can  be  observed 
resembling  those  in  the  vegetable  cells.  Excepting  as  regards 
the  nucleus,  it  is  much  the  same  as  the  Amoeba,  which  can  be 
readily  found  and  watched,  and  will  be  more  accurately  described. 

The  amosba  is  a  single  cell  or  mass  of  uncovered  protoplasm, 


92  MANUAL    OF    PHYSIOLOGY. 

containing  a  well-defined  nucleus^  within  which  is  a  nucleolus. 
There  is  also  generally  a  vacuole.  The  central  part  of  the  pro- 
toplasm is  densely  packed  with  coarse  granules,  but  the  outer, 
more  active  part  is  structureless  and  translucent  looking,  some- 
what like  a  fine  border  of  muffed  glass,  encasing  the  coarsely 
granular  middle  portion.  Such  an  animal  has  no  parts  differen- 
tiated for  special  purposes,  the  requirements  of  its  functions  being 
so  limited  that  the  protoplasm  itself  can  accomplish  them. 

Thus  the  processes  of  protoplasm,  which  flow  out  with  con- 
siderable rapidity  from  the  body,  frequently  encircle  particles  of 
nutrient  material,  and  then  closing  in  around  them,  press  them 
into  the  midst  of  the  granular  central  mass.  Here  they  sojourn 
some  time,  and  during  this  period  no  doubt  any  nutritive  pro- 
perties they  possess  are  extracted  from  them,  and  they  are  then 
ejected  from  the  plastic  substance.  This  form  of  assimilation 
demands  no  previous  preparation  of  the  food  such  as  we  shall  see 
takes  place  in  the  alimentary  tract  of  man,  and  in  the  special 
organs  of  the  higher  animals;  yet  it  is  a  form  of  digestion 
adequate  at  least  to  the  requirements  of  this  simple  organism. 
The  repeated  alteration  of  relationship  between  the  different  parts 
of  the  protoplasm,  and  the  surrounding  medium  during  the  flow- 
ing hither  and  thither  of  the  currents,  produces  not  only  a 
change  in  the  shape  and  position  of  the  animal,  but  also  acts  as 
a  means  of  distributing  the  nutriment  to  the  different  parts  of  the 
body,  and  of  collecting  and  carrying  to  the  surface  the  various 
products  of  tissue  decomposition  ;  thus  the  streaming  protoplasm 
does  the  work  of  a  circulating  fluid  such  as  we  see  in  the  more 
elaborate  organisms  for  the  distribution  of  nutriment  and  elimina- 
tion of  waste  materials.  The  surface  of  the  amoeba  is  sufficient 
to  allow  of  the  gas  interchange  necessary  for  life,  and  by  means 
of  the  ever-changing  material  exposed,  sufficient  oxygen  is  taken 
for  its  tissue  combustions,  and  so  a  function  of  respiration  is 
established.  The  growth  that  results  from  the  perfect  perform- 
ance of  these  vegetative  functions  proceeds  until  the  maximum 
size  is  attained,  arid  further  nutritive  activity  is  then  devoted  to 
reproduction.  When  growth  ceases,  commonly  the  cell  divides 
and  forms  two  distinct  individuals.  The  movements  which  form 


PARAMCECIUM. 


93 


FIG.  41. 


the  most  striking  operations  of  the  amoeba  are  the  same  as  those 
which  take  place  in  protoplasm,  except  that  they  are  more  rapid 
and  obvious.  The  clear,  outer  layer  first  flows  out  as  a  bud-like 
process,  and,  as  it  is  gradually  enlarging,  some  of  the  central 
granular  part  of  the  cell  suddenly  tumbles  into  its  midst,  where 
it  remains,  while  other  pseudopodia  are  being  thrown  out  in  the 
neighborhood,  and  the  same  changes  repeated  in  them.  It  is 
difficult  to  watch  the  motions  of  an  amoeba  without  being 
impressed  with  the  idea  that  it  is  not  only  endowed  with  sensi- 
bility, but  that  it  can  also  discriminate  between  different  objects, 
for  we  see  it  greedily  flowing  around  some  food  material,  while  it 
carefully  avoids  other  substances  with  which  it  comes  in  contact. 

If  a  glass  vessel  containing  several  amoebae  be  placed  in  a 
window,  they  will  be  found  to  cluster  on  the  side  of  the  glass 
most  exposed  to  the  light.  From  this  it  would  appear  that,  in 
some  obscure  way,  protoplasm  can  appreciate  light,  and  respond 
to  its  influence  by  moving  toward  it. 

This  single-celled  animal,  or  nucleated  mass  of 
protoplasm,  can  perform  all  the  functions  of  a 
higher  animal.  It  can  move  from  place  to  place 
and  assimilate  nutriment,  apparently  discriminat- 
ing between  different  materials.  It  distributes 
nutrient  stuffs  and  oxygen  throughout  its  body 
by  a  kind  of  tissue  circulation,  and  it  can  appre- 
ciate and  respond  to  the  most  delicate  form  of 
stimulus,  namely,  light,  which  subtle  motion  has 
no  effect  on  the  sensory  nerve  fibres  of  the  higher 
animals. 

In  some  unicellular  animals  certain  parts  of  the  Diagram  of  Para- 
cell  are  specially  modified  for  the  performance  of  dlgSuve  S  cavity? 
special  functions,  a  division  of  labor  thus  taking  filled  Bwlth  spSoCft 
place  which  insures  the  more  perfect  accomplish- 
ment of  the  different  kinds  of  activity.  In  one 
of  the  commonest  of  the  Infusoria  (Paramceeia 
bursaria),  which  swarm  in  dirty  water,  this  is 
well  exemplified.  The  outer  layer  of  the  flattened  body  is 
denser,  and  forms  a  kind  of  fibrillated  corticular  case  (ectosarc), 


protoplasm,  into 
which  food  is 
taken.  (6)  Mouth, 
(c)  Anus,  (d)  Con- 
tractile vesicle. 
(After  Lachmann.) 


94  MANUAL   OF   PHYSIOLOGY. 

which  is  covered  over  with  hair-like  processes  (vibratile  cilia), 
constantly  moving  in  a  certain  direction,  so  as  to  propel  the 
creature  rapidly  through  the  water.  The  internal  part  of  the  cell 
is  very  soft,  almost  fluid,  and  coarsely  granular  in  appearance, 
containing  many  bodies  which  have  obviously  been  introduced 
from  without.  This  soft  internal  protoplasm  (endosarc)  moves 
slowly  round  in  a  definite  direction,  completing  its  circuit  in  one 
or  two  minutes,  and  thus  carries  on  a  circulation  which  mixes  the 
various  matters  contained  in  it.  At  one  point  of  the  ectosarc,  or 
cortical  layer,  an  orifice  or  mouth  leading  to  an  oesophageal 
depression  is  found.  This  orifice  is  lined  by  moving  cilia,  which 
by  their  vibrations  drive  the  food  into  the  oesophagus,  whence  it 
is  periodically  jerked  into  the  soft  internal  protoplasm  or  endo- 
sarc, together  with  some  water,  and  thus  forms  a  food  vacuole, 
which  is  carried  round  in  the  circulation  of  the  ectosarc.  Besides 
a  well-marked  nucleus  and  nucleolus  in  the  central  part  of  the 
cell,  these  paramoecia  have  one  or  more  clear  spaces  placed  near 
the  surface  at  the  extremities  of  the  animal.  These  vacuoles 
suddenly  contract,  and  disappear  every  now  and  then.  When 
this  contraction  occurs,  fine  canals  radiating  from  the  contractile 
vacuole  are  distended  with  the  clear  fluid  which  has  probably 
entered  the  vacuole  from  without.  Thus  a  permanent  set  of 
water  vessels  carry  fluid  from  the  contractile  vacuole  throughout 
the  endosarc. 

In  such  an  animal  there  is  a  distinct  advance  of  function  com- 
pared with  the  amoeba  ;  a  more  elaborate  and  specialized  method 
of  feeding;  a  more  systematic  and  regular  circulation  of  nutri- 
ent matters ;  a  respiratory  distribution  of  water  by  the  contrac- 
tile vesicle  and  its  water  canals ;  more  rapid  motion  ;  and  more 
obvious  sensation. 

In  the  bell  animalcule,  or  Vorticella,  the  same  kind  of  division 
of  labor  exists,  but  in  one  of  its  commonest  conditions  it  is 
attached  by  a  thin  stalk  to  the  stalk  of  some  weed  or  other 
object.  Besides  the  ciliary  movement  we  here  find  that  the  gen- 
eral mass  of  the  protoplasm  can  suddenly  and  forcibly  contract, 
so  as  to  completely  alter  its  shape,  and  change  the  bell  into  a 
rounded  mass.  This  spasm  of  the  [body  is  commonly  associated 


PARAMCECIUM.  95 

with  a  wonderfully  rapid  contraction  of  the  stalk.  This  stalk 
consists  of  a  delicate  transparent  sheath,  in  the  centre  of  which 
is  a  thin  thread  of  pale  protoplasm.  The  rapid  contraction  of 
the  protoplasm  of  the  stalk  and  the  spasm  of  the  bell  occur  on 
the  application  of  the  least  mechanical  excitation,  such  as  a 
touch  to  the  cover  glass.  Here  in  a  single  cell  we  have  certain 
portions  set  apart  for  special  purposes,  most  of  which  are  the 
same  as  in  paramcecia.  But  the  animal  being  attached  requires 
a  special  way  of  escaping  from  its  enemies,  and  hence  we  find  it 
endowed  with  three  special  forms  of  motion.  Besides  the  ciliary 
and  streaming  protoplasmic  motion,  its  body  can  spasmodically 
change  its  shape,  and  the  stalk  contracts  with  a  velocity  compar- 
able with  that  of  the  most  specially  modified  contractile  tissue 
(muscle)  of  the  higher  animals,  by  means  of  which  their  rapid 
and  varied  movements  are  carried  out. 


96  MANUAL   OF   PHYSIOLOGY. 


CHAPTER  V. 

FOOD. 

The  continuation  of  life  depends  on  certain  chemical  changes 
which  are  accompanied  by  a  loss  of  substance  on  the  part  of  the 
active  tissues.  This  loss  must  be  made  good  by  the  assimilation 
of  material  from  without,  and  the  manner  in  which  assimilation 
takes  place  constitutes  one  great  point  of  difference  between 
Plants  and  Animals.  In  the  majority  of  the  former  (certain 
fungi  form  the  main  exceptions)  the  cells  in  those  portions  of  the 
plant  which  are  exposed  to  the  light  and  air  contain  a  peculiar 
green  substance  called  chlorophyll,  and  through  the  agency  of 
this  substance  they  are  able  to  obtain  from  the  inorganic  king- 
dom nearly  all  the  food  they  require.  Water,  with  such  salts  as 
may  happen  to  be  in  solution,  is  taken  up  by  the  roots,  and  car- 
ried through  the  stem  to  the  leaves  ;  here  the  active  chlorophyll- 
bearing  cells,  under  the  influence  of  the  sun's  rays,  cause  its 
elements  to  unite  with  the  carbon  dioxide  present  in  the  air,  and 
form  various  substances,  of  which  we  may  take  starch  or  cellu- 
lose as  an  example.  The  reaction  may  be  represented  chemic- 
ally, thus : — 

6C02  +  5H20  =  C6H1005  +  012. 

Starch  or  Cellulose. 

A  large  proportion  of  oxygen  is  thus  set  free  and  discharged 
into  the  atmosphere. 

The  most  striking  property  of  plant  protoplasm  is  the  powefc- 
of  using  the  energy  of  the  sun's  rays  to  separate  the  elements  of 
the  very  stable  compounds,  carbon  dioxide  and  water,  and  from 
the  elements  thus  obtained  to  make  a  series  of  more  complex  and 
unstable  compounds,  which  readily  unite  with  more  oxygen,  and 
change  back  to  carbonic  anhydride  and  water. 

The  carbon  compounds  made  in  and  by  the  protoplasm  of  the 
green  plants  are  some  of  the  so-called  "  organic  compounds," 


FOOD   STUFFS.  97 

which  enter  into  the  composition  of  both  plants  and  animals,  and 
form  an  essential  part  of  the  food  of  the  latter.  They  may  be 
divided  into  three  groups — 

.1.   Carbohydrates — bodies  so  called  from  the  presence  of  hydro- 
gen and  oxygen  in  proportion  to  form  water  ;  e.  g. : — 
Starch,  C6H10O5  =  C6(H2O)5. 
Grape  sugar  (dextrose),  C6rI12O6  =  C6(H2O)6. 
Cane  sugar  (sucrose),  C^H^On  =  (Ci2H2O)u. 

2.  Fats — compounds  of  carbon  and  hydrogen  with  a  less  pro- 

portion of  oxygen  than  the  starches,  e.  g. : — 

Olein  (principal  constituent  of  olive  oil),  C57Hi0tO6. 

3.  Albuminous  bodies  which  contain  nitrogen  in  addition  to 

carbon,  hydrogen  and  oxygen.  These  are  of  complex 
composition,  and,  as  a  rule,  cannot  be  represented  by 
chemical  formulae. 

Animals  cannot  thrive  on  the  simple  forms  of  food  obtainable 
from  the  inorganic  kingdom,  which  suffice  for  the  nutrition  of  a 
plant.  They  require  for  assimilation  materials  nearly  allied  in 
chemical  composition  to  their  own  tissues;  substances  to  be  used 
as  fuel  in  producing  the  activities  of  their  bodies.  In  short, 
they  require  as  food  the  very  organic  substances  which  plants 
spend  their  lives  in  making :  viz.,  starches,  fats  and  albuminous 
bodies.  These  substances  must  be  supplied  to  animals  ready 
made,  so  that  directly  or  indirectly,  through  the  medium  of  other 
animals,  all  these  complex  substances  are  the  result  of  work 
done  by  vegetable  life. 

The  chief  acts  of  animal  protoplasm  are  oxidations,  a  slow 
burning  away  of  its  substance,  which  results  in  the  production 
of  inorganic  materials  like  those  used  by  plants  as  food. 

Plants  use  simple  food  stuffs,  and  from  them  manufacture  com- 
plex combustible  materials,  and  thus  store  up  the  energy  of  the 
sun's  rays  in  their  textures. 

Animals,  on  the  other  hand,  use  complex  food  stuffs  to  renew 
their  tissues,  which  are  constantly  being  oxidized,  and  by  this 
means  the  energy  for  the  performance  of  their  active  functions 
is  set  free. 

Although  the  various  kinds  of  food  stuffs  used  by  animals  are 
9 


98  MANUAL  OF   PHYSIOLOGY. 

highly  organized  and  like  the  animal  tissues  in  composition,  yet 
they  cannot  be  admitted  at  once  into  the  economy  without  hav- 
ing undergone  a  special  preparation,  which  takes  place  in  the 
digestive  tract,  where  the  various  food  stuffs  are  so  changed  as  to 
allow  them  to  pass  into  the  fluids  of  the  body. 

We  shall  first  consider  the  food  stuffs,  next  their  preparation 
for  absorption  (digestion),  and  then  the  means  by  which  they  are 
distributed  to  the  tissues  (circulation).  The  final  step  in  tracing 
the  assimilation  of  the  food  is  to  follow  the  intimate  processes 
which  go  on  between  the  blood,  which  carries  the  nutriment,  and 
.the  different  tissues. 

CLASSIFICATION  OF  FOOD  STUFFS. 

There  are  two  portals,  namely,  the  lungs  and  the  alimentary 
canal,  by  which  new  materials  normally  enter  the  animal  body. 

Within  the  lungs  the  blood  comes  into  close  relation  with  the 
air,  and  takes  from  it  oxygen.  The  oxygen  is  then  carried  to 
the  various  tissues,  where  it  aids  in  the  combustion  accompany- 
ing the  life  and  functions  of  these  tissues.  Oxygen  is  the  most 
abundant  element  in  the  body,  taking  part  in  almost  every 
chemical  change,  and  its  continuous  supply  is  more  immediately 
necessary  for  life  than  that  of  any  other  substance,  yet  it  is  not 
counted  as  food,  because  tissue  oxidation  is  distinguished  from 
tissue  nutrition. 

The  details  of  the  union  of  oxygen  with  the  blood  will  be 
found  in  the  Chapter  (xix)  on  Respiration. 

It  is  then  only  to  the  liquid  and  solid  portions  of  the  material 
income  of  an  animal — that,  in  short,  which  it  must  busy  itself  to 
obtain — that  the  term  "  food  "  is  applied.  These  are  introduced 
into  the  alimentary  canal,  where  the  nutrient  materials  are  sepa- 
rated and  prepared  for  absorption,  while  the  portions  which  are 
not  useful  for  nutrition  are  carried  away  as  excrement.  We  are, 
therefore,  quite  prepared  to  hear  that  the  really  nutritious  food 
stuffs  are  composed  of  materials  which  are  chemically  like  the 
tissues,  although,  as  we  shall  see,  we  have  no  grounds  for  believ- 
ing that  the  different  chemical  groups  of  nutritive  stuffs  are 
exclusively  destined  to  replace  corresponding  substances  in  the 


CLASSIFICATION   OF    FOOD   STUFFS.  99 

body.  On  the  contrary,  we  have  good  reason  to  think  that 
within  the  body  the  conversion  of  one  group  into  another  is 
common. 

In  Chapter  in  the  tissues  of  the  animal  body  were  shown  to 
consist  of  chemical  compounds,  which  have  been  classified  into 
certain  groups.  It  has  also  been  stated  that  the  tissues  are  con- 
stantly undergoing  chemical  changes  inseparable  from  their  life, 
and  that  for  these  changes  a  supply  of  nutritive  material  is  neces- 
sary. 

The  nutriment  required  for  an  animal  is  made  up  of  substances 
which  may  be  divided  into  the  same  chemical  groups  as  the 
tissues  of  the  body,  viz.,  proteids,  fats,  carbohydrates,  salts  and 
water.  So  that  each  of  the  various  substances  which  we  make 
use  of  as  food,  contains  in  varying  proportions  several  of  the  dif- 
ferent kinds  of  nutrient  material,  either  naturally  or  artificially 
mixed  so  as  to  form  a  complex  mass,  the  important  item  water 
being  the  only  one  which  is  commonly  used  by  itself.  These 
substances  may  be  considered  to  be  the  chemical  bases  of  the 
food,  as  they  are  also  the  chemical  bases  of  the  animal  body. 

The  following  classification  shows  the  relationships  between  the 
chief  constituents  of  food,  from  a  chemical  point  of  view,  and 
their  distribution  in  the  various  substances  we  eat. 

I.  ORGANIC. 

1.  Nitrogenous — 

(A)  Albuminous — abundant  in  eggs,  milk,  meat, 
peas,  wheaten  flour,  etc. 

(B)  Albuminoid — in  soups,  jellies,  etc. 

2.  Non-nitrogenous — • 

(A)  Carbohydrates  (sugar,  starch) — abundant  in 
all  kinds  of  vegetable  food,  and  in  milk,  and 
present  in  small  quantity  in  meat,  fish,  etc. 

(B)  Fats — in  milk,  butter,  cheese,  fat  tissues  of 
meat,  some  vegetables,  oils,  etc. 

II.  INORGANIC. 

1.  Salts — mixed  with  all  kinds  of  food. 

2.  Water — mixed  with  the  foregoing  or  alone. 


100  MANUAL   OF   PHYSIOLOGY. 

The  nutritive  value  of  any  kind  of  food  depends  upon  a  variety 
of  circumstances,  which  may  be  thus  summed  up  : — • 

I.  Chemical  composition,  of  which  the  main  points  are — 

1.  The  proportion  of  soluble  and  digestible  matters  (true 

food  stuffs)  to  those  which  are  insoluble  and  indi- 
gestible, such  as  cellulose,  keratin,  elastic  tissue, 
etc. 

2.  The  number  of  different  kinds  of  nutrient  stuffs  pres- 

ent in  it. 

II.  Mechanical   Construction. — The  degree  of    subdivision  in 
which  the  substance  is  introduced  into  the  stomach  materially 
influences  its  nutritive  value,  since  the  smaller  the  particles  the 
greater  the  amount  of  surface  exposed  to  the  action  of  the  diges- 
tive juices. 

The  relation  of  the  nutrient  to  the  non-uutrierit  parts  is  also  of 
importance,  as  is  seen  where  the  nutritious  starch  of  various  vege- 
tables is  enclosed  in  insoluble  cases  of  cellulose,  which,  if  not 
burst  by  boiling,  prevent  the  digestive  fluids  from  reaching  the 
starch. 

III.  Digestibility. — This  depends  partly  upon  how  the   sub- 
stances affect  the  motions  of  the  intestines,  and  partly  upon  their 
construction.     Thus,  some  substances,  such   as  cheese,   though 
chemically  showing  evidence  of  great  nutritive  properties,  by 
their  impermeability  resist  the  digestive  juices,  and  are  not  very 
valuable  as  food. 

IV.  Idiosyncrasy. — In  different  animals  and  in  different  indi- 
viduals, and  even  in  the  same  individuals  under  different  circum- 
stances, food  may  have  a  different  nutritive  value. 

FOOD  REQUIREMENTS. 

Chemically,  foods  are  composed  of  a  limited  number  of  ele- 
ments similar  to  those  found  in  the  animal  tissues,  viz.,  carbon, 
oxygen,  nitrogen  and  hydrogen,  together  with  some  salts.  If 
nothing  more  were  needed  by  the  economy  than  a  supply  of  these 
elements  and  salts  in  a  proportion  like  'that  in  which  they  exist 
in  the  tissues,  such  could  be  easily  obtained  from  inorganic 
sources ;  but,  as  has  already  been  stated,  it  is  necessary  that  an 


FOOD   REQUIREMENTS. 


101 


animal  obtain  these  elements  associated  in  the  form  of  organic 
materials  of  complex  construction  (namely,  prot^ejds,  etc.). 
Allowing  the  necessity  of  organic  food,  it  might  be  .supposed  that 
since  the  elements  exist  in  proper  proportion  in  the,  proteids, 
an  abundant  supply  of  proteids  would  suffice  for  a\l  'nul/riiive 
purposes,  and  alone  form  an  adequate  diet.  Theoretically,  pro- 
teid  alone  ought  to  be  sufficient  for  nutrition.  It,  however,  has 
been  frequently  tested  by  experiment,  and  practically  decided, 


Proteids. 


Water. 


Explanation     .    . 


Human  milk 


Bread 


Diagram  showing  the  proportion  of  the  principal  food  stuffs  in  a  few  typical  comestibles. 
The  numbers  indicate  percentages.    Salts  and  indigestible  materials  omitted. 

that  an  animal  will  not  thrive  upon  a  free  supply  of  pure  proteid 
food  alone  ;  and  in  the  human  subject  such  exclusive  diet  would 
induce  dangerous  abnormal  conditions  in  a  short  time.  Since 
nitrogen  is  an  important  element  in  nearly  all  parts  of  the  body, 
we  could  hardly  expect  that  a  diet  composed  of  non-nitrogenous 
food  stuffs  alone  could  support  the  animal  economy.  In  short, 
the  results  of  numerous  experiments  show  that  no  one  group  of 
the  food  stuffs  enumerated  can  alone  sustain  the  body,  but  rather 


102  MANUAL   OF   PHYSIOLOGY. 

prove  that  a  certain  proportion  of  each  is  absolutely  necessary 
for  life. 

SPECIAL  FORMS  OF  FOOD. 

The  articles Jof  diet, we  make  use  of  are  animal  or  vegetable, 
according  to  the  source  from  which  they  are  derived.  It  will  be 
seen  that  a  varying  quantity  of  all  chemical  classes  of  food  stuffs 
is  present  in  most  kinds  of  food,  whether  animal  or  vegetable. 
The  diagram  on  the  preceding  page  shows  the  proportion  of  the 
more  important  food  stuffs  in  some  examples  of  the  materials 
commonly  used  as  food. 

Among  animal  foods  are  included  milk,  the  flesh  of  various 
animals,  and  the  eggs  of  birds.  These  may  be  more  fully 
described  as  typical  examples. 

Milk. — For  a  certain  period  of  their  lifetime  the  secretion  of 
the  mammary  gland  forms  the  only  food  of  all  mammals,  aiid  it 
is  the  one  natural  product  which  when  taken  alone  affords  ade- 
quate nutriment. 

It  consists  of  a  slightly  alkaline  watery  fluid,  containing  — 

1.  Proteids,  casein  and  albumin  in  solution. 

2.  Fats,  finely  divided  to  form  perfect  emulsion. 

3.  Carbohydrate,  sugar  in  solution. 

4.  Salts,  in  solution. 

5.  Water. 

Owing  to  the  action  of  certain  organisms  which  readily  propa- 
gate in  milk,  if  exposed  to  the  air  at  a  warm  temperature  for 
some  time,  it  loses  its  alkaline  reaction,  and  becomes  sour  from 
the  formation  of  lactic  acid  from  the  milk  sugar,  by  a  kind  of 
fermentation,  the  probable  equation  for  which  may  be  written 
thus : — 

C6H1206  =  2CsH.Os. 

Milk  Sugar.      Lactic  Acid: 

If  fresh  good  milk  be  allowed  to  stand,  the  fatty  particles  tend 
to  float  to  the  surface,  thus  forming  a  layer  of  cream. 

The  milk  of  different  animals  is  similar  in  all  essential  points, 
but  differs  slightly  in  the  relative  proportion  of  the  ingredients, 
as  may  be  seen  in  the  following  table : — 


MILK. 


103 


Human. 

Cow. 

Goat. 

Ass. 

Water  

889.08 

857.05 

863.58 

910.24 

Casein  .  .  .  \ 
Albumin  / 

39.24 

48.28 
5.76 

33.60 
12.99 

1  20.18 

Butter  

26.66 

43.05 

43.57 

12.56 

Milk  sugar 
Salts  

43.64 
1.38 

40.37 
5.48 

40.04 
6.22 

}  57.02 

Solids  

110.92 

142.95 

136.42 

89.76 

1000. 

1000. 

1000. 

1000. 

Milk  varies  both  in  the  amount  of  solids  in  solution  and  fat, 
according  to  the  age  and  general  condition  of  the  animal,  period 
of  lactation,  time  of  day,  etc. 

Since  human  milk  is  much  poorer  in  proteid,  fat  and  salts  (see 
Table),  and  richer  in  sugar,  than  that  of  the  cow  and  other 
domestic  animals,  it  is  necessary  to  dilute  the  latter  with  water, 
and  add  sugar,  when  it  is  substituted  for  human  milk  in  feeding 
infants. 

The  great  value  of  milk  as  nutriment  depends  upon  the  fact 
that  it  contains  every  class  of  food  stuff,  viz.,  proteids,  fat,  carbo- 
hydrates, salts  and  water,  in  the  proportion  demanded  by  the 
economy;  the  salts  in  milk  being  those  required  for  building  up 
the  bones  of  the  infant,  viz.,  phosphates  and  carbonates  of  lime, 
etc. 

The  normal  variations  in  these  proportions  are  not  very  great, 
but  as  adulteration  with  water  is  common,  a  knowledge  of  the 
method  of  testing  the  purity  of  milk  is  necessary. 

Milk  Tests. — The  specific  gravity  of  milk  gives  an  easy  measure 
of  the  solids  in  solution,  but  unfortunately  it  gives  no  accurate 
estimate  of  the  amount  of  fat  suspended  in  the  emulsion.  There- 
fore, to  test  milk  adequately  two  methods  must  be  employed : 
one  to  estimate  the  degree  of  density  of  solution,  and  the  other 
the  degree  of  opacity  of  the  emulsion. 

I.  To  test  the  density,  a  specially  graduated  form  of  hydro- 
meter is  generally  used.  This  is  graduated  so  as  to  indicate 
specific  gravities  from  1014  to  1042.  The  latter  being  the 


104 


MANUAL   OF   PHYSIOLOGY. 


maximum  density  of  pure  milk  (the  average  being  about  1030), 

and  the  former  being  about  the  density  of  pure  milk  when 

mixed  with  an  equal  bulk 
of  water.  Every  reduction 
of  3  in  the  specific  gravity 
may  be  said  to  correspond 
to  about  10  per  cent,  of 
water. 

g&;SQ-r»lQ?*"^3W£+          H.  The  degree  of  opa- 

°°?o   _^ 

^*sa  .o^.     A\^v 


city  is  estimated  by  the 
amount  of  water  required 
to  render  a  small  quantity 
of  milk  sufficiently  trans- 
lucent to  allow  a  candle 
flame  to  be  seen  through 
a  layer  of  the  mixture  one 
centimetre  thick.  One 


Microscopic  appearance  of  milk  in  the  early  stage 

of  lactation,  showing  colostrum  cells  (a).  ».  ..  />     Al 

cubic  centimetre  of  the 

milk  (which  has  been  shown  by  the  microscope  and  the  iodine 
test  not  to  contain  chalk  or  starch)  is  placed  in  a  test  glass  with 
flat  parallel  sides,  just  one  centimetre  apart,  and  water  is 
cautiously  added  from  a  graduated  pipette.  The  more  water 
required  the  richer  the  milk  is  in  fat;  good  fresh  milk  requires 
about  70  times  its  bulk  of  water  to  become  translucent. 

Another  method  employed  for  the  same  purpose  consists  in  the 
comparison  of  the  color  produced  by  a  layer  of  milk  1  mm.  thick 
in  a  black  cell  with  a  previously  prepared  standard  of  grayish 
colors. 

The  quantity  of  fat  may  also  be  estimated  by  placing  the  milk 
in  a  tall  graduated  vessel  for  twenty-four  hours,  at  the  end  of 
which  time  it  should  show  at  least  10  per  cent,  of  cream. 

Butter  is  made  from  milk,  or  better  from  cream,  by  breaking, 
by  agitation,  the  coating  of  proteid  which  before  churning  pre- 
vents the  oil  globules  from  running  together.  It  is  almost  com- 
pletely composed  of  fat,  the  larger  globules  having  run  together 
to  form  the  solid  butter,  which  can  be  removed,  leaving  some 


MEAT,  ETC.  105 

small  fat  globules  with  the  proteids,  milk  sugar,  lactic  acid,  and 
salts  in  the  water  forming"  buttermilk."* 

Cheese  is  another  form  of  food  made  from  milk  by  precipitating 
the  proteid  either  by  lactic  fermentation  or  the  addition  of  rennet 
— an  extract  of  calves'  stomach  which,  without  the  presence  of 
any  acid,  curdles  milk — and  draining  off  the  solution  of  milk 
sugar,  and  salts  (whey).  It  contains  most  of  the  proteid  and  a 
great  deal  of  the  fat  of  the  milk.  During  the  ripening  of  the 
cheese  more  fat  is  formed,  apparently  from  the  proteid  while 
leucin  and  ty rosin  also  appear. 

Meat. — We  use  the  flesh  of  the  vegetable-feeding  mammals 
and  birds  that  are  most  easily  obtainable,  and  many  kinds  of 
fish.  The  invertebrate  animals,  mostly  shellfish,  need  hardly  be 
mentioned  in  a  physiological  dietary,  and  are  not  spoken  of  as 
meat. 

As  it  comes  from  the  butcher,  meat  consists  of  many  of  the 
animal  tissues,  the  chief  ones  being  flesh  (muscle  tissue),  fat  and 
some  sinews  (fibrous  tissue).  The  fleshy  or  lean  part  of  meat 
is  chiefly  made  up  of  nitrogenous  materials,  and  contains :  (1) 
Several  proteids,  chiefly  the  globulin,  myosin;  (2)  gelatine  yield- 
ing substances ;  (3)  carbohydrates,  as  inosit  and  grape  sugar  ; 
(4)  small  quantities  of  fat ;  (5)  several  inorganic  salts ;  (6) 
extractives. 

Meat  may  be  eaten  raw,  but,  as  it  is  impossible  to  impart  to  it 
the  various  flavors  which  our  artificial  tastes  demand  without 
some  special  preparation,  it  is  generally  cooked  before  use. 
Moreover,  the  not  infrequent  occurrence  in  muscle  of  parasites 
which  would  prove  injurious  if  swallowed  alive,  makes  the 
exposure  of  meat  to  a  temperature  high  enough  to  ensure  their 
destruction  advisable. 

Apart  from  pleasing  the  taste,  it  is  of  great  importance  so  to 
prepare  meat  as  to  preserve  in  it  all  the  nutrient  parts,  many  of 
which  are  soluble  in  water,  and  therefore  are  apt  to  be  removed 
if  that  solvent  be  injudiciously  used.  Thus,  the  process  of  roast- 
ing, in  which  all  its  nutrient  parts  are  retained,  ought  to  be  more 

*  For  the  details  of  secretion  of  milk,  etc.,  see  Mammary  Gland. 


106  MANUAL   OF   PHYSIOLOGY. 

satisfactory  than  boiling,  by  which  the  salts,  extractives,  carbo- 
hydrates, gelatine,  and  some  albumin,  may  be  dissolved  by  the 
water.  However,  if  the  meat  be  plunged  into  water  which  is 
already  boiling,  the  proteids  near  the  surface  are  rapidly  coagu- 
lated, and  the  water  cannot  reach  the  central  parts  in  sufficient 
quantity  to  remove  even  the  soluble  ingredients.  The  whole  of 
the  albuminous  parts  may  be  thus  coagulated  as  the  temperature 
of  the  inner  parts  rises  to  boiling  point.  In  treating  meat  to 
obtain  "stock"  (bouillon)  for  the  foundation  of  soups,  the 
opposite  procedure  is  adopted.  Cold  water  is  used,  and  the  tem- 
perature slowly  and  gradually  raised,  but  not  quite  to  boiling 
point,  in  order  that  as  much  as  possible  of  the  soluble  materials 
may  be  extracted,  and  a  tasteless  friable  muscle  tissue  remains 
("bouilli").  As  the  fluid  is  generally  allowed  to  boil  in  order 
to  clear  it,  much  of  the  proteid  material  which  was  dissolved  in 
the  earlier  stage  is  coagulated  and  removed  with  the  scum. 
Although  "  stock  "  cannot  contain  any  great  proportion  of  the 
most  important  constituents  of  meat,  it  is  of  much  value  as  a 
nutriment  in  medical  practice,  possibly  on  account  of  some 
stimulating  action  of  its  ingredients  upon  the  motions  of  the 
intestines  and  heart.  A  strongly  albuminous  extract  of  meat, 
"  beef-tea,"  may  be  made  by  digesting  flesh  in  a  small  quantity 
of  water,  keeping  the  temperature  below  that  at  which  albumin 
coagulates,  and  adding  vinegar  and  salt  to  facilitate  the  forma- 
tion of  syntonin  and  the  solution  of  myosin.  The  salt  can  be 
then  removed  by  dialysis. 

Eggs. — Eggs  consist  of  two  edible  parts ;  one,  the  white,  com- 
posed of  albumin,  and  the  other,  the  yelk,  chiefly  made  up  of  fat. 

The  white  is  a  concentrated,  watery  solution  of  albumin,  held 
together  by  delicate  membranous  mesh  works.  Besides  the  albu- 
min it  contains  traces  of  fat,  sugar,  extractives  and  salts. 

The  yellow  fat  emulsion  of  the  yelk  contains  a  peculiar  proteid, 
vitellin,  some  grape  sugar,  and  some  inorganic  salts,  in  which 
combinations  of  phosphoric  acid  and  potassium  are  conspicuous. 
Raw  eggs  are  easy  of  digestion,  as  is  all  albumin  in  solution. 
Hard-boiled  eggs,  if  not  finely  divided  by  mastication,  are  very 
difficult  to  digest,  for  the  gastric  juice  cannot  penetrate  the  hard 


VEGETABLES.  107 

masses  of  coagulated  albumin  which  are  so  easily  and  commonly 
swallowed.  Eggs,  when  lightly  cooked,  are  easily  digested,  as 
the  albumin  is  only  partially  coagulated,  and  cannot  be  intro- 
duced in  large  masses  into  the  stomach.  Eggs  are  of  very  great 
nutritive  value,  as  they  contain  so  large  a  percentage  of  proteid, 
fat  and  salts. 

Vegetable  Food. — Vegetables  differ  from  animal  food — 

(1)  In   containing   a   much  greater   proportion  of  material 
which  for  man  is  indigestible  (cellulose),  and  a  less  proportion 
of  nutritive  material. 

(2)  Thfc  percentage  of  proteid  is  below  that  of  animal  food, 
and  the  proportion  of  carbohydrates  is  generally  much  greater, 
while  the  amount  of  fat  is  small,  but  varies  considerably.     In 
order,  therefore,  to  get  the  required  amount  of  nutritive  material 
from  a  purely  vegetable  diet,  it  is  necessary  to  consume  a  much 
greater  quantity,  and  the  amount  of  excrement  indicating  the 
indigestible  matters  is  proportionately  increased. 

Cereals. — The  most  valuable  forms  of  vegetable  foods  are  those 
obtained  from  the  seeds  of  certain  kindred  plants  (  Graminacece) — 
wheat,  rye,  maize,  oats,  rice,  etc. ;  which  when  ground  are  used 
either  as  "whole  meal,"  or,  the  integument  ("bran")  being 
removed,  as  flour.  They  contain  different  kinds  of  proteid. 
(1)  A  native  albumin  soluble  in  water  and  coagulable  by  heat, 
and  in  many  respects  like  animal  albumin ;  but,  as  it  cannot  be 
obtained  pure,  it  is  imperfectly  known.  (2)  Vegetable  fibrin, 
an  elastic  body,  which  coagulates  spontaneously  and  is  difficult 
to  separate.  (3)  Vegetable  glue  or  gliadin,  which  gives  the 
peculiar  adhesiveness  to  the  gluten,  as  the  proteid  mixture  obtain- 
able from  corn  is  commonly  called.  Cereals  also  contain  traces 
of  fat,  and  a  very  large  proportion  of  starch  and  some  salts. 

Green  Vegetables. — These  contain  some  starch,  sugar,  dextrin, 
salts,  and  minute  quantities  of  proteid,  and  are  of  small  nutritive 
value. 

Potatoes  contain  very  little  proteid,  but  a  considerable  quantity 
of  starch,  upon  which  their  nutritive  value  almost  entirely 
depends. 

The  following   tables  give   the  relative  proportions  of  the 


108 


MANUAL   OF   PHYSIOLOGY. 


various   nutritive  materials  contained  in  some  of  the  common 
cereals  and  vegetable  foods : — 


Wheat. 

Barley. 

Oats. 

Maize. 

Rice. 

Water  

13. 

14.48 

10.88 

12. 

9  20 

Proteid 

13  53 

12  96 

9  04 

7  91 

5  06 

Fats 

1  58 

2  63 

4 

4  83 

75 

Carbohydrates  .  . 

Salts 

69.61 

2 

67.96 
2  65 

73.49 
2  59 

73.19 
1  28 

84.47 
5 

Peas. 

Beans. 

Potatoes. 

Cauliflower. 

Water  

14  50 

12.85 

72.74 

79.18 

Proteid 

22  35 

22 

1  32 

50 

Carbohydrates 

56  61 

56  65 

23  77 

18 

Ex  tract!  ve 

1  18 

3  32 

97 

Fats 

]  96 

1  59 

15 

Salts 

2  37 

2  53 

1  05 

7 

The  most  striking  points  are  the  very  large  proportion  of  pro- 
teid  in  the  leguminous  fruits,  and  the  comparative  richness  of  all 
vegetables  in  starchy  food  stuffs. 

Water  is  the  great  medium  by  which  food  is  dissolved  and 
made  capable  of  ingestion.  Spring  water  always  has  a  certain 
quantity  of  lime  and  other  salts  in  solution,  and  in  proportion  to 
the  amount  of  salts  is  said  to  be  "  soft "  or  "  hard."  Water  is 
tasteless,  inodorous  and  colorless  when  pure.  Soft  water,  such 
as  rain  water,  is  pure,  but  not  so  agreeable  to  taste  as  spring 
water,  and  is  very  liable  to  contamination  in  passing  through 
gutters,  etc.,  previous  to  collection.  Standing  water  should  be 
avoided  for  drinking,  owing  to  the  probability  of  its  containing 
organic  matter. 

Since  water  is  known  to  be  a  common  means  of  communi- 
cating disease,  care  must  be  taken  as  to  the  source  of  drinking 
water,  and  we  should  be  able  to  form  an  opinion  as  to  its  purity 
when  its  source  is  not  known.  It  is,  perhaps,  impossible  to 
detect  in  it  the  specific  impurity  which  causes  any  disease,  but 


VEGETABLES. 


109 


FIG.  44. 


there  are  some  characters,  supposed  to  be  commonly  associated 

with  its  pathogenic  properties,  which  can  be  easily  recognized, 

and  should  be  familiar  to  a  student 

of  Physiology.     The  want  of  bril- 

liant limpidity  must  be  regarded 

with  suspicion.   Any  kind  of  smell, 

disagreeable  or  not,  indicates  im- 

purity.     The    reduction    (loss  of 

color)  of  permanganate  of  potash, 

when  added  in  small  quantity  to 

acidified  water,  indicates  the  pro- 

bable presence  of  organic  matter. 

A  high  percentage  of  chlorides  is 

often  associated  with  sewage  con- 

tamination. 

Salts.  —  Great     Varieties     of  Salts 

are  taken  into  the  system,  of  which 


gection  of 


8howing  starch  and 

al^urone  granules    imbedded    in  the 
protoplasm  of  the  cells.     (After  Sachs.) 

a.  Aieurone  granules. 

11      •  i  n  j.  n  ,-,  st.  Starch  granules. 

Chloride     OI      SOdlUm      lOrmS      the  i.  Intercellular  spaces. 

largest   proportion.     These   have, 

no  doubt,  very  important  functions  to  perform,  in  entering  into 
combination  with  the  various  tissues,  and  also,  probably,  in 
aiding  the  chemical  changes  of  parts  of  which  they  do  not  form 
a  normal  constituent.  They  help  to  render  certain  substances 
soluble,  and  stimulate  the  cells  of  certain  glands  to  more  active 
secretion,  e.  g.,  the  kidney  excretes  more  urea  when  there  is  an 
abundant  supply  of  common  salt  in  the  food. 


110  MANUAL   OF   PHYSIOLOGY. 


CHAPTER  VI. 

THE  MECHANISM  OF  DIGESTION. 

The  acts  of  digestion  may  be  divided  into  mechanical  and 
chemical  processes.  Under  the  mechanical  processes  come  the 
arrangements  for  the  subdivision,  onward  movement  and  general 
mixture  of  the  food.  The  chief  objects  of  the  chemical  changes 
may  be  said  to  be  the  change  from  the  insoluble  to  the  soluble 
form  of  certain  kinds  of  food  stuffs  (starch  and  proteids)  and 
the  finer  subdivision  of  others,  such  as  fats,  which  do  not  dissolve 
in  the  intestinal  secretions  or  in  the  juices  of  the  body. 

Attention  has  already  been  called  to  the  fact  that  there  are 
different  kinds  of  contracting  textures,  and  that  they  are  capable 
of  different  kinds  of  motion,  some  slow  and  steady,  some  rhyth- 
mical, some  sharp,  short  and  sudden.  It  must  also  be  remem- 
bered that  the  more  energetic  and  sudden  the  motions  are.  the 
more  marked  becomes  the  differentiation  of  the  tissue.  Thus  the 
active,  quick-contracting  skeletal  muscles  and  the  rhythmically 
acting  heart  are  made  up  of  tissue  which  is  very  distinct  in 
structure  and  in  mode  of  action  from  that  of  the  contracting  cells 
composed  of  ordinary  protoplasm,  while  in  the  slowly  moving 
internal  organs  we  meet  tissue  elements  which  in  different  ani- 
mals show  many  stages  of  gradation  between  simple,  undiffer- 
entiated  protoplasm  and  the  special  striated  muscle  tissue. 

It  is  necessary  that  in  the  first  stages  of  alimentation  the 
motions  should  be  quick  and  energetic  ;  so  the  mouth,  pharynx 
and  upper  part  of  the  oesophagus  are  supplied  with  striated  mus- 
cle tissue,  which  differs  in  function  and  structure  from  that  of 
the  rest  of  the  alimentary  canal.  In  the  stomach  and  intestines 
slower  and  more  gradual  kinds  of  motion  are  required,  and  here 
we  find  a  good  example  of  non-striated  muscle  tissue. 

Around  the  extremity  of  the  rectum  is  a  band  of  smooth  mus- 
cle, which  remains  in  a  condition  of  continuous  or  tonic  contrac- 
tion. 


DIGESTION. 


Ill 


For  further  details  concerning  the  muscle  tissue  the  student 
must  turn  to  the  Chapter  (xxiv)  on  that  subject.     Here,  how- 


FIG.  45. 


FIG.  46. 


Vertical  section  of  the  Canine  Tooth  of  a 
man.  (a)  Enamel;  (6)  Dentine;  (c) 
Pulp  cavity;  (rf)  Crusta  petrosa. 
(Cadiat.) 


Diagram  of  Alimentary  Tract,  etc.  An- 
gles of  mouth  slit  to  show  the  back  of 
the  buccal  cavity  and  the  top  of  the 
pharynx.  (c)  Cardiac ;  (p)  Pyloric 
parts  of  stomach  ;  «/)  Duodenum ;  (i) 
Jejunum  and  Ilinm;  (ac)  Ascending, 
(tc)  transverse,  and  (dc)  descending 
colon  ;  (r)  Rectum  ;  (a)  Anus. 


Structural  elements  of  the  Enamel  of 
Tooth. 

A.  Prisms  cut  across,  showing  the  hex- 

agonal section. 

B.  Isolated  prisms.    (KOltiker.) 


112 


MANUAL   OF   PHYSIOLOGY. 


ever,  it  may  not  be  out  of  place  to  describe  briefly  the  special 
character  of  the  muscles  found  in  the  wall  of  the  digestive  tube 
and  their  general  arrangement. 

MASTICATION. — In  man,  the  introduction  of  food  into  the 
mouth  is  generally  accomplished  by  artificial  means,  so  that  the 
biting  teeth  (incisors)  and  the  tearing  teeth  (canines)  (Fig.  46) 
are  comparatively  little  used  for  obtaining  a  suitable  morsel 
of  food.  In  the  mouth,  the  essential  act  of  chewing  or  mastica- 
tion is  accomplished  by  means  of  the  motions  of  the  lower  jaw, 
the  tongue  and  the  cheeks.  This  process  of  breaking  up  the 

solid  parts  of  the  food  ought 
to  be  continued  until  all 
hard  substances  are  ground 
into  a  soft  pulp. 

Structure  of  the  Teeth.— 
The  exposed  part  of  the 
teeth  is  covered  by  a  dense 
substance  of  flinty  hardness 
called  enamel,  which  is  de- 
veloped from  the  epithelium, 
and  consists  of  hexagonal 
prisms  set  on  end,  which  are 
really  modified  epithelial 
cells,  but  only  contain  about 
2  per  cent,  of  animal  matter 
(Fig.  47).  The  bulk  of  the 
tooth  is  made  up  of  dentine,  a 
substance  like  bone  in  compo- 
sition, pierced  by  numerous 
fine  canals — dentine  tubules 

Section  through  a  portion  of  the  Fang  of  a  Tooth. 

(a)  Dentine  tubules  near  the  surface  of  the   — which   radiate  toward  the 
fang ;  (b)  Granular  layer ;  (c)  Crusta  petrosa. 

surface,  from  the  pulp  cavity, 

in  the  centre  of  the  tooth.  Filaments  of  protoplasm  run  in  the 
dentine  tubules  from  the  tooth  cells,  which  line  the  pulp  cavity 
and  preside  over  the  nutrition  of  the  tooth.  The  cavity  contains 
vessels,  nerves,  etc.,  which  enter  at  the  root  of  the  tooth,  which 


DEGLUTITION.  113 

is  enclosed  in  a  kind  of  modified  bone  tissue  called  crusta 
petrosa. 

The  two  rows  of  grinding  teeth,  composed  of  molars  and  pre- 
molars,  of  the  lower  jaw  are  made  to  rub  against  the  correspond- 
ing teeth  in  the  fixed  upper  jaw  by  the  combined  vertical  and 
horizontal  movements  induced  by  the  action  of  the  powerful  mus- 
cles of  mastication,  the  temporal  muscles,  together  with  the  mas- 
seters  and  internal  pterygoids,  all  tending  by  their  contraction 
to  elevate  the  lower  jaw  and  bring  the  teeth  forcibly  together. 
This  action  is  opposed  by  the  digastric,  the  genio-  and  mylo-hyoid 
muscles,  which  by  their  combined  force  depress  the  jaw  and 
separate  the  teeth.  The  horizontal  movements  are  in  the  main 
accomplished  by  the  external  pterygoid  muscles,  which,  acting 
together,  pull  the  lower  jaw  forward  so  as  to  make  the  lower 
teeth  protrude  beyond  the  upper.  In  this  action  they  are 
opposed  by  the  digastric  and  hyoid  muscles.  One  external 
pterygoid  on  either  side  acting  alone,  advances  that  side  of  the 
lower  jaw  only,  and  thereby  causes  the  lower  teeth  to  incline 
toward  the  opposite  side  in  a  lateral  direction.  The  two  muscles 
acting  alternately  cause  a  horizontal  motion  from  side  to  side. 
Thus,  while  the  lower  teeth  are  pressed  firmly  against  the  upper 
ones,  they  are  at  the  same  time  made  to  glide  over  them,  either 
from  side  to  side  or  backward  and  forward.  By  these  move- 
ments the  bruised  food  is  soon  pushed  from  between  the  teeth, 
and  passes  toward  either  the  tongue  or  cheek.  The  morsel  is 
soon  replaced  between  the  teeth  by  the  action  of  the  tongue  on 
the  one  hand  and  the  buccinator  muscle  in  the  cheek  on  the 
other. 

While  the  process  of  mastication  is  going  on,  the  food  becomes 
thoroughly  moistened  with  the  fluid  secreted  within  the  mouth. 

DEGLUTITION. — The  next  step  is  swallowing.  When  the  food 
is  sufficiently  triturated  and  moistened,  it  is  collected  together  by 
means  of  the  tongue  and  placed  upon  the  upper  surface  of  that 
organ,  which  becomes  concave  and  presses  or  rolls  the  soft  pulp 
against  the  hard  palate  so  as  to  shape  it  into  an  oblong  mass  or 


10 


114 


MANUAL   OF   PHYSIOLOGY. 


bolus  (Fig.  51).  The  apex  of  the  tongue  is  now  raised  and 
pressed  against  the  hard  palate,  and  by  the  successive  elevations 
of  the  different  parts  of  the  dorsum  of  the  tongue  the  bolus  is 
gradually  pushed  backward  toward  the  isthmus  of  the  fauces. 


FIG.  49. 


Section  through  a  portion  of  Dentine  next  the  pulp  cavity  of  a  growing 
tooth,  (a)  An  isolated  odontoblast;  (b)  Growing  part;  (c)  Odonioblasts; 
(d)  Filaments  of  protoplasm  projecting  from  the  tubules  of  hard  dentine. 
(Beetle.) 


FIG.  50. 


The  Pterygoid  Muscles  seen  from  without  after  removal  of  the 
superficial  parts,  the  temporal  muscle,  the  zygomatic  arch,  and 
a  portion  of  the  lower  jaw  and  masseter.  (1)  External,  (2) 
Internal  pterjgoid  muscle. 


DEGLUTITION. 


115 


FIG.  51. 


The  root  of  the  tongue  with  the  hyoid  bone  is  at  the  same  time 
drawn  upward  and  forward,  so  that  the  bolus  easily  slips  down 
along  the  retreating 
slope  leading  from 
the  mouth  cavity, 
and  gets  within  the 
reach  of  the  constric- 
tors of  the  fauces. 
Immediately  before 
the  morsel  of  food  is 
grasped  by  the  mus- 
cles of  the  fauces,  the 
levator  palati  draws 
the  soft  palate  up- 
ward and  backward 
to  completely  close 
the  posterior  open- 
ings of  the  nasal  cav- 
ity, as  is  shown  by 
the  fact  that  during 
the  act  of  swallow- 
ing the  pressure  in 
the  nasal  cavity  is 
raised.  At  the  same 
moment  the  intrinsic 
muscles  of  the  lar- 
ynx, which  surround  Muscles  of  Tongue  and  Pharynx, 
ji  •  i  , , .  T.  1.  2,  3  Muscles  from  styloid  process  (6)  to  the  tongue,  hyoid 
tlie  rillia  glOttldlS  bone  (<J)  and  pharynx  respectively;  4,  5,  6,7,  8,  musclen 
i;u_  A.  •  L  of  tongue  ;  9,  10,  11,  constrictors  of  pharynx;  12,  cesoph- 
JlkC  d  COllStriCtor,  agus;  13,  is  placed  on  larynx  (e).  (Alien  Thomsm.) 

firmly     close     that 

opening  by  approximating  the  cords  and  arytenoid  cartilages. 
The  entire  larynx  is  at  the  same  time  drawn  up  behind  the 
hyoid  bone  by  the  thyro-hyoid  muscle.  The  rima  glottidis  is 
thus  tucked  in  under  the  cushion  of  epiglottis,  while  the  leaf  of 
the  epiglottis  is  pulled  down  over  the  larynx  by  the  oblique 
aryteno-epiglottidean  and  thyro-epiglottideau  muscles. 

While  the  closure  of  the  nasal  and  pulmonary  air  passages  is 


116 


MANUAL   OF   PHYSIOLOGY. 


FIG.  52. 


going  on,  the  bolus  has  passed  out  of  the  cavity  of  the  mouth 
and  has  been  caught  by  the  palato-glossal  and  palato-pharyngeal 
muscles,  which  force  it  into  the  pharynx  and  at  the  same  time 
close  the  isthmus  faucium  behind  the  descending  morsel.  The 
stylo-pharyngeis  and  the  pharyngeal  constrictors  now  grasp  the 
bolus  spasmodically,  and  the  latter  contract  in  rapid  succession, 
moving  the  bolus  onward,  and  drawing  themselves  over  it,  pass 
it  on  to  the  oesophagus,  where,  by  a  progressing  ring-like  con- 
traction of  the  circular 
muscles  and  a  simulta- 
neous shortening  of  the 
longitudinal  layer  of 
fibres,  the  mass  is  slowly 
squeezed  down  to  the 
cardiac  orifice  of  the 
stomach.  The  move- 
ments of  the  oesophagus 
are  essentially  peristal- 
tic in  character,  the 
peculiarities  of  which 
form  of  motion  will  be 
discussed  when  speak- 
ing of  the  intestinal 
movements. 

The  process  of  swal- 
lowing is  performed  by 
a  continuous  series  of 
coordinated  muscular 
movements,  quite  inde- 
pendent of  gravitation, 
as  may  be  seen  in  ani- 
mals drinking  with 
their  heads  downward. 
Although  these  com- 
plex sets  of  movements 
follow  each  other  regularly  and  without  any  check  or  interval, 
the  act  of  deglutition  is  commonly  divided  into  three  stages, 


Deep  Muscles  of  Cheek,  Pharynx,  etc. 
(1)  Orhicularis  oris;  (2)  buccinator;  (:i)  superior,  (4) 
middle  and  (5)  inferior  constrictors  of  the  pharynx  ; 
(fi)  ossophagus;  (7)  stylo'd  muscles  ciit  across:  '(8,  9, 
10)  muscles  attached  to  the  hyoid  bone  (d)  aud  thy- 
roid cartilage  (e).  (Allen  Thomson.) 


DEGLUTITION. 


117 


FIG.  53. 


between  which,  as  there  is  no  pause,  it  is  not  easy  to  draw  a  hard 
and  fast  line. 

The  first  stage  is  simply  the  initiatory  step  of  placing  the 
morsel  of  food  or  some  liquid  in  such  a  position  as  to  excite  the 
second  or  spasmodic  act  of  deglutition.  This  first  step  is  a  vol- 
untary act,  and  it  is  the  only  part  of  the  movements  of  swallow- 
ing over  which  we  can  exert  complete  control.  The  progress  of 
the  morsel  between  the 
tongue  and  palate  toward 
the  fauces  may  be  as  slow 
and  gradual  as  we  wish, 
but  the  moment  a  certain 
point  is  reached  volition 
is  at  an  end,  and  we  are  , 
unable  to  check  the  com- 
pletion of  the  act. 

By  the  second  stage  is 
meant  the  period  occupied 
by  the  passage  of  the  food 
bolus  through  the  pharynx 
and  past  the  top  of  the 
larynx.  Although  we  are 
not  able  to  influence  it  in 
any  way  by  our  will,  we 
are  conscious  of  the  food 
passing  in  this  region.  It 
is  a  rapid,  involuntary 
spasm  in  which  a  great 
number  of  muscles  take 
part,  all  of  which  are  made 
up  of  striated  muscle  tissue. 

The  third  stage  includes 
all  the  rest  of  the  time 
during  which  the  bolus  is  passing  from  the  grasp  of  the  lower 
pharyngeal  constrictor  and  along'the  oesophagus.  Not  only  has 
our  will  no  influence  over  this  stage  of  deglutition,  but  we  are 
hardly  conscious  of  its  taking  place,  since  no  sensations  accompany 


Transverse  section  of  (Esophagus. 
(tiorsley.) 

a.  Outer  fibrous  covering. 

b.  Bundles  of  longitudinal  muscle  cut  across. 

c.  Transverse  muscular  coat  cut  obliquely. 

d.  Sub-mucous  coat  with  glands  in  section. 

e.  Muscular  layer  of  the  mucous  membrane. 
/.  Mucous  membrane  with  cut  vessels. 

<7.  Stratified  epithelium. 


118  MANUAL   OF   PHYSIOLOGY. 

the  greater  part  of  it.  Thus  the  more  essential  movements  of  the 
act  of  swallowing  are  purely  reflex  and  involuntary,  though  we 
can  call  forth  this  series  of  reflexions  by  voluntary  stimulation  of 
a  certain  part  of  the  fauces  by  means  of  a  morsel  of  food  or  a  drop 
of  liquid,  and  without  such  a  stimulus  as  food  or  liquid  we 
cannot  by  our  will  excite  swallowing.  We  think  we  can  perform 
the  muscular  movements  of  swallowing  when  we  please,  without 
any  food  or  fluid,  but  in  this  we  are  mistaken,  as  careful  observa- 
tion of  our  own  performance  of  the  act  will  show. 

The  pharyngeal  spasm  is  always  preceded  by  the  deposition  in 
the  region  of  the  isthmus  faucium  of  some  drop  of  saliva  col- 
lected from  the  mouth  or  fauces  themselves.  In  fact,  without  a 
slight  preliminary  movement  of  the  posterior  part  of  the  tongue 
— which  might  be  called  the  last  act  of  mastication — the  more 
essential  stages  of  deglutition  cannot  be  excited. 

Nervous  Mechanism  of  Mastication  and  Deglutition. — The 
voluntary  influences  which  regulate  the  motions  of  the  muscles 
of  mastication  pass  along  the  efferent  branches  of  the  fifth  nerves 
(trigemini)  which  accompany  its  inferior  division.  The  muscles 
which  depress  the  jaw  to  open  the  teeth  and  the  intrinsic  muscles 
of  the  tongue  are  supplied  by  the  ninth  pair  of  nerves  (except 
the  posterior  belly  of  the  digastric,  which  has  a  branch  from  the 
facial,  and  the  mylohyoid  and  anterior  belly  of  the  digastric, 
which  are  supplied  from  the  third  division  of  the  fifth).  The 
coordination  of  the  movements  of  mastication  and  suction  seem 
to  reside  in  the  medulla  oblongata,  but  are  obviously  under  the 
control  of  the  will. 

The  afferent  impulses  which  excite  the  nerve  centres  in  the 
medulla,  and  give  rise  to  reflex  acts  which  cause  the  swallowing 
movements,  pass  from  the  mucous  membrane  of  the  fauces  along 

(1)  the   descending  palatine   branches  of  the   spheno-palatine 
ganglion  and  the  second  division  of  the  trigeminus,  also  along 

(2)  the  pharyngeal  branches  of  the  superior  laryngeal  branch  of 
the  vagus  to  the  medulla,  where  the  coordination  of  pharyngeal 
spasm  and  oesophageal  peristalsis  is  accomplished.     Thence  the 
efferent  impulses  pass  by  (1)  the  hypoglossal  to  the  hyoid  and 


GASTRIC   MOVEMENTS. 


119 


glossal  muscles,  (2)  the  glosso-pharyngeal  and  vagus  to  the 
pharyngeal  plexus  to  supply  the  constrictors,  aud  (3)  the  facial 
and  fifth  to  supply  the  fauces  and  palate,  as  indicated  by  their 
anatomical  distribution. 

The  act  of  deglutition  can  be  readily  excited  in  an  animal 
which  is  deprived  of  all  the  nerve  centres  down  to  the  medulla 
oblongata,  and  may  also  be  seen  in  those  human  monstrosities 
(anencephalous  foetus)  without  the  upper  part  of  the  brain  being 
developed,  but  which  can  notwithstanding  both  suck  and  swallow. 

The  movements  of  the  oesophagus  are  reflections  from  the  cen- 
tral nervous  system  (medulla),  both  sets  of  impulses  (possibly 
the  afferent  and  certainly  the  efferent)  passing  along  the  branches 
of  the  vagus. 

It  would  appear  that  the  normal  peristaltic  movements  of  the 
oesophagus  are  always  initiated  by  a  pharyngeal  spasm,  and  that 
they  form  an  inseparable  sequel  to  it.  Thus  the  wave  of  con- 
traction passes  along  the  entire  length  of  the  oesophagus  even 
when  the  bolus  is  stopped  mechanically,.and,  on  the  other  hand,  a 
body  introduced  into  the  oesoph- 
agus without  passing  through 
the  pharynx  excites  no  peristal- 
tic wave,  and  remains  motion- 
less. 

But  it  has  been  observed,  in 
apparent  contradiction  to  the 
foregoing  statement,  that  the 
oesophagus  when  removed  from 
the  body,  and  therefore  quite 
independent  of  the  pharynx  and 
its  nervous  connections,  can  be 
excited  to  move  peristaltically. 
In  this  case  the  medulla  or 
vagus  can  have  no  part  in  bring- 
ing about  this  wave  of  move- 
ment. To  explain  this  discrepancy,  it  may  be  urged  that  the  local 
nerve  and  muscle  mechanism  in  the  tissues  of  the  oesophagus  are 
capable  by  themselves  of  carrying  out  peristaltic  contraction 


FIG.  54. 


Diagram  of  Wall  of  the  Stomach,  show- 
ing the  relativethicknessofthe  mucous 
membrane  (a,  b,  c)  and  the  trans- 
verse (e),  oblique  (/)  and  longitu&iual 
muscle  fibres. 


120  MANUAL   OF   PHYSIOLOGY. 

independently  of  the  central  nerve  organs,  but  that  this  power  is, 
under  ordinary  circumstances,  held  inr  check  by  the  vagus.  The 
inhibition  is  temporarily  suspended  as  a  sequence  of  pharyngeal 
spasm,  and  consequently  a  wave  of  peristaltic  contraction  is 
excited  in  the  cesophageal  muscles,  either  in  response  to  the  direct 
stimulus  of  a  passing  bolus,  or  as  a  result  of  impulses  reflected 
along  the  vagus  channels  from  the  medulla. 

Motion  of  the  Stomach. — The  stomach  and  greater  part  of 
the  intestinal  tract  move  freely  within  the  abdomen,  being 
covered  by  the  smooth  serous  lining  of  that  cavity,  which  also 
keeps  in  position,  so  as  to  restrict  their  movements,  those  parts, 
such  as  the  duodenum,  into  which  the  ducts  of  large  glands  open. 
When  the  stomach  is  empty  it  hangs  with  the  great  curvature 
downward,  and  the  muscular  coats  are  quiescent.  On  being 
filled  it  is  passively  rotated  on  its  long  axis,  so  that  the  greater 
curvature  is  turned  forward,  here  meeting  with  less  resistance, 
and  the  lesser  curvature  is  turned  backward  to  its  line  of 
attachment.  In  the  main,  the  motions  of  the  stomach  are  peri- 
staltic. They  become  very  active  about  fifteen  minutes  after 
the  introduction  of  food,  and  gradually  become  more  and  more 
energetic  until  the  end  of  stomach  digestion,  which  lasts  about 
five  hours. 

The  result  of  the  peristaltic  motion  is  to  move  the  food,  par- 
ticularly the  part  next  the  gastric  wall,  along  the  great  curva- 
ture toward  the  pylorus.  A  back  current  toward  the  cardiac 
extremity  has  been  noticed  running  along  the  lesser  curvature 
and  the  median  axis  of  the  food  mass.  At  the  same  time  a 
peculiar  rotatory  motion  of  the  gastric  wall  takes  place,  similar 
to  that  of  rolling  a  ball  between  the  palms  of  the  hands,  so  that 
the  food  is  twisted  in  a  given  direction,  and  the  deeper  lying 
portion  is  brought  into  contact  with  the  mucous  membrane. 

While  the  fimdus  keeps  up  considerable  pressure  on  the  con- 
tents of  the  stomach,  the  indistinct  peristaltic  action  of  the  cen- 
tral parts  is  intensified,  on  nearing  the  pylorus,  into  a  strong 
circular  contraction,  which  proceeds  as  a  definite  wave  toward 
the  pyloric  valve,  through  which  it  gradually  forces  the  more  or 


VOMITING.  121 

less  digested  food.  At  first  only  the  fluid  parts  are  allowed  to 
pass,  but  toward  the  later  stages  of  digestion  the  fatigued  pyloric 
muscle  admits  solid  masses  into  the  duodenum. 

Nerve  Influence  on  Stomach  Motions. — The  stomach  has 
nerve  connections  with  the  cerebro-spinal  axis  through  the  vagi, 
and  the  splanchnic  branches  of  the  sympathetic,  and  in  the  walls 
of  the  organ  itself  are  numerous  ganglion  cells.  The  sympathetic 
connections  do  not  seem  to  have  any  influence  on  the  muscular 
coats,  for  neither  their  stimulation  nor  section  has  any  marked 
effect  on  their  movements.  If  the  vagi  be  severed,  stomach  con- 
tractions still  occur,  but  no  form  of  local  stimulation  produces 
the  normal  gastric  motions,  even  if  the  organ  be  quite  full  of 
food,  therefore  it  would  appear  that  the  local  nerve  centres  are 
not  sufficient  to  excite  the  normal  rhythmical  muscular  action. 
Moreover,  stimulation  of  the  cut  vagi  leading  to  the  stomach 
causes  active  movements  when  the  stomach  is  full.  It  is  not 
merely  the  presence  of  food  that  produces  the  movements,  as  is 
shown  by  the  fact  that  the  motions  increase  as  the  contents  of 
the  stomach  diminish,  but  conditions  incidental  to  digestion 
(hypenernia  etc.),  probably  also  act  as  a  stimulus. 

Vomiting  is  the  ejection  of  the  contents  of  the  stomach  by 
means  of  a  convulsive  action  of  the  respiratory  and  abdominal 
muscles  associated  with  an  abnormal  contraction  of  the  stomach 
wall,  which  aids  in  opening  the  cardiac  orifice  while  it  keeps  the 
pylorus  firmly  closed. 

The  act  of  vomiting  is  commonly  preceded  by  (1)  a  feeling  of 
sickness  or  nausea,  (2)  a  great  secretion  of  saliva,  (3)  retching. 
The  latter  consists  in  a  violent  inspiratory  effort,  in  the  midst  of 
which  the  root  of  the  tongue  and  the  larynx  are  raised  and  the 
rima  glottidis  suddenly  closed  so  as  to  prevent  air  entering  the 
windpipe.  The  iuspiratory  muscles  still  acting,  and  the  phar- 
ynx and  upper  part  of  the  oesophagus  being  held  open,  air  is 
drawn  into  the  gullet  and  dilates  this  tube  nearly  as  far  as  the 
opening  into  the  stomach.  A  contraction  of  the  muscle  fibres 
radiating  from  the  oesophagus  over  the  stomach  then  opens  the 
cardiac  orifice  and  allows  some  gas  to  escape.  Now  the  act  of 
11 


122  MANUAL   OF   PHYSIOLOGY. 

vomiting  is  completed  if  at  this  moment — the  mouth  and  phar- 
ynx being  open,  the  larynx  closed,  the  oesophagus  on  the  stretch, 
the  cardiac  orifice  relaxed,  and  the  pylorus  firmly  closed — the 
expiratory  muscles  forcibly  contract,  and,  pressing  upon  the 
abdominal  cavity,  give  a  sudden  stroke  to  its  contents  so  as  to 
empty  the  stomach.  The  wall  of  the  stomach  also  contracts 
evenly  throughout,  but  not  with  any  forcible  anti-peristaltic 
action  such  as  would  greatly  aid  in  the  operation  of  rapidly 
ejecting  the  vomit.  The  chief  object  attained  in  the  adult  by 
the  action  of  the  muscular  coat  of  the  stomach  seems  to  be  the 
relaxation  of  the  cardiac  orifice.  In  children,  when  the  fundus 
is  little  developed,  and  the  fibres  radiating  over  the  stomach 
from  the  oesophagus  are  numerous  and  strong,  the  act  of  vomit- 
ing requires  less  effort  on  the  part  of  the  respiratory  muscles ; 
the  frequent  puking  of  suckling  infants  being  accomplished  by 
the  gastric  muscle  alone.  When  the  vomit  is  emitted,  the 
hyoidean,  laryngeal,  and  neck  muscles  relax,  and  the  air  is 
forcibly  driven  out  of  the  partially  distended  lungs  so  as  to 
clear  away  any  remaining  particles  from  the  upper  part  of  the 
air  passages. 

Vomiting  is  usually  caused  by  irritation  of  the  stomach  itself, 
and  may  be  induced  by  either  mechanical,  electrical,  or  chemical 
stimulation  of  the  mucous  membrane.  In  this  way  some  emetics, 
such  as  mustard,  sulphate  of  copper,  etc.,  act.  It  may  also  be 
caused  by  intestinal  irritation,  as  when  a  hernia  is  strangulated 
or  the  mucous  membrane  irritated  by  intestinal  worms. 

Gentle  stimulation  of  the  fauces  and  neighborhood  of  the  root 
of  the  tongue  commonly  induces  vomiting.  In  the  early  stages 
of  pregnancy  the  unusual  condition  of  the  uterus  causes  frequent 
vomiting,  which  is  known  as  "  morning  sickness."  The  irritation 
of  a  calculus  passing  through  the  ureter,  or  a  gallstone  impacted 
in  the  bile  duct,  usually  excite  vomiting.  Injuries  of  the  brain, 
and  psychical  impressions,  particularly  those  excited  by  the  sense 
of  smell  or  unusual  disturbance  of  equilibrium,  may  give  rise  to 
vomiting.  Moreover,  a  number  of  medicaments,  as  apomorphin, 
emetin,  etc.,  cause  vomiting  if  introduced  into  the  blood. 

From  the  foregoing  facts  it  appears  that  vomiting  is  a  complex 


INTESTINAL   MOVEMENTS.  123 

and  irregular  muscular  act,  which  may  be  induced  by  the  stimu- 
lation of  various  parts  of  the  internal  surfaces  of  the  body,  par- 
ticularly those  which  receive  branches  from  the  vagus  nerve. 

One  would,  therefore,  be  inclined  to  suppose  that  some  afferent 
nerve  channels  exist  in  the  vagus  which  bear  impulses  to  a  vom- 
iting nerve  centre  and  excite  it,  so  as  to  cause  it  to  send  forth 
peculiar  and  irregular  impulses  to  the  respiratory,  gastric,  and 
other  muscles,  and  give  rise  to  their  characteristic  spasm. 

In  short,  it  would  seem  to  be  a  reflex  act,  the  afferent  impulses 
of  which  pass  to  the  medulla  oblongata  by  the  vagus,  and  the 
efferent  impulses  are  conveyed  by  the  ordinary  spinal  nerves  to 
the  respiratory  muscles  by  the  vagus  to  the  pharyngeal,  laryn- 
geal  and  gastric  muscles,  and  by  the  fifth,  seventh  and  ninth 
nerves  to  the  palatine,  facial  and  hyoidean  muscles.  This  vom- 
iting nerve  centre  must  lie  in  the  medulla,  in  very  close  relation- 
ship to  the  respiratory  centre,  with  which  it  nearly  corresponds. 
This  centre  may  bring  about  the  whole  sequence  of  events  known 
as  vomiting,  when  stimulated  either  directly  by  poisons  contained 
in  the  blood,  indirectly  through  the  vagus,  or  even  from  the 
higher  centres  by  emotions  or  ideas.  Section  of  the  vagi  ren- 
ders vomiting  impossible,  as  it  cuts  off  both  the  commonest  source 
of  stimulus  going  to  the  centre,  and  also  the  important  efferent 
impulses  which  cause  the  muscle  coat  of  the  stomach  to  contract 
and  to  open  the  cardiac  orifice. 

Movements  of  the  Intestines. — The  muscular  coats  are  some- 
what differently  arranged  in  the  small  and  the  large  intestines, 

i  but  have  the  same  general  relation  to  each  other,  viz.,  a  thin 
longitudinal  layer  lying  externally,  next  the  serous  membrane, 
and  a  layer  of  circular  fibres  considerably  thicker  lying  inter- 
nally under  the  mucous  membrane.  In  the  large  intestine  the 

;  external  longitudinal  fibres  are  collected  into  three  bands  placed 
at  equal  distances  one  from  another,  which,  being  rather  shorter 

1  than  the  remainder  of  the  intestine,  throw  the  intermediate  part 

i)  into  a  series  of  pouches. 

It  is  in  the  small  intestine  that  peristaltic  motion  of  the  most 

;    typical  kind  occurs.     A  wave  of  contraction  passes  from  the 


124 


MANUAL   OF   PHYSIOLOGY. 


the 
the 


pylorus  along  the  circular  fibres,  so  as  to  look  like  a  broad  ring 
of  constriction  progressing  slowly  downward. 

The  longitudinal  fibres  at  the  same  time  contract  so  as  to 
shorten  the  piece  of  intestine  immediately  below  the  ring  of  con- 
striction, and  also  cause  a  certain  amount  of  rolling  movement 
of  those  loops  of  intestine  which  are  free  enough  to  move. 

This   motion  takes  place   periodically  in   proportion    to   the 

amount  and  character  of 
contents  of  the  intestine, 
food  passing  over  the  mucous 
membrane  being  to  all  appear- 
ance the  stimulus  which  nor- 
mally calls  forth  and  intensifies 
the  action. 

The  activity  of  the  peristaltic 
movements  varies  with  many 
circumstances  besides  the  con- 
tents of  the  intestines.  Of  these 
the  most  noticeable  is  the  amount 
and  character  of  the  blood  flow- 
ing through  the  vessels  of  the 
intestinal  wall.  Thus  stoppage 
of  the  blood  current  by  tying 
the  arteries,  or  deficiency  of  oxy- 
gen and  excess  of  carbonic  acid, 
causes  inordinate  activity  of  the 
peristaltic  action.  Direct  irrita- 
tion of  the  serous  surface  of  the  intestine  with  mechanical, 
chemical,  or  electrical  stimuli  also  causes  an  increase  in  the 
movements  of  the  intestine. 

The  great  activity  of  the  motion  observed  when  the  abdominal 
cavity  of  a  recently  killed  animal  is  opened  depends  partly  on 
the  exposure  to  cool  air,  and  partly  on  the  venous  character  of 
the  blood  in  the  vessels  no  longer  oxidized  by  respiration. 

The  irregular  and  impetuous  action  of  the  intestine  which 
follows  the  constriction  or  strangulation  of  a  hernial  protrusion, 
probably  depends  chiefly  on  the  mechanical  stimulation,  but  also 


Diagram  of  a  longitudinal  section  of  the 

Wall  of  the  Small  Intestine, 
a.  Villi. 
6.  Lieberklihn's  Glands. 

c.  Tunica  muscularis  mucosse,  below 

which     lies    Meissner's     nerve 
plexus. 

d.  Connective  tissue  in  which  many 

blood  and  lymph  vessels  lie. 

e.  Circular  muscle    fibres  cut  across 

with  Auerbach's  nerve  plexus  be- 
low it. 

/.  Longitudinal  muscle  fibres. 

g.  Serous  coat. 


INTESTINAL   NERVE   MECHANISM.  125 

is  intimately  related  to  interference  with  the  blood  supply  conse- 
quent on  the  pressure  exerted  by  the  constricting  band.  Pro- 
longed overwork  often  induces  immobility  of  the  intestinal  wall, 
and  hence  we  find  the  purging  and  vomiting  which  accompany  a 
temporary  heruial  constriction  followed  by  inability  of  the  intes- 
tine to  propel  its  contents.  These  points  have  also  been  proved 
by  results  of  experiments  on  the  lower  animals. 

The  movements  of  the  large  intestines  are  the  same  as  the 
small,  but  not  so  obvious,  owing  to  the  modified  sacculated  shape 
of  this  part  of  the  alimentary  canal.  The  contractions  of  the 
colon  begin  at  the  ileo:csecal  valve  where  the  peristaltic  wave  of 
the  ileum  ceases.  The  normal  intestinal  motions  thus  pass  in  an 
almost  uninterrupted  wave  from  the  pylorus  to  the  end  of  the 
gut,  but  when  special  sources  of  irritation  exist,  a  wave  may 
originate  in  almost  any  intermediate  part  of  the  intestine.  A 
reversed  **  anti-peristaltic  motion,"  as  it  is  called,  only  occurs  as 
a  result  of  some  intense  local  stimulation,  such  as  the  strangula- 
tion of  a  hernia,  etc. 

The  motion  produced  by  the  substances  contained  in  the  intes- 
tine depends  on  their  character.  The  solid  parts  excite  more 
rapid  movements,  and  the  more  fluid  portions  but  slightly  influ- 
ence the  intestinal  peristalsis. 

Thus  the  solids  which  make  their  way  through  the  pylorus  are 
seldom  to  be  found  in  the  jejunum,  no  matter  at  what  period 
after  a  meal  the  animal  be  killed,  whereas  the  folds  of  the 
mucous  membrane  are  always  bathed  in  a  fluid,  creamy  material 
during  the  entire  period  of  digestion,  and  even  for  a  considerable 
time  after  all  the  food  has  left  the  stomach. 

Mechanism  of  Defecation. — This  is  a  point  of  much  import- 
ance, for  the  evacuation  of  the  lower  bowel  is  intimately  con- 
nected with  feelings  of  comfort  and  health,  and  in  illness  the 
insuring  of  its  accomplishment  forms  an  essential  part  of  the 
physician's  duty. 

The  movements  of  the  intestine  cause  the  various  excretions 
and  indigestible  parts  of  the  food,  to  pass  toward  the  sigmoid 
flexure  of  the  colon,  where  their  onward  motion  is  checked  for  a 


126  MANUAL   OF   PHYSIOLOGY. 

time  by  the  strong  circular  muscle  of  the  rectum  (called  the 
superior,  or  tertius  sphincter  by  Hyrtl),  which  does  not  carry  on 
the  peristaltic  wave.  The  materials  here  get  packed  into  a  more 
or  less  solid  mass,  which  is  gradually  augmented  after  each 
meal. 

The  lower  outlet  of  the  alimentary  canal  is  closed  by  two  dis- 
tinct sphincter  muscles.  One  thin  external  superficial  muscle, 
made  up  of  striated  fibres,  belongs  to  the  perineal  group,  and  has 
little  influence  on  the  closure  of  the  anus.  The  deep  or  internal 
sphincter,  which  is  much  stronger,  surrounds  the  gut  for  rather 
more  than  an  inch  (3  centimetres,  HeuleXin  height,  and  is  one- 
quarter  inch  thick.  It  is  made  of  smooth  muscle,  and  therefore 
capable  of  prolonged  (tonic)  contraction.  It  would  appear,  how- 
ever, that  this  strong  sphincter  is  merely  a  supernumerary  guard 
to  the  anal  orifice,  but  rarely  called  into  action,  for  during  the 
interval  of  rest  between  the  acts  of  defecation,  the  faeces  do  not 
come  in  contact  with  the  portion  of  intestine  surrounded  by  this 
muscle.  The  rectum  for  quite  one  inch  above  the  sphincter  is 
perfectly  empty,  being  kept  free  from  feculent  particles  partly 
by  a  fold  of  the  intestinal  wall  and  partly  by  the  repeated  action 
of  the  voluntary  muscles  in  the  neighborhood,  which,  by  intensi- 
fying the  angle  that  exists  at  this  point  and  flattening  this  inch 
of  rectum,  can  squeeze  back  the  approaching  matters.  Any  one 
familiar  with  the  digital  examination  of  the  unevacuated  rec- 
tum, knows  that  no  faeces  are  met  with  for  about  two  inches. 

Considerable  accumulation  may  take  place  in  the  sigmoid 
flexure  without  much  discomfort  ensuing,  but  when  the  rectum 
is  distended,  an  urgent  sensation  of  wanting  to  empty  it  is 
experienced,  and  the  voluntary  movements  mentioned  above  are 
performed  by  the  levator  ani  and  the  neighboring  perineal 
muscles,  with  the  object  of  preventing  any  substance  reaching 
the  part  of  the  rectum  immediately  above  the  sphincter. 

If  the  rectum  be  distended  with  fluid,  the  occasional  anal 
elevation  does  not  suffice  to  keep  it  back,  and  a  continuous  and 
combined  action  of  the  sphincters  and  levator  ani,  etc.,  is  neces- 
sary to  ward  off  the  expulsion  of  the  contents. 

When  the  lower  bowel  is  habitually  emptied  at  the  same  hour 


INTESTINAL   NERVE   MECHANISM. 


127 


daily — a  habit  which  should  be  carefully  exercised — the  sensa- 
tions of  requirement  to  go  to  stool  occur  with  great  punctuality,  or 
can  be  readily  induced  by  the  will,  so  that  normal  defecation  is 
reputed  to  be,  and  practically  is,  a  voluntary  act.  But  not 
completely  so,  for,  somewhat  like  swallowing,  the  later  stages  of 
defecation  consist  essentially  of  a  series  of  involuntary  reflex 
events  which  we  can  initiate  by  the  will,  but  when  it  is  once 
started,  are  powerless  to  modify  until  the  reflex  sequence  is  com- 
pleted. 

Under  ordinary  circumstances,  the  evacuation  of  the  faeces  is 
commenced  by  the  voluntary  pressure  exercised  on  the  abdominal 
contents  by  the  respiratory  muscles.  The  diaphragm  is  depressed, 


FIG.  56. 


FIG.  57. 


Auerbach's   plexus   from    between    the 
muscle  coats  of  the  intestine  with  low     A  nodal  point  of  Auerbach's  plexus  under 
power.  high  power,  showing  the  nerve  cells. 

the  outlet  of  the  air  passages  firmly  closed,  and  the  expiratory 
muscles  thrown  into  action,  while  at  the  same  moment  the  muscles 
which  close  the  pelvic  outlet  relax,  and  allow  the  anus  to  descend, 
so  that  the  inferior  angle  of  the  rectum  is  straightened,  and  a 
voluntary  inhibition  of  the  sphincter  is  brought  about.  This 
voluntary  expiratory  effort  seldom  requires  to  be  continued  for 
more  than  three  or  four  seconds  before  some  fecal  matter  reaches 
the  part  of  the  rectum  just  above  the  sphincter.  When  this  has 
occurred,  no  further  abdominal  pressure  is  necessary  (except 
when  the  masses  of  faeces  are  large  and  hard),  for  the  local 
stimulus  starts  a  series  of  reflex  acts  which  carry  on  the  operation. 


128  MANUAL   OF   PHYSIOLOGY. 

These  consist  of  an  increased  peristaltic  contraction  of  the  colon 
and  sigmoid  flexure,  the  waves  of  which  pass  along  the  rectum. 
These  waves  are  accompanied  by  synchronous  rhythmical  relaxa- 
tion of  the  sphincter,  which  replaces  its  normal  condition  of  tonic 
contraction. 

^The  effect  of  the  voluntary  effort,  and  the  amount  of  the 
abdominal  pressure  required,  depend  upon  the  consistence  of  the 
faeces.  When  quite  fluid,  they  constantly  tend  to  come  in  contact 
with  the  sensitive  point  of  the  rectum,  and  a  voluntary  effort  is 
required  to  prevent  the  reflex  series  of  events  from  taking  place  ; 
a  momentary  relaxation  of  the  sphincter  with  voluntary  abdom- 
inal pressure  is  sufficient  to  eject  the  contents  of  the  bowel.  Oil 
the  other  hand,  when  the  faeces  are  firm,  time  is  required  in  order 
that  the  slowly  acting  smooth  muscle  may  pass  the  mass  onward. 
In  common  constipation,  the  difficulty  is  to  get  the  solid  mass 
down  to  the  sensitive  exciting  point,  in  which  casern  few  drachms 
of  warm  fluid,  used  as  an  enema,  may  awaken  the  necessary 
reflex  movements. 

Nervous  Mechanism  of  the  Intestinal  Motion. — Many  points 
in  the  nervous  control  exerted  over' the  intestinal  muscles  are 
obscure.  We  know  that  intestinal  movements  which  are  peri- 
staltic in  their  nature  occur  in  a  portion  of  intestine  removed 
from  the  body,  and  thus  separated  from  all  central  nervous  con- 
trol. We  know,  also,  that  there  are  abundant  nerve  elements 
in  the  walls  of  the  intestines  which  have  all  the  characters  of 
ganglion  cells,  and  therefore  probably  act  as  nerve  centres. 
(Figs.  56,  57.) 

With  regard  to  these  local  nervous  agencies,  anatomists  have 
made  out  two  distinct  sets,  both  of  which  have  the  form  of  a  net- 
work of  nerve  fibrils  studded  with  cell  elements  at  their  nodal 
points.  One  of  these,  a  closely-meshed  plexus  with  flattened 
cords  and  ganglionic  masses  at  their  points  of  union,  lies  between 
the  longitudinal  and  circular  layers  of  muscle  (Figs.  56,  57), 
forming  the  plexus  myentericus  exterior  of  Auerbach,  and  most 
probably  controls  the  movements  of  these  layers  of  muscle.  The 
other  lies  internal  to  the  circular  muscle,  in  close  relation  to  the 


INTESTINAL   NERVE   MECHANISM. 


129 


muscularis  mucosse,  and  is  called  the  plexus  myentericus  interior 
of  Meissner ;  the  meshes  of  which  are  looser  and  more  irregular, 
and  the  cords  and  ganglia  more  rounded  and  finer  than  those  of 
Auerbach's  plexus.  (Figs.  58,  59.) 

The  blood  flowing  through  these  nerve  centres  in  all  proba- 
bility acts  as  a  sufficient  stimulus,  under  ordinary  circumstances, 
to  produce  some  peristaltic  motions,  and  hence  we  may  say  that 
they  are  automatic.  When  food  comes  into  the  intestine  it 
increases  the  flow  of  blood  as  well  as  mechanically  irritating  the 
intestinal  wall.  The  intestinal  vessels  remain  engorged  so  long 
as  the  process  of  digestion  goes  on.  Food  seems  to  act  more 
effectually  than  insoluble  mechanical  stimuli,  for  when  insoluble 


FIG.  58. 


FIG.  59. 


Meissner's  plexus,  low  power. 


Meissner's  plexus  (high  power),  showing  cells 
grouped  at  nodal  points. 


substances  are  placed  in  the  gut,  they  at  first  call  forth  active 
movements ;  but  these  do  not  last  long,  for  the  stimulus  is  not 
of  itself  adequate  to  excite  prolonged  action,  except  it  be  asso- 
ciated with  continuing  congestion  dependent  upon  other  causes, 
such  as  the  vaso-motor  changes  accompanying  the  general  diges- 
tive process,  and  the  absorption  of  the  prepared  food  stuffs. 

With  regard  to  the  influence  of  other  nerves,  it  seems  to  be 
admitted  on  all  sides  that  the  vagus  acts  as  an  exciting  nerve, 
since  stimulation  of  its  peripheral  part  causes  increased  action, 
and  it  is  probable  that  its  great  efferent  channel  for  impulses  is 
reflected  through  the  brain. 


130  MANUAL   OF   PHYSIOLOGY. 

On  the  other  hand,  the  splanchnic  nerves,  which  come  from 
the  thoracic  sympathetic,  are  said  to  be  inhibitors  of  the 
myenteric  plexuses.  This  may  be  explained  by  their  effect  on 
the  small  vessels — which  they  no  doubt  control — causing  a 
change  in  the  blood  supply.  Be  this  as  it  may,  the  splanchnic 
seem  to  have  considerable  influence  on  the  intestinal  movements. 
When  stimulated,  they  commonly  check  the  intestinal  motions, 
but  may  sometimes  (as  when  the  movements  have  ceased  after 
death)  give  rise  to  new  movements. 

On  account  of  this  double  action,  it  has  been  said  that  there 
are  two  kinds  of  fibres,  (1)  inhibiting,  which  are  easily  excited, 
and  during  life  have  greater  influence,  and  (2),  exciting,  which, 
though  less  excitable,  retain  their  irritability  longer. 

However,  most  of  these  effects  may  be  explained  by  referring 
them  to  vasomotor  changes. 

With  regard  to  defecation,  we  know  that  a  nerve  centre  exists 
in  the  lumbar  portion  of  the  spinal  cord,  which  governs  the 
sphincter,  and  seems  to  keep  up  its  tonic  contraction.  This 
centre  may  be  either  excited  to  increased  action  or  inhibited,  by 
peripheral  stimuli  or  by  central  influences  from  the  brain. 

Thus  the  local  application  of  warmth  causes  inhibition  of  the 
centre,  and  thereby  relaxation  of  the  sphincter,  while  cold  gives 
rise  to  increased  central  action,  causing  contraction  of  the 
sphincter  muscle  (a  point  to  be  remembered  when  examining  or 
operating  within  its  grasp).  Besides  the  voluntary  variations 
which  we  can  bring  about  in  the  activity  of  this  lumbar  centre, 
many  other  central  influences,  such  as  emotions,  may  operate 
upon  it.  Thus,  terror  inhibits  the  centre  and  loosens  the  sphincter 
independently  of  our  will. 


MOUTH   DIGESTION.  ^     131 

CHAPTER  VII. 
MOUTH  DIGESTION. 

The  cavity  of  the  mouth  is  lined  by  a  bright  red  mucous  mem- 
brane, which  is  continuous  with  the  skin  at  the  lips.  It  varies  in 
structure  in  different  parts  of  the  buccal  cavity,  and  in  its  gene- 
ral construction  more  resembles  the  outer  covering  of  the  body 
than  the  mucous  membrane  lining  the  alimentary  tract.  It  con- 
sists of  (1)  a  superficial  part,  com  posed  of  thick  stratified  epithe- 
lium, the  upper  cells  of  which  are  flat,  scaly  and  tough,  and  are 
placed  horizontally,  while  in  the  deeper  layers  the  cells  are  soft, 
rounded  or  elongated,  having  their  long  axis  perpendicular  to 
the  surface  ;  and  (2)  a  deeper  part,  composed  of  fibro-elastic 
tissue,  which,  over  the  alveoli  of  the  teeth,  is  amalgamated  with 
the  periosteum  and  forms  the  dense,  tough  gums. 

The  mucous  membrane  of  the  inouth  is  covered  with  papillse 
which,  on  the  dorsum  of  the  tongue, 
attain  great  magnitude  and  variety 
of  shape  and  epithelial  covering. 
In  man,  three  kinds  are  described  : 
(1)  Narrow  pointed,  filiform.  (2) 
Blunt  and  clubbed  at  the  apex,  fun- 
giform.  (3)  Broad  complex  papil- 
Ise,  drcumvallate,  surrounded  by  a 
fossa,  of  which  there  are  but  a  lim- 
ited number  (about  a  dozen).  Diagram  taken  from  a  small  portion 

rpi  •    i  of  sacculated  gland  from  Cockroach, 

ihe   Special   secreting   organs   or     showing  branching  duct  and  saccules. 

glands,  which  pour  their  juices  into 

the  mouth,  have  all  the  same  general  type  of  structure,  though 
they  vary  much  in  the  detail  as  to  the  variety  and  character  of 
their  cells.  They  are  known  as  the  acinous  or  sacculated  glands, 
from  their  being  made  up  of  numerous  acini,  or  minute  elongated 
sacs  or  tubules,  arranged  at  the  end  of  a  repeatedly  branching 


132 


MANUAL   OP   PHYSIOLOGY. 


duct,  like  grapes  on  the  terminals  of  the  successive  little  branches 
growing  from  the  central  stalk  to  form  a  bunch.  In  the  glands 
the  saccules  are  packed  together  closely  around  the  ducts,  and  by 
mutual  pressure  are  made  to  assume  various  shapes.  The  wall 
of  the  saccule  is  formed  of  a  very  delicate,  clear,  transparent 
membrane,  on  the  outside  of  which  are  numerous  flattened,  branch- 
ing stellate  cells,  the  branches  of  which  anastomose  one  with 


Section  of  the  Subraaxillary  Gland  of  the  Dog,  showing  the 
commencement  of  a  duct  in  the  alveoli.    X  425.    (Sch&fer.) 

a.  One  of  the  alveoli,  several  being  grouped  round  the  duct- 

let  (d'). 

b.  Basement  membrane  in  section. 

d.  Larger  duct  with  columnar  epithelium. 
*.  Half-moon  group  of  cells. 

another,  and  appear  to  penetrate  the  membrane  in  order  to  reach 
the  inside  of  the  acini. 

The  cavity  of  the  little  sacs  is  almost  completely  filled  with 

^rge  polygonal  gland  cells,  so  that  only  a  very  narrow  space 

-d"'    '  \the  centre.     (Fig.  61.)     From  this  space  there  is  free 

v^'^munication  to  the  main  duct  of  the  gland  by  means  of  the 

proper  ductlet  of  each  saccule.     In  the  saccules  of  a  few  glands* 


MUCOUS   AND   SALIVARY    GLANDS. 


133 


FIG.  62. 


viz.,  some  of  the  so-called  mucous  salivary  glands,  another  kind 
of  cell  element  is  seen  between  the  gland  cells  just  described 
and  the  wall  of  the  sac,  their  outer  side  following  accurately  the 
concave  boundary  of  the  saccule,  their  inner  side  impinging  upon 
the  gland  cells.  They  thus  acquire  a  more  or  less  half-moon 
shape.  These  demi-lune  cells  will  be  again  referred  to  (page 
143). 

Between  the  saccules  are  numerous  blood  vessels  which  branch 
and  form  a  network  of  capillaries  on  the  outside  of  each  little 
sac.  Numerous  nerves  are  also  found,  which,  according  to  some 
observers,  have  ganglionic  cell  connections  in  the  gland  sub- 
stance, and  send  terminals  into  the  gland  cells  direct. 

Although  this  account  of  the  nerve  terminations  in  the  secret- 
ing cells  of  other  glands  has  met  with  doubt,  it  is  certain  that  in 
the  lower  animals  nerve  terminals  have  been  traced  into  gland 
cells,    and    upon    physiological 
grounds,  as  will  presently  appear, 
we  are  forced  to  believe  that  a 
similar  connection  must  exist  in 
mammalia. 

The  ducts  are  lined  with  short 
cylindrical  epithelium  \ch 
does  not  appear  to  haw  any 
secreting  function  >A11  the 
glands  are  made  upofnumerous 
packets  of  lobules  bound  together 
in  one  mass  and  united  by  their 
ducts.  Each  of  these  lobules  is 
itself  a  perfect  gland.  The 
smaller  mouth  glands  are  also 
separable  into  lobules,  and  hence 
are  called  compound  acinous 
glands. 

The  mouth  glands  are  divided 

into  two  sets,  which  produce  different  kinds  of  se  -  '.-s- 
(1)  Mucous  glands,  which  secrete  mucus,  and  (2)  Salivary  y^i^', 
which  produce  watery  saliva.  The  functional  distinction  is 


A  dissection  of  the  side  of  the  face,  show- 
ing the  Salivary  Glands. 
a.  Sublingual  gland. 
6.  Submaxillary  glands  with  their  ducts 

opening  on  the  floor  of  the  mouth 

beneath  the  tongue  at  (d). 
c.  Parotid    gland    and    its   duct,    which 

opens  on  the  inner  side  of  the  cheek. 


134  MANUAL   OF    PHYSIOLOGY. 

seldom  absolute,  for  most  salivary  glands  have  a  mixed  secre- 
tion, and  various  gradations  of  transition  from  purely  salivary 
to  purely  mucous  glands  are  met  with. 

The  proper  mucous  glands  are  small,  varying  in  size  from  a 
pin's  head  to  a  pea.  They  are  found  in  groups  under  the  mucous 
membrane  in  various  parts  of  the  mouth,  and  from  their  posi- 
tions are  called  labial,  buccal,  etc.  Their  cells  contain  a  clear 
mucilaginous  substance. 

The  great  Salivary  glands  are  the  three  large  glands  which  are 
known  as  the  parotid,  submaxillary  and  sublingual.  On  account 
of  their  great  size  they  form  striking  anatomical  objects,  being 
large  masses  of  irregularly  arranged  glandular  packets,  which 
might  be  spoken  of  as  lobes,  to  distinguish  them  from  the 
smaller  packets  or  lobules.  Their  ducts  are  of  considerable  size, 
and  have  strong  walls  made  of  dense  fibrous  tissue,  containing 
many  elastic  fibres,  and  in  one  of  them,  the  submaxillary, 
smooth  muscle  tissue  has  been  demonstrated. 

The  parotid  duct  (^Steno's)  opens  into  the  mouth  about  the 
middle  of  the  cheek  just  opposite  the  second  molar  tooth.  The 
submaxillary  has  also  a  single  duct  (Wharton's),  which  opens 
beneath  the  tongue  beside  the  frsenum.  The  sublingual  gland 
has  several  ducts,  some  of  which  open  into  that  of  the  submax- 
illary, and  others  unite  to  enter  the  mouth  beside  Wharton's 
duct. 

In  different  animals  and  in  different  glands  of  the  same  animal 
a  variable  amount  of  mucus  is  secreted  by  these  glands,  which 
are  all  called  salivary,  though  the  parotid  alone  deserves  the 
name  in  the  strictest  sense  of  the  term,  owing  to  the  freedom  of 
its  secretion  from  mucus. 

THE  CHARACTERS  OF  MIXED  SALIVA. 

The  liquid  in  the  mouth  is  a  mixture  of  the  secretion  of  the 
salivary  glands  with  that  of  the  small,  purely  mucous  glands. 

It  is  a  slightly  turbid,  tasteless  fluid,  of  a  distinctly  alkaline 
reaction,  of  1004-1008  specific  gravity,  and  so  tenacious  that  it 
can  be  drawn  into  threads.  The  amount  secreted  by  an  adult 
human  being  during  24  hours  varies  greatly  according  to  cir- 


COLLECTION    OF    THE   SECRETION. 


135 


FIG.  63. 


cumstances,  and  has  been  variously  estimated  by  different 
authors,  by  whom  the  wide  limits  of  200-2.000  grms.  (7-70  oz.) 
have  been  assigned  as  the  daily  amount. 

Saliva  contains  about  .5  per  cent,  of  solids.  Of  these  the 
greater  part  are  organic  ;  namely,  (1)  Muein,  from  the  submax- 
illary,  sublingual  and  small  mucous  glands,  which  can  be  pre- 
cipitated by  acetic  acid.  To  this  substance  the  viscidity  of  the 
saliva  is  due.  (2)  Traces  of  albumin,  precipitable  by  concen- 
trated nitric  acid  and  boiling.  (8)  Traces  of  globulin,  precipi- 
tated by  carbonic  acid.  (4)  Ptyalin,  a  peculiar  ferment. 

The  inorganic  constituents  are  salts,  among  which  an  incon- 
stant amount  of  potassium  sulphocyanate  is  found,  a  substance 
which  does  not  exist  in  the  blood. 

There  are  also  many  morphological  elements;  of  these  the 
majority  are  accidental,  being 
the  remains  of  food,  etc. ;  others 
are  more  or  less  characteristic  ; 
namely,  (1)  Salivary  corpuscles, 
which  are  rounded  protoplasmic 
masses  containing  nuclei  and 
coarse  granules  which  show 
Brownian  movements.  (2)  Epi- 
thelial scales,  from  the  surface  of 
the  mucous  membrane  of  the 
mouth.  (3)  Various  forms  of 
bacteria,  which  propagate  readily 
amid  the  decaying  particles  of 

food  in  the  mouth.  No  bacteria  or  other  fungi  exist  in  the  ducto* 
of  the  glands  or  saliva  taken  from  the  ducts  with  the  necessary 
aseptic  precautions. 

COLLECTION  OF  THE  SECRETION. 

Ordinary  mixed  saliva  may  be  easily  collected  by  chewing 
some  insoluble  material,  such  as  a  bit  of  rubber  tubing,  and  col- 
lecting the  fluid  which  the  motion  causes  to  be  poured  into  the 
mouth. 

The  collection  of  the  secretion  of  the  different  glands  requires 


The  form  elements  from  mixed  saliva 
from  tip  of  tongue,  showing  (e)  large, 
irregular,  scaly  epithelial  cells,  (c) 
round  salivary  corpuscles,  several  (b) 
bacteria  and  (m)  niicrococci. 


136  MANUAL   OF   PHYSIOLOGY. 

more  delicate  methods.  It  may  be  collected  separately  by  placing 
a  cannula  in  the  duct  of  each  gland. 

Parotid  saliva  obtained  in  this  way  is  found  to  have  no  struc- 
tural elements  nor  mucus,  and  is  a  thin  fluid  dropping  easily,  not 
capable  of  being  drawn  into  threads.  It  contains  some  serum, 
albumin  and  globulin,  potassium  sulphocyanate,  and  ptyalin. 
The  portion  first  secreted  is  commonly  acid,  and  it  never  becomes 
strongly  alkaline.  Its  specific  gravity  is  1003-1004.  On  stand- 
ing it  becomes  turbid  from  the  precipitation  of  carbonate  of 
lime,  which  existed  as  bicarbonate. 

The  submaxillary  secretion  is  more  strongly  alkaline  than  that 
of  the  parotid;  it  contains  structural  elements  and  mucin,  but  is 
not  so  viscid  as  the  general  mouth  fluid. 

The  sublingual  is  much  more  viscid  than  either  of  the  others, 
is  more  strongly  alkaline,  and  contains  much  mucus  and  many 
salivary  corpuscles. 

THE  METHOD  OF  SECRETION  OF  SALIVA. 

Under  ordinary  circumstances  very  little  saliva  is  secreted, 
only  sufficient  being  poured  into  the  mouth  to  keep  the  surface 
moist.  When,  however,  food  is  introduced  into  the  mouth,  and 
the  process  of  mastication  commences,  the  secretion  goes  on  more 
or  less  rapidly,  according  to  the  stimulating  or  non-stimulating 
character  of  the  food. 

The  activity  of  a  salivary  gland  is  at  once  brought  about  by 
means  of  special  nervous  agencies  when  a  stimulus  is  applied  to 
the  mouth.  We  know  that  the  nervous  mechanism  which  regu- 
lates this  secretion  is  called  a  reflex  act.  The  stimulus  traveling 
from  the  surface  of  the  mouth  to  the  nerve  centres  is  reflected 
thence  to  the  glands.  We  speak,  then,  of  afferent  nerves,  which 
carry  the  impulses  to  the  nerve  centre,  and  efferent  nerves,  which 
carry  them  from  the  centre. 

If  we  review  the  ordinary  circumstances  giving  rise  to  a  flow 
of  saliva,  there  will  be  no  difficulty  in  determining  the  nerves 
which  act  as  the  afferent  channels  in  the  simple  reflex  act. 

Stimulation  of  the  mucous  membrane  of  the  tongue  and 
mouth,  whether  chemically,  as  with  irritating  condiments,  or 


SECRETION    OF   SALIVA. 


137 


mechanically,  as  by  the  motions  of  mastication,  is  generally 
transmitted  to  the  centre  by  the  sensory  branches  of  the  fifth 
cranial  nerve,  which  supply  the  mouth,  and  by  the  branches  of 
the  glosso-pharyngeal. 

The  stimulus  of  the  sense  of  taste  is  sent  by  the  nerves  of  that 
sense,  mainly  the  glosso-pharyngeal,  to  the  taste  centre  in  the 
cortex  cerebri,  and  from  thence  to  the  secreting  centre  by  means 
of  inter-central  fibres. 

FIG.  64. 


Diagram  of  Nerves  of  the  Submaxillary  Gland.   The  dark 

lines  show  the  course  of  the  nerves  going  to  the  gland, 
(vn)  Portio  dura;  (v)  Inferior  maxillary  division  of  the 
fifth  cranial  nerve;    (o)    Submaxillary  ganglion;    (s) 
Sympathetic  round  facial  artery  (A)  ;   (s.  c.  o.)  Superior 
cervical  ganglion. 

The  stimulating  of  the  olfactory  region  with  certain  odors 
induces  salivation  through  a  channel  of  a  similar  kind  passing 
along  the  olfactory  nerve  to  the  brain,  and  thence  to  the  special 
salivary  centre.  Even  in  the  absence  of  taste  or  smell,  mental 
emotion  may  be  excited  by  seeing  or  thinking  of  food,  and  may 
cause  activity  of  the  salivary  glands,  here  the  inter-central  chan- 
nel is  the  only  one  occupied  in  bearing  the  impulse  to  the  special 
secreting  centre. 


138 


MANUAL   OF   PHYSIOLOGY. 


FIG.  65. 


DUCT. 


Irritation  of  the  gastric  mucous  membrane  stimulates  the  sali- 
vary glands,  as  may  be  seen  with  a  gastric  fistula,  or  by  the 
sudden  flow  of  saliva  which  commonly  precedes  vomiting.  In  this 
case  the  impulses  are  carried  by  the  gastric  branches  of  the  vagus. 
The  stimulation  of  the  central  end  of  the  cut  sciatic  is  said  to 
cause  an  increase  in  the  flow  of  saliva,  so  that  it  would  appear 
that  even  an  ordinary  sensory  nerve  can  excite  the  centre  to 

action.  Lastly,  many  drugs, 
when  introduced  into  the 
blood,  cause  a  flow  of  saliva; 
among  these  are  pilocarpin, 
physostigmin  and  digitalin, 
while  atropin  and  daturin, 
on  the  other  hand,  check 
the  action  of  the  glands. 

From  this  we  learn  that 
the  nerve  centre  controlling 
the  activity  of  the  salivary 
glands  receives  impulses 
from  many  distant  and  di- 
verse sources,  or  may  be 
influenced  directly  by  the 
quality  of  the  blood  flowing 
through  the  nerve  centre 
itself. 

The  channels  traversed  by 
the  efferent  impulses  going  to 
the  salivary  glands  have 

Diagram  of  Nerves  supplying  the  Parotid  Gland,    been     demonstrated     by     CX- 
The  dark  lines  indicate  the   course  of  the  .  T         .,  « 

nerves  of  the  gland.  penmen t.     In    the    case   oi 

(V)  ^S^S^SSS^^lSaJS^  and  its  the  submaxillary,  the  route 

(laG^SupeiioT cervical    ganglion    sending  a   is   especially  distinct  and  in- 
branch  to  the  carotid  plexus  around  the   gtructive,    SO    that    from    this 

gland  we  obtain  most  of  our 

knowledge  concerning  the  direct  influence  of  nerve  impulses  on 
the  gland  cells.  This  question,  therefore,  will  be  treated  some- 
what in  detail. 


S.C.G. 


NERVE    MECHANISM   OF   SALIVARY   SECRETION.  139 

There  are  two  sets  of  nerves  going  to  the  salivary  glands,  one 
belonging  to  the  sympathetic  and  the  other  to  the  cerebro-spinal 
systems,  both  of  which  have  been  proved  to  exert  a  certain 
amount  of  influence  on  the  action  of  the  glands,  the  share  taken 
by  each  apparently  differing  in  different  animals. 

The  sympathetic  branches  for  the  submaxillary  and  sublingual 
gland  come  from  the  plexus  which  embraces  the  facial  artery, 
those  for  the  parotid  come  from  the  plexus  surrounding  the 
internal  maxillary  as  that  artery  traverses  the  gland.  Both  of 
these  nervous  plexuses  are  derived  from  the  superior  cervical 
part  of  the  sympathetic  nerve. 

The  cerebro-spinal  fibres  for  the  submaxillary  and  sublingual 
glands  lie  in  the  complex  nerve  known  as  the  chorda  tympani, 
which  comes  from  the  portio  dura  of  the  seventh,  and  joins  the 
lingual  branch  of  the  fifth.  They  pass  thence  through  the  sub- 
maxillary ganglion  to  the  glands. 

The  cerebro-spinal  parotid  branches  pass  through  the  lesser 
superficial  petrosal  nerve  from  the  tympanic  plexus  to  the  otic 
ganglion,  and  thence  to  the  auriculo-temporal  nerve  which  sends 
twigs  to  the  gland.  (Fig.  65.) 

I. — The  effects  of  experimental  stimulation  of  the  cerebro- 
spinal  glandular  branches  are,  so  far  as  we  know,  alike  for  all 
the  glands.  But  owing  to  the  greater  facility  with  which  the 
submaxillary  gland  can  be  reached,  and  its  nerve  isolated, 
research  has  been  chiefly  devotee^  to  it,  by  operating  on  the 
chorda  tympani  and  the  other  nerves  supplying  the  gland. 

It  has  been  found  that  section  of  this  nerve,  or  of  the  portio 
dura  near  its  origin,  removes  the  possibility  of  exciting  the 
glands  to  action  by  stimulating  the  mouth,  so  that  the  cerebro- 
spinal  and  not  the  sympathetic  are  the  channels  traversed  by  the 
reflected  impulse  on  its  way  to  the  gland  from  its  centre. 

The  reflex  stimuli  which  were  supposed  to  be  elicited  through 
the  medium  of  the  submaxillary  ganglion,  probably  depended 
on  the  escape  of  the  stimulating  electric  current  used,  and  the 
reflexion  from  a  sporadic  ganglion,  such  as  the  submaxillary,  has 
never  been  satisfactorily  demonstrated. 

It  has  further  been  shown  that  direct  stimulation  of  the  chorda 


140  MANUAL   OF    PHYSIOLOGY. 

tympani  nerve,  although  it  be  cut  off  from  its  central  connec- 
tions, causes  a  copious  secretion  of  thin  watery  saliva,  and  this 
increased  secretion  is  accompanied  by  a  great  dilatation  of  the 
small  arteries  going  to  the  gland,  so  that  a  pulsation  may  be  seen 
in  the  small  veins,  and  the  blood  retains  its  bright  arterial  color 
when  leaving  the  organ. 

These  two  chief  results  of  stimulation,  activity  of  the  secreting 
cells  and  vascular  dilatation,  are  distributed  by  different  nervous 
agencies,  as  appears  from  the  action  of  atropia,  which  stops  the 
secretion  of  saliva,  but  does  not  prevent  the  dilatation  of  the 
vessels  on  stimulation  of  the  chorda  tympani ;  from  which  we 
conclude  that  its  effect  is  restricted  to  a  mechanism  engaged 
exclusively  in  controlling  the  activity  of  the  gland  cells. 

Stimulation  of  the  chorda  tympani  causes  the  secretion  to  be 
carried  on  with  great  energy.  The  fluid  was  found  to  enter  the 
duct  with  a  pressure  equal  to  200  mm.  (about  8  inches)  of  mercury, 
while  the  blood  pressure  in  the  carotid  artery  of  the  animal  was 
only  112  mm.  (about  4J  inches)  mercury;  that  is  to  say,  the 
force  by  means  of  which  secretion  is  driven  outward  is  nearly 
twice  as  great  as  the  pressure  in  the  blood  vessels  in  the  gland. 
The  secretion  of  saliva  cannot  then  be  a  question  of  mere  filtra- 
tion, for  if  the  physical  agency — pressure — alone  were  acting, 
the  saliva,  if  produced,  would  be  forced  into  the  lymph  or  blood 
vessels  when  the  pressure  in  the  duct  exceeded  that  in  the 
vessels. 

The  force  and  rate  with  which  the  secretion  is  produced  vary 
with  the  strength  of  the  stimulation.  The  flow  of  saliva  steadily 
increases  within  certain  limits  as  the  stimulus  gets  stronger.  It 
is  not  only  the  quantity  of  the  secretion  that  depends  on  the 
amount  of  nerve  impulse,  but  also  its  quality ;  that  is  to  say, 
with  a  fresh  gland,  not  wearied  by  previous  experiment,  the 
amount  of  solids  in  the  saliva  increases  as  the  stimulus  is 
increased,  so  that  not  only  is  the  activity  of  the  gland  cells  under 
the  control  of  nerve  influence,  but  the  kind  of  work  they  perform 
is  also  regulated  by  the  intensity  of  nerve  impulse  they  receive. 

It  has  been  found  that  the  increase  in  the  blood  flow  is  second- 
ary to  the  secretion  called  forth  by  stimulation  of  the  chorda 


NERVE  MECHANISM    OF   SALIVARY   SECRETION. 


141 


tympani.  This  is  shown  by  the  fact  that  even  when/th^  (blood 
supply  is  cut  off  by  any  means  (strong  sympathetic  |timulation, 
ligature  of  the  vessels,  or  even  decapitation),  an  amount  of  saliva 
can  be  made  to  flow  from  the  gland  which  could  not  have  been 
stored  up  in  its  cells  prior  to  the  stimulation  of  this  nerve. 

II. — With  regard  to  the  influence  exerted  by  the  sympathetic 
branches,  the  most  obvious  result  of  stimulation  of  these  is  a 
contraction  in  the  arterioles,  and  a  consequent  diminution  of  the 
amount  of  blood  flowing  through  the  gland.  The  glands  look 


FIG.  66. 


A. 


Sections  of  Orbital  Gland  of  the  Dog.    (Heidenhain.) 

(A)  After  prolonged  period  of  rest.  (B)  After  a  period  of  activity. 

A)  the  secreting  cells  are  clear,  being  swollen  up  In  (B)  the  accumulated  material  has  been 
ith  mucigen,  and  the  half-moon  cells  are  very  discharged  from  the  cells,  and  the  alveoli 
stinct  and  darkly  stained.  are  shrunken. 

pale,  and  the  blood  leaving  them  is  intensely  venous  in  charac- 
ter ;  the  exact  opposite,  in  fact,  to  the  result  obtained  by  stimu- 
lation of  the  cerebro-spinal  nerves.  But  the  sympathetic  has 
also  an  effect  on  the  gland  cells,  as  it  produces  an  increased  flow 
of  saliva.  In  the  dog  the  secretion  of  "  sympathetic  saliva  "  is 
only  temporary  and  scanty,  having  high  specific  gravity,  and 
being  overloaded  with  the  solids.  In  the  cat  and  rabbit  "  sym- 
pathetic saliva  "  is  scanty,  and  not  thicker  than  the  "  chorda 


142  MANUAL   OF   PHYSIOLOGY. 

saliva"  of  the  same  animal.  So  far  as  regards  the  blood  vessels, 
the  chorda  is  directly  opposed  to  the  sympathetic.  To  explain 
this  antagonism  we  may  either  assume  the  existence  of  local 
nerve  centres  governing  the  muscular  coats  of  the  arterioles,  and 
suppose  that  the  sympathetic  stimulates  and  the  chorda  inhibits 
the  activity  of  these  centres,  or,  what  seems  more  simple,  in  the 
absence  of  anatomical  evidence  that  such  a  centre  exists,  we  may 
attribute  to  the  arterial  muscle  cells  themselves  an  automatic 
tonic  power  of  contraction  which  can  be  increased  by  the  sym- 
pathetic and  diminished  by  the  chorda  tympani.  It  is  singular 
that,  if  all  the  nerves  leading  to  the  gland  be  cut,  a  copious  secre- 
tion of  watery  saliva  begins  after  some  hours,  and  lasts  for  some 
weeks,  after  which  the  cells  undergo  atrophic  changes,  and  the 
gland  becomes  reduced  in  size.  The  explanation  of  the  appear- 
ance of  this  so-called  "  paralytic  saliva"  is  not  clearly  made  out. 
Possibly  the  removal  of  some  trophic  nerve  influences  induces 
abnormal  nutritive  changes  which  cause  stimulation  of  the  cells, 
and  ultimately  lead  to  their  degeneration. 

The  histological  investigation  of  the  elements  of  these  glands 
in  the  various  stages  of  secretion  throws  considerable  light  on 
the  behavior  of  the  cells  during  their  periods  of  activity  and 
rest. 

It  is  now  certain  that  the  different  stages  are  accompanied  by 
constant  structural  changes  in  the  cells,  which  doubtless  are  inti- 
mately connected  with  secretory  activity.  During  the  period  of 
rest,  that  is,  the  time  when  the  gland  is  not  discharging  its  secre- 
tion, the  cells  slowly  undergo  a  change  in  their  appearance,  which 
is  the  more  obvious  in  proportion  to  the  ease  with  which  the 
material  they  secrete  is  recognized  in  the  protoplasm.  Thus,  in 
mucous  glands,  or  in  mucus-yielding  salivary  glands,  the  changes 
are  conspicuous ;  while  in  those  which  give  a  watery  secretion 
they  are  less  easily  seen. 

As  an  example  we  may  take  a  mucous  gland,  such  as  the  orbi- 
tal gland  of  the  dog,  and  follow  the  changes  which  occur  in  one 
of  its  cells,  during  the  period  which  may  be  called  its  cycle  of 
activity.  (Fig.  66.) 

Immediately  after  the  prolonged  and  active  discharge  of  the 


NERVE   MECHANISM   OF   SALIVARY   SECRETION. 


143 


secretion  of  the  gland,  the  cells  have  all  the  characters  of  ordi- 
nary protoplasmic  units,  and  the  distinction  between  the  poly- 
gonal cells  and  those  next  the  wall  of  the  acinus  (demi-lune 
cells)  is  made  out  with  great  difficulty,  because  all  the  cells  stain 
evenly  with  carmine,  and  have  no  special  characters  except  those 
belonging  to  active  protoplasm. 

During  rest  certain  changes  gradually  appear  in  those  gland 
cells  which  are  next  the  lumen  of  the  saccule.  They  appear  to 
swell  toward  the  lumen,  and  at  the  same  time  become  clear  and 
resist  staining  with  carmine,  their  protoplasm  becoming  impreg- 
nated with  mucus-like  material  (mucigen),  while  the  demi-lune 
cells  remain  protoplasmic  and  stain  easily,  and  are  thereby  readily 
distinguished  from  the  cell  in  the  cavity  of  the  saccule. 


FIG.  67. 


Cells  of  the  Alveoli  of  a  Serous  or  Watery  Salivary  Gland.    (Langley.) 

(A)  After  rest.  (B)  After  a  short  period       (c)  After  a  prolonged  period 

of  activity.  of  activity. 

If  the  discharge  of  secretion  be  induced  either  by  normal 
reflex  excitation,  or  by  direct  stimulus  of  the  chorda  tympani 
nerve,  the  cells  discharge  the  contained  specific  material,  some  of 
them  probably  being  destroyed  by  the  act.  If  the  active  secre- 
tion be  continued  for  some  time,  the  cells  return  to  their  former 
protoplasmic  state,  and  those  which  have  been  worn  out  are 
replaced  by  others  from  the  demi-lune  or  marginal  cells. 

In  the  glands  which  do  not  produce  any  mucus  the  brilliant 
look  of  the  cells  after  rest  is  wanting,  but  a  corresponding  change 
occurs.  The  secreting  protoplasm  becomes  extremely  granular 
during  the  resting  period,  and  again  clear  after  the  discharge  of 
the  secretion.  (Fig.  67.) 


144  MANUAL   OF    PHYSIOLOGY. 

Thus  it  would  appear  that  during  the  so-called  period  of  rest, 
when  little  or  no  fluid  is  poured  into  the  duct,  the  gland  cells  are 
busy  at  their  manufacturing  process,  diligently  adding  to  their 
stock  in  hand,  in  order  to  be  ready  for  a  sudden  demand  which 
they  could  not  meet  by  merely  concurrent  work. 

To  sum  up,  then,  we  may  conclude  : — 

1.  That  the  manufacture  of  the  specific   materials  of  the 

secretion  is  accomplished  as  the  result  of  the  intrinsic 
power  of  the  protoplasm  of  the  gland  cells. 

2.  That  a  vital  process  is  called  forth  in  the  gland  cells  by 

the  action  of  nerve  impulses,  because — (a)  The  force 
with  which  the  secretion  is  expelled  cannot  be  accounted 
for  by  the  blood  pressure.  (6)  The  quantity  and  qual- 
ity of  the  secretion  is  modified  by  the  intensity  of  the 
nerve  stimulation,  (c)  The  temperature  of  the  blood 
is  raised,  (d)  Structural  changes  in  the  cells  can  be 
observed. 

3.  The  normal  stimulus  to  secretion  passes  from  the  centre 

in  the  medulla  oblongata  to  the  salivary  glands  along 
cerebro-spinal  nerves. 

4.  This   centre   for  salivary  secretion,  which  at  ordinary 

times  is  moderately  active,  may  be  excited  to  energetic 
action  by  impulses  coming  from  taste,  smell  and  or- 
dinary sensory  nerve  terminals  (particularly  in  the 
mouth),  as  well  as  by  those  which  emanate  from  mental 
emotions. 

CHANGES  UNDERGONE  BY  FOOD  IN  THE  MOUTH. 

Food  when  taken  into  the  mouth  undergoes  two  processes, 
which  are  inseparable  and  simultaneous  in  action ;  viz.,  mastica- 
tion and  insalivation. 

The  mechanism  of  mastication  has  already  been  discussed,  so 
far  as  its  triturating  power  is  concerned.  In  its  final  object  of 
forming  the  subdivided  food  into  a  bolus  which  can  be  easily 
swallowed,  it  is  much  aided  by  insalivation,  particularly  in 
chewing  dry  food ;  and  in  this  latter,  the  moistening  of  the  par- 
ticles, so  as  to  make  them  adhere  together,  is  the  most  necessary 


CHANGES  UNDERGONE  BY  FOOD.  145 

act  of  mouth  digestion,  and  is  next  in  importance  to  the  subdi- 
vision accomplished  by  the  teeth.  The  saliva,  also,  covers  the 
bolus  with  a  coating  of  viscid  fluid,  so  that  it  can  be  more  easily 
propelled  down  the  oesophagus.  Deglutition  of  solids  is  difficult 
without  an  adequate  supply  of  saliva. 

While  in  the  mouth  the  saliva  dissolves  a  great  quantity  of  the 
more  readily  soluble  materials,  such  as  sugar  and  salt,  which 
may  be  either  mingled  with  the  insoluble  substances,  and  swal- 
lowed together  with  the  bolus,  or  separately  in  a  fluid  form. 
Solution,  then,  is  an  important  item  in  mouth  digestion. 

In  many  carnivorous  animals  the  use  of  the  mouth  fluid  is 
chiefly  mechanical,  dissolving  some  insignificant  part  of  the  food, 
and  aiding  mastication  and  deglutition.  In  man,  however,  and 
other  animals  that  make  use  of  much  vegetable  food,  it  has  a 
chemical  function,  and  acts  on  the  insoluble  starch,  converting 
it  into  soluble  sugar. 

The  active  principle  which  brings  about  this  change  is  Ptyalin. 
This  is  one  of  a  series  of  ferments  to  which  most  of  the  chemical 
changes  in  digestion  are  due. 

As  a  group  they  are  remarkable  for  the  following  characters 
in  which  they  differ  from  most  chemical  agents:  (1)  They  effect 
alterations  in  the  substances  on  which  they  act,  while  they  them- 
selves do  not  undergo  any  perceptible  change  or  diminution. 
(2)  They  exist  in  such  small  quantities  that  as  a  rule  their  pre- 
sence can  only  be  shown  by  the  effects  they  produce.  (3)  They 
are  most  active  at  the  body  temperature,  but  are  "killed"  by 
that  at  which  albumen  coagulates. 

Ptyalin  acts  on  starch,  and  hence  is  spoken  of  as  an  amylolytic 
ferment ;  its  action  consists  in  causing  the  starch  to  unite  chem- 
ically with  one  molecule  of  water,  thus : — 

C6H1005  +  H20  =  C6H1206 

Starch.  Grape  Sugar. 

During  this  process,  which  takes  at  the  least  a  few  minutes  to 
complete,  various  stages  can  be  detected :  first,  two  substances  are 
formed  which  together  are  commonly  spoken  of  as  dextrin  ;  one, 
erythro-dextrin,  which  gives  a  red  color  with  iodine,  and  easily 
passes  into  soluble  sugar;  and  the  other,  achroo-dextrin,  gives 
13 


146  MANUAL   OF   PHYSIOLOGY. 

no  color  with  iodine,  and  is  with  difficulty  converted  into  sugar. 
As  it  gives  no  color  with  the  ordinary  test,  its  presence  is  often 
overlooked. 

The  sugar  thus  formed  has  been  called  Ptyalose,  which  can  be 
converted  into  ordinary  grape  sugar  (glucose)  by  the  action  of 
sulphuric  acid.  Some  say  the  product  is  maltose. 

The  presence  of  starch,  either  in  its  soluble  or  insoluble  form, 
is  easily  recognized  by  the  blue  color  given  by  free  iodine,  which 
color  disappears  on  heating  to  about  100°  C.,  but  reappears  on 
cooling. 

Very  many  tests  have  been  recommended  for  the  detection  of 
sugar.  The  most  generally  applicable  one  is  Trommer's.  The 
liquid  is  made  strongly  alkaline  with  potash,  and  a  few  drops  of 
a  dilute  solution  of  cupric  sulphate  is  added,  a  clear  blue  solution 
results,  which,  on  being  raised  to  the  boiling  point,  deposits  an 
orange  precipitate  of  cuprous  oxide.  Fehling's  and  Pavy's  solu- 
tions are  modifications  of  the  above  test  adapted  for  quantitative 
analysis. 

When  yeast  is  added  to  a  solution  of  grape  sugar,  the  sugar  is 
converted  into  alcohol  and  carbon  dioxide.  This  may  be  seen  in 
an  inverted  test  tube.  The  CO2  rises  to  the  top,  and  can  be  used 
as  an  indication  of  the  quantity  of  sugar  present.  Experiments 
may  be  carried  out  with  saliva  obtained  directly  from  any  of  the 
glands,  but  the  mixture  of  the  secretion  of  all  is  found  to  be 
more  efficacious  than  that  of  any  single  one.  The  ordinary 
mouth  fluid,  filtered,  serves  well  for  ordinary  experiments. 

An  effective  glycerine  solution  of  ptyalin  may  be  obtained  by 
steeping  chopped  salivary  glands  in  alcohol,  and  then  extracting 
for  some  days  with  glycerine  and  water. 

The  following  facts  must  be  borne  in  mind  concerning  the 
amylolytic  action  of  ptyalin : — 

1.  The  extremely  small  amount  of  the  ferment  required  to 

make  the  fluid  effective. 

2.  There  is  no  appreciable  diminution  in  the  amount  of  fer- 

ment, so  that  it  cannot  be  said  to  be  used  up  in  the 
process. 

3.  The  action  takes  place  most  readily  in  alkaline  solutions, 


FUNCTIONS   OF   THE   SALIVA.  147 

such  as  the  saliva,  slowly  in  neutral  solution,  and  not 
at  all  in  acids  of  the  strength  of  .2  per  cent,  of  hydro- 
chloric acid. 

4.  Temperature  has  a  marked  effect  on  the  process.     Cold 

(0°  C.),  quite  checks  the  action ;  heat  (75°  C.)  destroys 
the  power  of  the  ferment,  which  is  most  active  at  the 
body  temperature  (35°-40°  C.). 

5.  Strong  acids  or  alkalies  destroy  the  amylolytic  power  of 

ptyalin. 

6.  The  ferment  has  but  little  effect  on  raw  starch,  its  cel- 

lulose coating  protecting  it ;  but  it  acts  rapidly  on  well- 
boiled  starch. 

7.  Ptyalin  is  more  active  in  weak  starch  solutions,  and  is 

much  impeded  in  its  action  by  an  accumulation  of 
sugar. 

To  recapitulate,  we  find  that  the  following  changes  take  place 
in  the  mouth : — 

(1)  Solid  food  is,  or  should  be,  finely  subdivided ;  (2)  dry 
food  is  moistened,  (3)  rolled  into  a  bolus,  (4)  and  lubri- 
cated ;  (5)  the  soluble  part  is  dissolved,  and  rendered 
capable  of  being  tasted ;  (6)  and  part  of  the  indiffusible 
starch  is  converted  into  soluble  diffusible  sugar  by  the 
action  of  a  ferment  called  ptyalin. 

In  the  short  time  occupied  by  the  passage  of  food  through  the 
oesophagus  no  special  change  takes  place  in  it,  so  we  may  pass 
at  once  to  the  gastric  digestion,  which  will  occupy  the  next 
chapter. 


148 


MANUAL    OF    PHYSIOLOGY. 


FIG.  68. 


CHAPTER  VIII. 
STOMACH  DIGESTION. 

The  surface  of  the  stomach  is  covered  by  a  single  layer  of 
cylindrical  epithelial  cells  which  also  line  the  orifices  of  the 
numerous  glands  with  which  the  mucous  membrane  is  thickly 

studded.  This  single  layer  of 
cylindrical  cells  commences  ab- 
ruptly at  the  cardiac  orifice  of 
the  stomach,  and  is  marked  off 
from  the  stratified  squamous 
cells  lining  the  oesophagus  by 
a  sharp  line  of  demarcation. 
The  glands  of  the  stomach  are 
tubes  with  conical  orifices  which 
often  divide  into  two  or  three 
tubular  prolongations.  The  out- 
let or  orifice  is  covered  by  the 
common  cylindrical  epithelium 
of  the  surface  of  the  stomach, 
and  the  fundus  is  filled  with 
specific  granular  cells.  The 
glands  dip  down  to  the  delicate 
eub mucous  tissue,  the  branching 
tubes  lying  parallel  and  exceed- 
ingly close  together.  A  dense 
network  of  capillary  blood  ves- 
sels may  be  demonstrated  by 

injection  to  surround  the  tubes  and  closely  invest  the  thin  base- 
ment membrane  which  forms  the  boundary  of  the  glands  and  the 
^basis  of  attachment  of  the  glandular  cells.  A  close-meshed  net- 
work of  absorbent  vessels  also  surrounds  the  tubules  of  the 
glands,  and  leads  to  the  larger  vessels  in  the  submucous  tissue. 
In  the  cardiac  end  of  the  stomach  two  distinct  kinds  of  cells 


Diagram  of  a  Section  of  the  Wall  of  the 
Stomach. 

a.  Orifices   of  glands    with   cylindrical 

epithelium. 

b.  Fundus  of  glands  with  spherical  and 

oval  epithelium. 

c.  Tunica  mnscularis  mucosae. 

d.  Submucous    tissue   containing   blood 

vessels,  etc. 

e.  Circular,  (/)  oblique,  and  (g)  longi- 

tudinal muscle  coats. 
h.  Serous  membrane. 


1 


STOMACH    DIGESTION. 


149 


are  found  in  the  deeper  part  of  the  gland  tubes.  Much  the 
more  numerous  are  small,  pale,  spheroidal  cells,  which  occupy 
the  lumen  of  the  gland  and  form  the  regular  cell  lining  of  its 
cavity.  These  cells  have  been  called  the  "  chief  cells  "  (Haupt- 
zellen),  "  central "  or  spheroidal  cells. 

The  cells  of  the  other  form  are  comparatively  few,  being  alto- 
gether wanting  in  some  of  the  glands.  They  are  larger  and 
more  striking  than  the  central  or  spheroidal  cells  between  which 

FIG.  69. 


Diagram  showing  the  relation  of  the  ultimate  twigs  of  the  blood  vessels  (y  and  A),  and 
of  the  absorbent,  radicals  (i,)  to  the  glands  of  the  stomach,  and  the  different  kinds 
of  epithelium,  viz.,  above  cylindrical  cells;  small,  pale  cells  in  the  lumen,  outside  of 
which  are  the  dark  ovoid  cells. 

and  the  basement  membrane  they  lie  scattered  here  and  there 
over  the  fundus  of  the  gland,  making  the  delicate  membrane 
bulge.  They  stain  more  easily,  and  have  darker  granules  than 
the  central  cells.  On  account  of  their  position  they  have  been 
called  "  parietal,"  "  marginal  or  border  cells  "  (Belegzellen),  and 
from  their  oval  shape,  which  equally  well  distinguishes  them 
from  the  other,  "  ovoid  cell*."  (See  Fig.  69.) 


150  MANUAL   OF    PHYSIOLOGY. 

There  is  a  different  class  of  glands,  the  so-called  mucous,  found 
chiefly  near  the  pyloric  end  of  the  stomach,  in  which  there  is 
but  one  kind  of  cell  throughout,  and  this  seems  to  differ  in  char- 
acter from  both  the  varieties  in  the  other  glands,  resembling 
rather  the  cylindrical  epithelium  covering  the  surface  of  the 
stomach  and  dipping  into  the  conical  orifices  which  lead  to  the 
glands. 

The  difference  between  the  two  kinds  of  glands  found  in  the 
stomach,  both  as  regards  their  distribution  and  way  of  branching, 
and  the  cells  which  line  the  deeper  parts  of  the  tubes,  is  found 
to  vary  in  different  animals.  The  difficulty  of  obtaining  fresh 
specimens  of  the  human  stomach  makes  it  still  uncertain  whether 
the  same  differences  exist  in  the  human  subject.  The  varieties 
of  opinion  and  drawings  published  suggest  that  various  stages  of 
gradation  from  one  kind  of  gland  to  another  are  met  with  in  the 
stomach  of  even  the  same  animal. 

Experimental  research  does  not  show  decisively  that  the  ana- 
tomical differences  denote  differences  of  function. 

CHAKACTERS   OF   GASTRIC   JUICE. 

The  gastric  juice  is  a  clear,  colorless  fluid  with  strongly  acid 
reaction.  It  contains  .5  per  cent,  of  solids,  its  specific  gravity 
being  1002.  The  amount  secreted  in  the  day  is  extremely  vari- 
able, and  depends  upon  the  quantity  and  character  of  the  food  ; 
in  well-fed  dogs  it  has  been  estimated  to  be  one-tenth  of  the  body 
weight. 

It  contains : — 

1.  About  .2  per  cent,  of  free  hydrochloric  acid  in  man,  but 

in  the  dog  considerably  more.  The  lactic,  formic, 
butyric,  and  other  acids  which  have  been  found  in  the 
gastric  juice  probably  depend  on  the  decomposition  of 
some  of  the  ingesta. 

2.  Pepsin,  the  specific  substance  which  gives  the  gastric  juice 

its  digestive  qualities,  is  a  nitrogenous  ferment  which, 
with  the  foregoing  acid,  acts  on  proteids.  About  .3 
per  cent,  is  present  in  the  secretion  of  the  human 
stomach. 


GASTRIC   SECRETION.  151 

3.  Associated  with  the  pepsin  are  other  less-known  ferments, 

one  of  which  curdles  milk  without  the  presence  of  any 
acid. 

4.  A  variable  quantity  of  mucus  is  found  in  the  secretion  of 

the  stomach. 

5.  It  contains  .2  per  cent,  of  inorganic  salts,  chiefly  chlorides 

of  sodium,  potassium  and  calcium. 

Method  of  Obtaining  Gastric  Secretion. — Formerly,  attempts 
were  made  to  obtain  gastric  juice  by  inducing  a  dog,  while  fast- 
ing, to  swallow  a  sponge,  and  withdrawing  it  when  saturated 
with  the  gastric  secretion ;  or  a  fasting  dog,  allowed  to  swallow 
insoluble  materials,  was  killed,  and  the  secretion  collected  from 
the  stomach. 

It  is  best  obtained  directly  from  a  fistulous  opening  in  the 
abdominal  wall  communicating  with  the  stomach.  A  gastric 
fistula  was  first  made  accidentally  in  a  man  by  injury.  A  case 
in  which  the  surgical  treatment  of  a  gunshot  wound  of  the 
stomach  left  a  permanent  fistula,  allowed  the  gastric  secretion 
to  be  carefully  investigated,  and  proved  a  valuable  subject  for 
experimental  research. 

It  is  not  a  difficult  matter  to  reach  the  stomach  by  making  an 
artificial  opening  through  the  wall  of  the  abdomen,  and,  having 
brought  the  serous  surface  of  the  gastric  wall  into  firm  connection 
with  the  serous  lining  of  the  abdominal  wall,  to  open  the  stomach. 
The  juxtaposition  of  the  parts,  as  well  as  the  patency  of  the 
fistula,  can  be  secured  by  a  suitable  flanged  cannula  closed  with 
a  well-fitting  cork.  By  removing  the  cork  the  gastric  juice  may 
be  obtained  in  small  quantities,  and  various  kinds  of  food  may 
be  introduced  through  the  cannula,  and  the  changes  occurring 
in  them  studied. 

For  experimental  purposes  an  artificial  gastric  juice  may  be 
used.  This  can  be  made  from  the  gastric  mucous  membrane  of 
a  dead  animal  (pig)  by  extracting  the  pepsin  from  the  finely- 
divided  glandular  membrane,  with  a  weak  acid  (less  than  .2  per 
cent.)  or,  better,  with  a  large  quantity  of  glycerine,  and  subse- 
quently adding  HC1  to  the  extent  of  .2  per  cent. 


152  MANUAL    OF   PHYSIOLOGY. 

MODE   OF   SECRETION. 

The  gastric  juice  is  not  secreted  in  large  quantity  when  the 
stomach  is  empty,  but  only  when  the  mucous  membrane  is  irri- 
tated with  some  chemical  or  mechanical  stimulus.  The  swal- 
lowing of  alkaline  saliva  acts  as  a  gentle  stimulus  and  causes 
secretion,  so  that  the  surface  of  the  stomach  becomes  acid.  When 
the  lining  membrane  of  the  stomach  is  mechanically  stimulated 
through  a  fistula  it  becomes  red,  and  drops  of  secretion  appear 
at  the  point  of  stimulation,  but  the  amount  of  secretion  thus 
produced  is  very  scanty  when  compared  with  that  called  forth 
by  chemical  irritants. 

Thus,  ether,  alcohol  and  pungent  condiments  produce  copious 
secretion*.  Weak  alkaline  solutions  also  cause  secretion,  but  the 
most  perfect  form  of  stimulant  seems  to  be  a  mass  of  food  satu- 
rated with  the  alkaline  saliva. 

In  all  probability  the  secretion  of  the  gastric  juice  is  under  the 
control  of  a  special  nerve  mechanism,  and  the  way  in  which  the 
state  of  activity  follows  stimulation  of  the  part  seems  to  point  to 
its  being  a  simple  reflex  act.  However,  the  nervous  connections 
(vagi  and  splanchnics)  between  the  stomach  and  central  nervous 
system  may  all  be  severed  without  any  marked  effect  on  the 
secretion,  other  than  that  which  would  naturally  follow  the 
changes  in  the  amount  of  blood  supply,  which,  of  course,  is  greatly 
altered  by  cutting  the  vasomotor  nerves  —  the  splanchnics. 
Whether  this  be  so  or  not,  there  must  be  some  connection  with 
the  nerve  centres,  for  sudden  emotions  check  the  secretions,  and 
the  sensations  caused  by  the  sight  or  smell  of  food  give  rise  to 
gastric  secretion. 

It  has  been  suggested  that  Meissner's  submucous  ganglionic 
network  may  act  as  a  reflex  centre  and  regulate  the  secretion. 
But  as  the  reflection  from  local  ganglionic  centres  has  not  yet 
been  definitely  demonstrated,  we  are  hardly  entitled  to  assume 
that  it  occurs  here,  and  since  the  stimulus  comes  into  close  con- 
tiguity with  the  secreting  cells,  it  seems  quite  as  probable  that 
these  elements  are  excited  to  activity  by  direct  stimulation  of 
their  protoplasm. 

As  in  the  salivary  glands,  so  in  the  gastric  tubes,  the  cells 


GASTRIC   SECRETION.  153 

show  some  structural  changes  which  accompany  with  great  regu- 
larity their  periods  of  rest  and  activity,  and  therefore  may  be 
concluded  to  be  the  indications  of  the  internal  processes  belong- 
ing to  the  production  of  the  specific  materials  of  the  secretion. 

It  appears  probable  that  the  chief  secretory  activity  resides  in 
the  small  central  cells,  and  not  in  the  large  ovoid  border  cells, 
since  no  distinct  changes  can  be  seen  in  the  latter,  and  the 
smaller  gland  cells  seem  to  contain  the  pepsin;  for  if  the  mucous 
membrane  be  treated  with  weak  hydrochloric  acid,  these  central 
gland  cells  are  rapidly  dissolved  by  a  process  of  digestion,  while 
the  border  cells  simply  swell  up  and  become  more  transparent. 
So  that  the  outer  ovoid  cells  have  no  title  to  their  former  name 
of  "  peptic  cells." 

The  central  cells  of  the  gastric  glands  are  finely  granular,  pale, 
protoplasmic  masses,  and  continue  so  dtfring  the  time  when  the 
stomach  is  empty  and  the  glands  not  secreting.  In  the  earlier 
stages  of  digestion  these  cells  swell  up  and  become  turbid  and 
coarsely  granular,  and  stain  more  readily  with  the  aniline  dyes. 
As  the  digestive  process  goes  on  the  cells  again  diminish  in  size, 
but  are  found  to  contain  a  large  quantity  of  peculiar  granules, 
which  are  discharged  from  the  cell  before  its  return  to  the  ordinary 
state  of  rest.  The  cells  are  said  to  be  rich  in  pepsin  in  propor- 
tion to  their  size;  when  swollen  during  active  digestion  they 
contain  much  pepsin,  when  small,  during  hunger,  they  contain 
but  little. 

It  would  therefore  appear  that  the  pepsin  of  the  gastric  juice 
is  produced  as  a  distinct  and  new  manufacture  by  the  central 
cells  of  the  peptic  glands,  and  not  by  the  other  cells.  Structural 
changes  have  also  been  followed  out  in  the  so-called  mucous 
glands  and  in  glands  without  any  of  the  ovoid  border  cells, 
which,  taken  with  the  fact  that  the  alkaline  secretion  of  the 
pyloric  end  of  the  stomach,  where  the  mucous  glands  abound, 
is  capable  of  rapidly  digesting  proteid  if  acid  be  added  to  it, 
tends  to  show  that  in  these  so-called  mucous  glands  pepsin  is  also 
produced. 

The  acid  is  found  chiefly  on  the  surface  of  the  stomach.  The 
mode  of  its  production  seems  distinct  from  that  of  pepsin,  but  is 


154  MANUAL    OF    PHYSIOLOGY. 

not  well  understood.  Possibly  the  surface  epithelial  cells  store 
up  in  their  protoplasm  and  render  inert  the  small  quantities  of 
HC1  which  are  constantly  being  set  free  from  the  NaCl  by  the 
action  of  the  newly-formed  weak  organic  acids  (lactic,  etc.).  The 
amount  of  HC1  thus  slowly  accumulated  in  time  becomes  con- 
siderable and  is  discharged  by  the  cells  at  appropriate  periods. 

Although  the  fact  that  the  deeper  part  of  the  glands  do  not 
give  an  acid  reaction,  while  the  neck  and  orifices  of  the  gland 
are  distinctly  acid,  would  support  the  former  view,  there  is  some 
reason  for  believing  that  the  manufacture  of  acid  from  the  alka- 
line blood  is  really  an  active  process  carried  out  by  some  gland- 
ular cells. 

It  has  been  suggested  that  the  cell  elements  which  produce  the 
acid  are  the  ovoid  border  cells,  from  whence  it  rapidly  passes  to 
the  orifice  of  the  glands.  This  view  is  supported  by  the  alka- 
linity of  the  pyloric  end  of  the  stomach  where  the  border  cells 
are  not  found.  In  some  animals  the  distinct  distribution  of  the 
different  cell  elements  and  the  accompanying  reaction  of  the 
secretion  are  well  marked. 

ACTION  OF  THE  GASTRIC  JUICE. 

The  gastric  juice  has  in  the  absence  of  mucus  no  effect  on  the 
carbohydrates,  and  probably  the  amylolytic  fermentation  set  up 
by  the  saliva  is  impeded,  if  not  completely  checked,  by  the  free 
acid  in  the  stomach  as  soon  as  the  bolus  is  moistened  by  the 
gastric  fluid. 

Fats  are  not  affeqted  by  the  gastric  juice,  but  are  simply  melted 
in  the  stomach. 

Upon  the  albuminous  bodies  the  gastric  digestion  produces 
a  marked  effect.  The  proteids  being  colloid  bodies,  cannot 
readily  pass  through  an  animal  membrane  by  the  process  called 
dialysis  ;  it  has  therefore  been  assumed  that  they  cannot  be  ab- 
sorbed through  the  lining  membrane  of  the  stomach.  They  are 
often  eaten  in  an  insoluble  form.  To  convert  the  insoluble  and 
indiffusible  albumins  into  a  soluble  and  diffusible  substance 
would  obviously  be  a  great  step  toward  their  absorption.  This 
Dower  is  ascribed  to  the  gastric  juice.  The  steps  of  the  process 


GASTRIC    DIGESTION.  155 

may  be  accurately  followed  in  a  suitable  glass  vessel,  irrespective 
of  the  stomach,  by  using  artificial  gastric  juice,  and  attending  to 
the  various  conditions  necessary  for  its  action.  The  power  of 
artificial  gastric  juice  carefully  prepared  from  the  mucous  mem- 
brane of  an  animal's  stomach  differs  in  no  essential  respect  from 
that  of  the  natural  secretion  in  the  stomach,  if  all  the  circum- 
stances which  aid  the  action  of  the  gastric  ferments  be  applied 
in  the  experiment.  This  action  consists  in  a  conversion  of  co- 
agulated albumins  into  the  peculiar,  soluble  and  more  diffusible 
form  of  proteid  known  as  "  peptones." 

The  change  is  not  effected  immediately,  but  certain  stages  may 
be  recognized  in  which  the  two  chief  constituents  of  the  gastric 
juice,  the  acid  and  the  pepsin,  seem  to  have  special  parts  to  play. 

Shortly  after  the  introduction  of  a  proteid,  such  as  boiled 
fibrin,  into  gastric  fluid  at  the  temperature  of  the  body,  the 
masses  of  fibrin  swell  up,  become  transparent,  and  eventually  are 
easily  shaken  to  pieces  and  dissolved. 

The  first  step  in  the  process  seems  to  be  brought  about  by  the 
free  acid,  and  consists  in  the  formation  of  acid  albumin.  This 
can  be  shown  by  neutralizing  the  fluid  during  the  process  and 
thereby  causing  a  precipitate  of  acid  albumin.  The  amount  of 
this  precipitate  will  depend  upon  how  far  the  conversion  into 
peptone — which  is  not  precipitated  by  neutralization — has  pro- 
gressed. Thus,  in  the  earlier  stages,  nearly  all  the  proteid  used 
will  be  thrown  down  by  neutralization,  while  only  a  compara- 
tively small  amount  is  precipitated  in  the  later  stages. 

The  formation  of  acid  albumin  may  be  effected  with  weak  acid 
without  the  other  constituents  of  the  gastric  juice,  and  therefore 
the  preliminary  step  may  be  attributed  to  the  unaided  action  of 
the  acid  ;  but  since  this  stage  in  the  formation  of  peptone  is  con- 
stant, and  the  material  may  possibly  be  distinguishable  from  the 
ordinary  acid  albumin,  it  has  been  called  parapeptone. 

While  the  parapeptone  is  being  formed  by  the  acid,  the  pepsin 
is  engaged  in  changing  it  into  the  final,  soluble,  diffusible  and 
uncoagulable  product — peptone.  The  pepsin  by  itself  cannot 
convert  proteid  into  peptone,  as  may  be  seen  in  the  want  of  effi- 
cacy of  a  neutral  solution  of  pepsin,  in  which  neither  peptone 


156  MANUAL   OF   PHYSIOLOGY. 

nor  parapeptone  is  formed.  In  other  words,  pepsin  solution  can 
only  change  parapeptone  or  acid  albumin  into  peptone.  It 
would  appear  probable,  however,  that  it  possesses  this  property  to 
an  unlimited  extent,  since  it  undergoes  no  change  itself,  and  with 
fresh  supplies  of  acid  a  very  minute  quantity  of  pepsin  can  con- 
vert an  indefinite  amount  of  proteid  into  peptone. 

The  rapidity  with  which  proteid  is  converted  varies  according 
to  the  circumstances  under  which  it  is  placed  as  well  as  the  kind 
of  proteid  used.  If  the  same  proteid  be  used,  the  following  cir- 
cumstances will  be  found  to  influence  the  rapidity  of  the  pro- 
cess : — 

1.  The    temperature.      As    already    stated,    the    optimum 

degree  of  heat  for  the  change  is  about  that  of  the 
body,  35°-40°  C. 

The  activity  of  the  gastric  juice  diminishes  when  the 
temperature  rises  above  or  falls  below  this  standard. 
The  minimum  at  which  it  is  capable  of  producing  any 
effect  is  about  1°  C.  and  the  maximum  is  below  70°  C. 
Boiling  permanently  destroys  the  function  of  pepsin. 

2.  The  percentage  of  acid  as  well  as  the  kind  of  acid  has  a 

marked  effect.  Though  the  action  will  go  on  with  other 
acids,  hydrochloric  is  the  most  effective,  and  that  of 
a  strength  of  .2  per  cent. 

3.  A  condensed  solution  of  peptone  or  large  quantities  of 

salts  in  solution  impede  the  action,  a  certain  degree  of 
dilution  being  necessary  for  the  process.  In  strong 
solutions  of  proteid,  the  peptones  must  be  removed  by 
dialysis  in  order  to  allow  of  the  continuance  of  the 
action.  This  occurs  in  the^stomach  by  means  of  the 
blood  and  absorbent  vessels. 

4.  The  degree  of  subdivision  to  which  the  proteid  has  been 

subjected  materially  influences  the  rapidity  of  its  con- 
version into  peptone.  The  more  finely  subdivided  the 
substance  the  greater  will  be  the  relative  extent  of  sur- 
face exposed  to  the  action  of  the  digestive  fluids.  When 
large  masses  of  coagulated  albumen,  such  as  boiled 
white  of  egg,  are  introduced  into  the  stomach,  the  gas- 


GASTRIC    DIGESTION.  157 

trie  fluid  cannot  reach  the  central  portions,  and  their 
digestion  must  await  the  completion  of  that  of  the 
exterior  part. 
5.  Motion  aids  the  action  of  the  foregoing  factors. 

All  these  requisites  are  present  during  normal  digestion. 

The  temperature  of  the  stomach  is  38°  C.  (=  100°  R).  Hy- 
drochloric acid  is  present  in  the  proportion  of  about  .2  per  cent. 
As  quickly  as  the  peptones  are  formed  they  can  be  removed  by 
absorption  from  the  stomach,  and  thus  the  needful  dilution  is 
accomplished.  Finally,  if  the  mouth  has  done  its  duty,  the 
pieces  of  proteid  have  been  reduced  to  a  pulp,  composed  of 
minute  particles.  These  are  kept  in  constant  motion  by  the  gas- 
tric walls,  and  thus  are  repeatedly  brought  in  contact  with  fresh 
supplies  of  the  digestive  fluid. 

There  can  be  little  doubt  that  the  conversion  of  proteid  into 
peptone  is  normally  brought  about  by  the  pepsin,  which  acts  as 
a  ferment,  in  some  way  or  other  facilitating  a  process  which 
without  it  is  extremely  difficult  to  accomplish.  Proteids  may, 
however,  give  rise  to  peptone  without  the  presence  of  any  pepsin, 
if  they  be  treated  with  strong  acids,  alkalies,  boiling  under  high 
pressure,  putrefactive  and  other  fermentative  actions.  This, 
together  with  the  analogy  suggested  by  the  chemical  details  of 
the  amylolytic  action  of  saliva,  which  one  may  say  depends  on  a 
molecule  of  water  being  taken  up,  suggests  that  the  change  of 
proteid  into  peptone  is  also  hydrolytic,  the  peptones  being  simply 
an  extremely  hydrated  form  of  proteid.* 

So  far  we  have  found  that  the  action  of  the  gastric  juice  affects 
proteids  alone.  Its  action  on  other  constituents  of  food  varies. 
Gelatinous  material  is  dissolved  by  the  gastric  digestion  and  ren- 
dered incapable  of  forming  a  jelly  ;  its  conversion  into  peptone 


*  Though  proteids  will  not  diffuse  through  a  dead  animal  membrane  when  distilled 
water  is  used,  a  fair  amount  of  diffusion  takes  place  if  a  suitable  solution  of  common 
salt  be  employed  instead  of  water.  It  must  also  be  remembered  that  the  gastric  mucous 
membrane  is  a  living,  active  structure,  and  that  the  fluid  into  which  the  albumins  have 
to  diffuse  may  be  regarded  as  a  salt  solution.  It  is  therefore  quite  probable  that  a  con- 
siderable quantity  of  albumin  may  be  absorbed  as  such.  The  fact  that  peptone  cannot 
be  found  in  any  quantity  in  chyle  or  portal  blood  tends  to  prove  that  the  albumin  does 
pass  through  the  stomach  wall  without  being  changed  into  peptone. 


158  MANUAL    OF   PHYSIOLOGY. 

has,  however,  not  been  established.  The  connective  tissue  of 
meat  and  adipose  tissue  is  therefore  soon  removed,  and  the  muscle 
fibres  fall  asunder,  the  sarcolemma  is  dissolved,  and  the  muscle 
substance  converted  into  true  peptone.  The  delicate  sheets  of 
elastic  tissue,  such  as  basement  membranes  and  those  of  small 
vessels,  are  dissolved,  but  larger  masses  of  yellow  elastic  tissue 
are  not  affected  by  the  gastric  digestion.  The  horny  part  of  the 
epidermis,  hairs,  etc.,  are  quite  unaltered,  and  also  the  mucus, 
which  passes  along  the  alimentary  tract  without  change.  Bone 
dissolves  slowly,  the  animal  part  being  attacked  at  the  surface 
by  the  gastric  juice  and  the  acid  slowly  removing  the  salts. 

The  action  of  the  gastric  juice  on  milk  is  peculiar.  On  reach- 
ing the  stomach,  milk  is  curdled  by  a  special  ferment  formed  in 
the  gastric  mucous  membrane.  This  ferment,  known  as  "  Ren- 
net," is  made  from  the  stomach  of  the  calf,  and  used  in  the 
manufacture  of  cheese.  The  precipitation  of  the  casein  (alkali 
albumin),  which  gives  rise  to  the  curdling  of  the  milk,  is  not 
brought  about  by  the  hydrochloric  acid  (although  the  acidity 
would  be  quite  sufficient),  because  neutralized  gastric  juice  has 
the  same  effect.  It  appears  that  a  special  ferment  (not  pepsin) 
which  directly  affects  the  casein  and  causes  its  coagulation,  must 
exist.  It  is  not  due  to  common  lactic  ferment,  for  though  lactic 
acid  is  produced,  it  is  formed  too  slowly  to  account  for  the  very 
rapid  coagulation  of  milk  which  occurs  in  the  stomach. 

The  gastric  juice  has  little  effect  on  vegetable  food  in  general, 
though  well-masticated  bread  may  be  very  materially  altered, 
owing  to  the  action  of  the  saliva  on  the  starch  continuing  until 
the  mass  is  broken  up,  and  the  gastric  juice  then  dissolving  the 
proteids  (gluten).  The  greater  part  of  the  substance  of  bread, 
however,  leaves  the  stomach  in  an  imperfectly  digested  state. 

In  short,  the  amount  of  change  which  any  given  form  of  food 
will  undergo  in  the  stomach  will  depend  on  the  amount  and 
exposed  condition  of  the  proteid  it  contains. 

In  recapitulating  the  chief  events  of  gastric  digestion,  it  must  be 
remembered  that  while  the  food  is  yet  in  the  mouth  the  secretion 
of  the  gastric  juice  commences,  and  is  greatly  increased  by  the 
arrival  of  a  bolus  of  food  and  a  quantity  of  frothy  alkaline  saliva. 


GASTRIC    DIGESTION.  159 

As  the  stomach  is  filled,  more  and  more  secretion  is  produced, 
and  as  some  food  is  absorbed  an  additional  stimulus  is  applied. 
Being  kept  in  motion  in  a  large  quantity  of  liquid  which  dis- 
solves the  cases  in  which  the  food  particles  are  contained,  the 
bolus  of  food  soon  falls  asunder  and  each  of  its  ingredients  is 
fully  exposed  to  the  action  of  the  gastric  juice.  The  acid  reac- 
tion of  the  gastric  fluid  neutralizes  the  alkalinity  of  the  saliva, 
so  that  the  action  of  the  ptyalin  is  hindered,  and  the  starch 
granules  float  about  quite  unaffected  by  the  pepsin  or  hydro- 
chloric acid.  The  heat  of  the  stomach  melts  the  fats,  and  the 
motion  breaks  up  the  oily  fluid  into  smaller  masses.  They  are 
then  mingled  with  the  general  liquid,  which  becomes  more  and 
more  turbid  owing  to  the  admixture  of  starch  granules,  fat  glo- 
bules, dissolved  parapeptones,  and  minute  particles  of  partially 
digested  proteids.  This  dull-gray,  turbid  fluid  is  called  chyme. 
The  proteids  (the  class  of  food  stuffs  affected  by  the  gastric 
digestion)  are  changed  more  or  less  rapidly  according  as  their 
particles  are  small  and  uncovered,  or  large  and  massed  together, 
so  that  they  are  more  or  less  readily  reached  by  the  gastric  juice, 
and  also  in  proportion  to  the  facility  with  which  they  form  acid 
albumin.  The  chyme  contains  but  little  peptones,  so  we  may 
conclude  that,  when  formed,  they  are  rapidly  absorbed,  as  are 
also  the  soluble  sugar  and  ordinary  fluids  taken  with  the  food. 
The  chyme  begins  to  leave  the  pylorus  soon  after  gastric  diges- 
tion has  begun,  some  passing  into  the  duodenum  in  about  half  an 
hour.  The  materials  which  resist  the  gastric  secretion,  or  are 
affected  very  slowly  by  it,  are  retained  many  hours  in  the 
stomach,  and  the  pylorus  may  refuse  exit  to  such  materials  for 
an  indefinite  time,  so  that  after  causing  much  uneasiness  they 
are  finally  removed  by  vomiting.  However,  many  solid  masses, 
unchewed  vegetables,  etc.,  escape  through  the  pylorus  when  it 
opens  to  let  out  the  chyme. 


160  MANUAL    OF    PEIYSIOLOGY. 


CHAPTER  IX. 

PANCREATIC  JUICE. 

The  copious  secretions  of  two  of  the  largest  glands  of  the  body 
— the  pancreas  and  the  liver — are  poured  into  the  duodenum. 
This  is  the  widest  part  of  the  small  intestine,  and  the  extent  of 
the  surface  of  its  lining  membrane  is  increased  by  crescentic, 
shelf-like  projections  called  valvulce  conniventes,  so  that  its  secret- 
ing follicles  are  numerous.  In  its  walls  are  also  small  racemose 
glands  not  found  in  other  parts  of  the  alimentary  tract. 

The  pancreas  is  a  large  compound  sacculated  or  acinous  gland, 
composed  of  numerous  irregular  packets  of  gland  tissue  attached 
by  its  lateral  branchlets  to  the  main  central  duct.  The  saccules 
are  elongated,  and  have  the  same  general  construction  as  those 
of  the  serous  salivary  glands  already  described,  but  they  are  less 
closely  held  together  by  the  intervening  connective  tissue,  and 
thus  the  pancreatic  tissue  does  not  show  such  a  regular  and  com- 
pact arrangement  on  section  as  the  salivary  glands.  A  single 
layer  of  irregular  or  slightly  conical  secreting  cells  in  the  sac- 
cule,  shows  a  difference  of  structure  in  its  central  or  peripheral 
sides,  so  that  an  external  or  homogeneous  zone,  and  an  internal 
granular  zone,  may  be  distinguished.  Each  zone  corresponds  to 
one-half  of  the  cells,  the  clear  half  being  next  the  boundary,  and 
the  granular  half  next  the  lumen  of  the  saccule.  The  relative 
width  of  these  zones  varies  with  the  digestive  process,  so  that  the 
nuclei  which  are  situated  between  them  sometimes  appear  to 
be  in  the  outer  clear  zone,  and  sometimes  in  the  inner  gran- 
ular zone.  The  outer  zone  colors  readily  with  carmine,  while 
the  inner  zone  remains  unstained. 

The  large  duct  which  passes  down  the  axis  of  the  gland,  receiv- 
ing tributaries  on  all  sides,  is  surrounded  with  a  layer  of  loose 
connective  tissue  which  forms  its  outer  coat.  The  proper  coat  of 
the  duct  is  composed  of  elastic  tissue,  lined  by  a  single  layer  of 
cylindrical  epithelium. 


PANCREATIC   JUICE. 
COLLECTION  OF  PANCREATIC  JUICE.   x- 

From  a  temporary  fistula  the  secretion  of  the  pancreas  can  be 
obtained  in  sufficient  quantity  to  determine  its  character  and 
properties.  It  is  difficult  to  establish  a  satisfactory  permanent 
fistula:  the  secretion  soon  alters  its  characters,  becoming  thin, 
and  losing  its  efficacy,  probably  owing  to  an  altered  or  abnormal 
state  of  the  gland. 

An  artificial  pancreatic  juice  may  be  extracted  by  water  from 
the  gland  taken  a  few  hours  after  death  from  an  animal  killed 
during  active  digestion  (a  couple  of  hours  after  eating)  and  care- 
fully minced.  This  extract,  used  with  proper  precautions,  will 
have  the  same  effect  as  the  secretion  itself. 

A  glycerine  solution  containing  the  active  principles  of  the  pan- 
creatic secretion  may  be  made  from  the  pancreas  by  treating  the 
minced  gland  for  a  couple  of  days  with  absolute  alcohol,  remov- 
ing the  alcohol,  and  allowing  it  to  soak  for  a  week  in  sufficient 
glycerine  to  cover  it.  This  glycerine  extract,  filtered,  contains 
but  little  else  than  pancreatic  ferments. 

Characters  of  the  Secretion. — The  pancreatic  juice  is  a  very 
thick,  transparent,  colorless,  strongly  alkaline  fluid,  which  turns 
to  a  jelly  if  cooled  to  0°  C.  It  often  contains  about  ten  per 
cent,  of  solids  when  obtained  from  a  temporary  fistula,  but  it 
may  have  as  little  as  two  per  cent. 

Of  these  a  considerable  proportion  are  organic,  namely  : — 

1.  Albumin,  which  is  coagulated  by  boiling. 

2.  Alkali  albumin,  precipitated  by  acetic  acid  or  by  adding 

magnesium  sulphate  to  saturation. 

3.  Leucin  and  tyrosin. 

4.  Fats  and  soaps. 

5.  Salts,  particularly  sodium  carbonate,  to  which  it  owes  its 

alkalinity. 

6.  Three   ferments,  to  which  it  owes  its  specific  action  on 

the  food  stuffs. 

Mode  of  Secretion. — The  pancreas  does  not   continue   in  a 
state  of  activity  during  the  interval  between  the  periods  of  active 
digestion.     When  the  gland  is  at  rest  it  is  of  a  pale  yellow  color, 
14 


162  MANUAL   OF   PHYSIOLOGY. 

and  is  flaccid,  but  during  active  digestion  it  becomes  more 
turgid,  and  assumes  a  pinkish  color,  from  the  increased  flow  of 
blood.  The  secretion  commences  immediately  after  taking  food, 
and  rises  rapidly  for  a  couple  of  hours,  then  falls  and  rises  again 
in  the  later  hours  of  digestion,  five  to  seven  hours  after  a  meal ; 
then  it  gradually  falls  for  eight  to  ten  hours,  and  ceases  com- 
pletely when  digestion  is  at  an  end.  The  first  rise  which  accom- 
panies the  introduction  of  food  into  the  stomach  is  certainly 
brought  about  by  nervous  agencies  of  a  similar  nature  to  those 
of  the  stomach,  the  secretion  of  which  follows  closely  upon  mas- 
tication. The  second  accompanies  the  passage  of  the  undigested 
food  through  the  small  intestines,  and  may  be  most  conveniently 
explained  as  the  result  of  reflex  nervous  stimulation  of  the  gland 
cells. 

The  great  complexity  of  the  nervous  mechanism  of  the  glands 
of  the  intestinal  tract  makes  it  difficult  to  ascertain  the  exact 
channels  traversed  by  the  afferent  and  efferent  impulses.  The 
following  observations,  if  accurate,  would  tend  to  prove  that  cer- 
tain inhibitory  impulses  pass  from  the  stomach  along  the  vagus 
to  the  medulla,  and  are  thence  reflected  to  the  gland  by  its  vaso- 
motor  nerves.  During  vomiting,  or  when  the  central  end  of  the 
divided  vagus  is  stimulated,  the  secretion  of  the  pancreas  ceases. 
Section  of  the  nerves  which  surround  the  blood  vessels  distrib- 
uted to  the  pancreas  causes  considerable  (paralytic)  flow  of 
secretion  which  stimulation  of  the  vagus  cannot  check. 

No  nerve  channels  have  been  demonstrated  to  carry  exciting 
impulses  direct  to  the  glands,  as  the  chorda  tympani  does  to  the 
submaxillary ;  but  the  direct  stimulation  of  the  gland  itself,  or 
of  the  medulla  oblongata,  is  said  to  induce  activity  of  the  gland. 

Structural  Changes  in  the  Cells  during  Secretion. — During  the 
period  of  rest,  i.  e.,  no  secretion  flowing  from  the  duct,  and  the 
gland  being  pale,  the  gland  cells  in  the  acini  undergo  a  change 
which  may  be  compared  with  that  observed  in  the  cells  of  the 
serous  salivary  glands.  The  division  of  the  row  of  cells  lining 
the  acinus,  into  a  central  granular  and  outer  clear  zone,  has 
already  been  mentioned. 

Immediately  after  very  active  secretion,  the  central  granular 


CHANGES   IN   PANCREATIC   CELLS.  163 

zone  is  reduced  to  a  minimum,  owing  to  the  paucity  of  granules  ; 
and  the  outer  zone  occupies  the  greater  part  of  the  cell,  the 
entire  substance  of  which  stains  readily  and  looks  like  ordinary 
protoplasm.  After  rest,  however,  the  granules  reappear,  and 
after  the  lapse  of  a  short  quiescent  period,  the  inner  granular 
zone  has  again  encroached  on  the  outer,  owing  to  the  accumula- 
tion of  granules  which,  rapidly  increasing,  fill  the  greater  part  or 
the  cells,  and  cause  them  to  bulge  inward  and  occlude  the  lumen 
of  the  gland.  As  digestion  proceeds,  the  cells  undergo  a  slight 
change  in  form,  so  that  each  individual  cell  is  more  distinctly 
seen,  and  its  angles  are  retracted,  giving  a  notched  appearance 

FIG.  70. 


One  Saccule  of  the  Pancreas  of  the  Rabbit  in  different  states  of  activity. 

A.  After  a  period  of  rest,  in  which  case  the  B.  After  the  gland  has  poured  out  its  secre- 

outlines  of  the  cells  are  indistinct,  and  tion,    when    the    cell    outlines    (d)  are 

the  inner  zone,  i.e.,  the  part  of  the  cells  clearer,  the  granular  zone  (a)  is  smaller, 

(a)  next  the  lumen  (c),  is  broad  and  and   the   clear    outer   zone   is    wider, 

filled. with  fine  granules.  (KUhne  and  Lea.) 

to  the  margin  of  the  acinus.  The  blood  supply  during  this 
period  is  much  increased,  red  arterial  blood  flowing  from  the 
veinlets  of  the  gland.  At  the  same  time  the  granules  are  dimin- 
ished in  number,  escaping  at  the  free  central  margin  of  the 
cells  into  the  lumen,  toward  which  they  appear  to  crowd,  leaving 
the  outer  zone  once  more  clear  and  free  from  granules,  while  the 
lumen  of  the  saccule  and  of  the  ducts  is  filled  with  secretion. 

The  examination  of  a  single  cell  shows  that  during  the  period 
of  rest  with  a  comparatively  poor  supply  of  blood,  it  receives  its 
normal  nutrition,  which  is  accompanied  by  an  accumulation  of 


164  MANUAL   OF   PHYSIOLOGY. 

granules  in  the  protoplasm  next  the  free  side  of  the  cell.  Dur- 
ing secretion  these  granules  are  pushed  out  of  the  cell,  and  seem 
in  some  way  to  form  the  secretion. 

It  will  be  seen  immediately  that  one  of  the  most  important 
functions  of  the  pancreatic  juice  is  the  formation  of  peptone  from 
proteid,  which  operation  is  carried  out  by  a  special  ferment  called 
trypsin.  It  has  been  found  that  this  ferment  can  only  be  obtained 
from  the  active  pancreas,  and  that  the  wider  the  inner  granular 
zone  of  the  cells  is,  the  richer  in  ferment  is  the  glycerine  extract 
made  from  the  gland.  But  it  has  also  been  found  that  if  a  glycer- 
ine extract  be  at  once  made  from  an  actively  secreting,  abso- 
lutely fresh  gland,  i.  e.,  removed  from  the  dead  animal  while 
still  warm,  the  extract  is  found  to  be  quite  inert  toward  proteids, 
while  an  extract  made  from  a  portion  of  the  same  pancreas 
which  has  been  kept  some  hours  after  death  is  very  active ;  and 
a  portion  of  the  fresh  pancreas  pounded  in  a  mortar  with  a  little 
weak  acid  so  as  to  develop  the  trypsin  acts  in  an  alkaline  solution 
and  forms  peptone  energetically. 

We  must  therefore  conclude  that  the  special  proteolytic  fer- 
ment of  the  pancreas  does  not  exist  prior  to  the  period  at  which 
the  secretion  is  poured  out  from  the  gland  cells. 

Although  a  definite  relation  seems  to  exist  between  the  amount 
of  granules  in  the  active  cells  and  the  degree  of  efficacy  of  the 
secretion,  the  ferment  does  not  appear  in  full  force  for  some  time 
after  the  height  of  the  gland  activity  has  been  established,  and 
it  is  likely  that  the  presence  of  an  acid  helps  in  the  birth  of  the 
ferment. 

It  has  therefore  been  assumed  that  the  granules  of  the  gland 
cells  give  rise,  not  to  the  proteolytic  ferment,  but  to  a  ferment- 
producing  substance  which  is  called  Zymogen. 

So  that  if  we  trace  the  history  of  the  pancreatic  proteolytic 
ferment,  we  shall  find  that,  so  far  as  this  trypsiu  is  concerned, 
there  can  be  no  question  as  to  whether  it  preexists  in  the  blood 
and  is  removed  thence  by  the  gland  or  not,  because  by  studying 
the  process  the  final  elaboration  of  the  secretion  is  seen  to  take 
place  after  it  has  got  into  the  ducts  or  into  the  intestinal  cavity. 
Thus  the  blood  gives  nutriment  to  the  protoplasm  of  the  gland 


PANCREATIC    DIGESTION.  165 

cells.  The  protoplasm  of  the  cells,  by  its  intrinsic  chemical  pro- 
cesses, manufactures  peculiar  granules.  These  granules  give  rise, 
among  other  things,  to  zymogen,  which  in  the  presence  of  an 
acid  begets  trypsin. 

PANCEEATIC  DIGESTION. 

The  pancreatic  juice  is,  of  all  digestive  fluids,  the  most  general 
solvent.  It  acts  upon  the  three  great  classes  of  food  stuffs  which 
require  modification  to  enable  them  to  pass  through  the  barrier 
that  intervenes  between  the  intestinal  cavity  and  the  blood  cur- 
rent. It  changes  proteids  into  peptones,  emulsifies  fatty  sub- 
stances, and  converts  starch  into  soluble  sugar.  The  ferments  to 
which  its  activity  is  due  may  be  separately  described. 

I.  Action  of  Pancreatic  Juice  on  Proteids. — The  ferment  which 
produces  peptone  is  trypsin.  Some  of  the  conditions  required  for 
its  perfect  operation  are  the  same  as  those  necessary  for  the  action 
of  the  gastric  ferment,  pepsin;  namely,  a  certain  degree  of  dilu- 
tion, and  a  temperature  of  about  40°  C.  But  it  differs  from 
pepsin  in  the  most  important  characteristic  of  its  action.  While 
the  presence  of  an  acid  is  absolutely  necessary  for  peptic  proteo- 
lysis,  we  find  that  an  alkaline  reaction  is  required  for  this  action 
of  the  pancreatic  ferment,  and  as  the  peptic  peptone  has  to  pass 
through  preliminary  stages  in  which  it  closely  resembles  acid 
albumin,  so  the  tryptic  peptone  is  first  produced  from  alkali  albu- 
min, which  has  been  formed  as  a  preliminary  step  by  the  alkali 
of  the  pancreatic  juice.  The  addition  of  the  sodium  carbonate 
aids  the  action,  and  indeed  seems  to  play  a  part  which  closely 
corresponds  to  that  taken  by  the  hydrochloric  acid  in  gastric 
digestion. 

The  change  to  alkali  albumin  and  peptone  as  accomplished  by 
the  trypsin,  is  not  accompanied  by  any  swelling  of  the  albumin 
such  as  occurs  in  the  formation  of  the  acid  albumin  in  the  stom- 
ach, but  the  proteid  is  gradually  eroded  from  the  surface  and 
thus  diminished  in  size. 

Moreover,  the  alkali  albumin  is  not  made  directly  into  pep- 
tone, but  passes  through  a  stage  in  which  it  resembles  globulin, 
and  is  soluble  in  solutions  of  sodium  chloride. 


166  MANUAL   OF   PHYSIOLOGY. 

Besides  these  differences  between  the  mode  of  action  of  pepsin 
and  trypsin  in  producing  peptones,  trypsin  has  a  peculiar  power 
upon  proteids,  which  has  no  analogue  in  the  peptic  action. 
While  the  pancreatic  peptone  is  being  produced,  a  further  change 
occurs,  which  gives  rise  to  the  formation  of  two  crystallizable 
nitrogenous  bodies  known  as  leucin  and  tyrosin,  the  former  belong- 
ing to  the  fatty  acid,  and  the  latter  to  the  aromatic  acid  group. 
These  substances,  which  are  commonly  found  together  as  a  result 
of  the  decomposition  of  peptones,  seem  inseparable  from  pan- 
creatic digestion,  and  increase  in  amount  toward  the  later  stages 
of  the  process. 

The  amount  of  peptone  produced  reaches  a  maximum  in  about 
four  hours,  after  which  the  proportion  of  the  different  unknown 
decomposition  products  appears  to  increase  at  the  expense  of  the 
peptone.  Among  these  substances  must  be  named  indol  and 
skatol,  the  materials  from  which  the  process  of  pancreatic  diges- 
tion derives  its  peculiarly  disagreeable  odor. 

This  breaking  up  of  the  surplus  proteid  food  into  bodies  which 
cannot  be  of  much  utility  in  the  economy,  and  which,  as  will 
appear  hereafter  (compare  Chapter  xxin),  are  but  a  step  in  the 
direction  of  their  elimination,  is  probably  an  important  part  of 
the  pancreatic  function,  as  it  relieves  the  economy  of  a  surcharge 
of  albuminous  substances. 

Small  quantities  of  phenol  are  also  found  in  conjunction  with 
the  above. 

II.  Action  on  Fat. — The  action  of  the  pancreatic  juice  on  fats 
is  of  two  kinds.  (1)  Saponification. — By  the  action  of  a  special 
ferment  (steapsiri)  a  small  proportion  of  the  neutral  fats  is  split 
up  into  glycerine  and  the  corresponding  fatty  acids.  The  acids 
thus  produced  readily  unite  with  the  alkali  present,  to  form  a 
little  soap.  The  chemistry  of  the  change  will  be  found  at  p.  79, 
and  may  be  shortly  stated,  taking  olein  as  an  example.  Olein  is 
a  compound  of  oleic  acid  and  glycerine.  Olein  in  presence  of 
this  ferment  and  soda  gives  glycerine  and  oleic  acid,  and  the 
latter  combines  with  soda  to  form  soap.  This  process  materially 
aids  in  the  next.  (2)  Emulsification. — Which  means  that  the 
fat  is  reduced  to  a  state  of  very  fine  subdivision,  as  it  exists  in 


PANCREATIC    DIGESTION.  167 

milk.  The  production  of  this  condition  is  facilitated  by  (a),  the 
albumin  in  solution ;  (6),  the  alkalinity  of  the  fluid ;  (c),  the 
presence  of  soap  alluded  to  above ;  and  (c?),  the  motion  of  the 
intestines.  This  process  of  em  unification  may  be  imitated  by 
adding  about  one-quarter  volume  of  rancid  linseed  oil  to  a  solu- 
tion of  sodic  carbonate  and  shaking  in  a  test  tube.  It  will  be 
found  that  the  addition  of  a  little  soap  and  albumin  will  make 
the  emulsion  more  perfect  and  more  permanent. 

III.  Action  on  Starch. — The  amylolitic  power  of  the  pancreatic 
juice  depends  on  a  separate  ferment  (Amylopsin).  Its  action 
seems  to  be  identical  with  that  of  the  saliva,  with  the  exception 
that  it  is  more  rapid  and  energetic,  and  is  said  to  affect  raw  as 
well  as  boiled  starch.  This  power  is  found  to  exist  in  the  extract 
of  the  gland,  whether  it  has  been  removed  from  a  fasting  or  from 
a  recently  fed  animal,  and  therefore  does  not  depend  upon  the 
gland  being  engaged  in  active  secretion. 


168 


MANUAL   OF    PHYSIOLOGY. 


CHAPTER  X. 

BILE. 

The  liver  has  two  chief  functions,*  which  are  so  distinct  in 
their  ultimate  object  that  they  may  be  conveniently  described 
separately.  One  of  these,  namely,  the  secretion  of  bile,  is  mainly 
excrementitious. 

Bile  is  one  of  the  fluids  connected  with  digestion,  being  poured 
into  the  intestine,  and  therefore  is  treated  under  this  heading ; 

FIG.  71. 


Section  of  the  Liver  of  the  Newt,  in  which  the  hile  ducts 
have  been  injected,  and  can  be  seen  to  form  a  network  of 
fine  capillaries. 

but  its  influence  upon  digestion  is  not  so  great  as  was  formerly 
supposed. 

The  other  function  of  the  liver  is  nutritive,  consisting  in  the 
formation  of  glycogen.  The  glycogenic  function  of  the  liver 
belongs  to  the  history  of  the  nutrient  materials  after  their  absorp- 
tion, and  is  of  the  first  importance  in  attaining  the  elaboration 
of  the  blood,  and  will  therefore  be  reserved  for  the  chapter  on 
that  subject. 


*  The  formation  of  urea  may  also  be  mentioned  here,  for  there  is  no  doubt,  as  will  be 
seen  later  on  in  speaking  of  the  excretions,  that  the  liver  has  an  important  share  in  pro- 
ducing this  substance. 


STRUCTURE   OF   THE   LIVER.  169 

Among  the  most  striking  peculiarities  of  the  liver  may  be 
mentioned  the  following  facts  :  (1)  It  has  a  receptacle,  the  gall 
bladder,  for  storing  the  secretion  until  required.  (2)  It  has  a 
double  blood  supply.  It  receives  by  the  hepatic  artery  a  small 
supply  of  fresh  arterial  blood  as  well  as  all  that  coming  from 
the  spleen,  pancreas  and  intestinal  canal,  collected  by  the  tribu- 
taries of  the  great  portal  vein,  and  distributed  by  its  branches  to 
the  liver.  (3)  A  regular  network  is  formed  by  the  minute  chan- 
nels (bile  capillaries),  which  freely  anastomose  between  the  cells. 
(4)  Although  in  the  embryo,  and  in  many  animals,  the  liver  is 
a  compound  saccular  gland,  the  arrangement  of  the  duct  radi- 
cles and  the  saccules  is  so  modified  in  the  higher  animals  and 
man,  that  their  relationship  is  no  longer  apparent,  and  the  struc- 
ture is  best  understood  by  following  its  vascular  groundwork. 

STRUCTURE  OF  THE  LIVER. 

On  the  surface  of  the  liver  are  seen  with  the  naked  eye  small 
rounded  markings  about  the  size  of  a  pin's  head,  which  give  the 
organ  a  mottled  appearance.  This  is  much  more  striking  in 
some  animals  (giraffe,  bear,  pig)  than  others,  but  is  easily  recog- 
nizable in  the  livers  of  all  mammalia.  These  little  areas  are  the 
surfaces  of  the  lobules  of  the  liver.  They  are  surrounded  by  a 
dark-red  boundary,  and  their  centre  is  marked  by  a  dark  spot, 
between  these  is  a  paler,  yellowish  zone.  The  dark  parts  corre- 
spond to  the  blood  vessels,  and  have  a  constant  relation  to  the 
lobules. 

The  entire  liver  is  made  up  of  these  little  lobules,  and  each 
one  of  them  has  the  same  construction  and  blood  supply,  and 
therefore  forms  in  itself  a  little  liver  perfect  in  all  its  structural 
arrangements,  so  that  the  description  of  one  such  unit  will  suffice 
to  give  an  idea  of  the  structure  of  the  gland.  For  other  details, 
anatomical  works  must  be  referred  to. 

The  branches  of  the  large  portal  vein  and  those  of  the  small 
hepatic  artery  pursue  the  same  course  through  the  gland,  and 
are  enclosed  in  a  sheath  of  connective  tissue  (capsule  of  Glisson), 
which  also  forms  the  bed  of  the  hepatic  duct  and  its  numerous 
tributaries.  If  these  branching  vessels  be  followed  to  their  final 
15 


170 


MANUAL   OF    PHYSIOLOGY. 


ramifications,  they  are  found  to  pass  around  and  between  the 
neighboring  lobules.  The  branches  of  the  portal  vein  in  this 
situation  receive  the  name  of  the  interlobular  veins.  They  anas- 


Section  of  Lobule  of  Liver  of  Rabbit  in  whicn  the  blood  and  bile  capillaries  have  been 
injected.    (Cadiat.) 

a.  Intralobular  vein.    6.  Interlobular  veins,    c.  Biliary  canals  beginning  in  fine 
capillaries. 


tomose  freely  with  the  terminal  veinlets  in  the  vicinity,  so  as  to 
form  a  network  round  each  lobule.     From  this  a  number  of 


STRUCTURE   OP   THE    LIVER. 


171 


capillary  vessels  pass  into  the  lobule,  and,  lying  between  the 
gland  cells,  form  a  network  with  long  meshes  radiating  from  the 
centre.  These  are  the  lobular  blood  capillaries.  The  vessels  of 
this  radiating  capillary  network  become  larger  as  they  unite  and 
converge  to  the  centre  of  the  lobule,  where  they  open  into  a  cen- 
tral vein  which  lies  in  immediate  apposition  with  the  gland  cells. 
This  vein  is  called  the  intralobular  vein,  and  is  the  radicle  of  the 
efferent  or  hepatic  vein,  which  carries  the  blood  of  the  liver  to 
the  inferior  vena  cava. 

The  ultimate  ramifications  of  the  hepatic  artery  can  be  traced 
to  various  destinations.     Some  pass  into  the  walls  of  the  accom- 

FIG.  73. 


Cells  of  the  Liver.  One  large  mass  shows  the  shape  they  assume  by  mutual  pressure, 
(a)  The  same  free,  when  they  become  spheroid.  (6)  More  magnified,  (c)  During 
active  digestion  containing  refracting  globules  like  fat. 

panying  vein  and  duct  and  the  connective  tissue  which  sur- 
rounds these  vessels.  Many  of  the  arterial  capillaries  unite  with 
offshoots  from  the  interlobular  venous  plexus,  and  thus  reinforce 
the  lobular  capillaries.  Other  branches  form  a  lobular  capillary 
plexus,  which  joins  the  capillaries  of  the  vena  porta,  together 
with  that  from  the  walls  of  the  vein  and  duct. 

The  blood  flowing  to  the  liver  in  the  great  vena  porta  and  the 
hepatic  artery  is  thus  conducted  by  those  vessels  to  the  bound- 
aries between  the  lobules  (interlobular  veins),  and  thence  streams 
through  the  converging  lobular  blood  capillaries  to  the  intra- 


172 


MANUAL   OF   PHYSIOLOGY. 


lobular  vein,  and  is  collected  from  the  latter  by  the  sublobular 
tributaries  of  the  hepatic  vein,  by  which  it  is  conducted  back  to 
the  general  circulation,  and  enters  the  heart  by  the  inferior  vena 
cava. 

Between  the  meshes  of  the  lobular  capillaries  the  gland  cells 

FIG.  74. 


Section  of  injected  liver  showing  the  position  of  the  portal  branches  (interlobular  veins, 
VP)  and  radicals  of  hepatic  veins  (intralobular  veins,  HV)  connected  by  lobular  ca- 
pillaries. 

Below  is  a  portion  of  the  same  highly  magnified,  (a)  Liver  cell  with  (n)  nucleus ;  (6) 
Blood  capillaries  cut  across  passing  along  angles  of  cells ;  (c)  Bile  capillaries  between 
flattened  sides  of  cells.  (Huxley.) 


STRUCTURE   OF   THE    LIVER. 


173 


are  tightly  packed.  These  are  large,  soft,  polyhedral  cells,  with 
one,  two,  or  even  more  nuclei,  and  no  trace  of  a  limiting  mem- 
brane. Owing  to  the  shape  of  the  capillary  meshes,  the  cells  are 
placed  in  rows  radiating  from  the  centre  of  the  lobule  toward 
the  periphery. 

The  blood  capillaries  are  said  to  pass  along  the  angles  and 
edges  of  these  cell  blocks  so  as  not  to  come  into  close  relation  to 
the  bile  capillaries  (Fig.  71).  The  finely  granular  protoplasm 
of  the  liver  cells  is  capable  of  undergoing  some  slight  change  in 
form  while  alive.  In  the  protoplasm  are  situated  varieties  of 
granules,  the  commonest  being  bright,  refracting  fat  globules, 
which  vary  in  amount  with  the  different  stages  of  digestion  ; 


FIG.  75. 


Section  of  the  Liver  of  the  Newt,  in  which  the  bile  ducts  have  been  injected,  and  can  be 
seen  through  the  transparent  liver  cells  to  form  a  network  of  fine  capillaries. 

others  of  a  yellow  color  seem  connected  with  the  coloring  matter 
of  the  bile ;  and  a  third  variety,  less  refracting  and  colorless,  is 
said  to  be  related  to  the  glycogen. 

Between  the  cells  of  the  lobules  there  can  be  demonstrated 
very  fine  anastomosing  canals,  which  appear  to  be  formed  by  the 
juxtaposition  of  grooves  which  lie  in  the  middle  of  the  flat 
surface  of  two  neighboring  cells.  Every  liver  cell  is  related  to 
such  a  canal,  and  consequently  a  very  dense  network  with  pecu- 
liarly regular  polygonal  meshes  is  present,  each  mesh  corre- 
sponding in  size  to  one  cell. 

These  fine  intercellular  canals  are  called  lobular  bile  capillaries, 


174  MANUAL   OF   PHYSIOLOGY. 

and  must  not  be  confounded  with  lobular  blood  capillaries,  the 
diameter  of  which  is  about  ten  times  as  great  as  the  former,  and 
which  have  a  definite  boundary  wall,  while  the  bile  capillaries 
have  no  other  boundary  than  the  substance  of  the  liver  cell,  and 
therefore  are  not  really  vessels. 

These  fine  intercellular  bile  passages  are  described  as  commu- 
nicating with  the  interlobular  ducts  directly  opening  into  the 
ducts  without  any  marked  increase  in  the  size  or  change  of 
arrangement.  The  interlobular  ducts  which  follow  the  course  of 
the  artery  and  portal  vein  are  composed  of  a  delicate  basement 
membrane  lined  with  a  thin  layer  of  epithelium  which  in  the 
larger  vessels  shows  a  cylindrical  character.  The  large  bile 
ducts  have  a  firm  fibro-elastic  coat  lined  with  a  definite  mucous 
membrane  covered  with  cylindrical  epithelium  lying  upon  a 
vascular  submucosa,  in  which  are  scattered  numerous  mucous 
glands  of  saccular  form. 

The  amount  of  connective  tissue  in  the  liver  of  man  and  most 
domestic  animals  is  very  small,  but  in  the  pig,  bear,  giraffe,  and 
some  others,  it  is  easily  recognized  around  the  lobules,  sending 
delicate  supporting  processes  between  them.  This  connective 
tissue  passes  into  the  organ  with  the  portal  system  of  vessels  form- 
ing a  loose  sheath  derived  from  the  capsule  of  Glisson,  and  is 
distributed  with  the  subdivisions  of  those  vessels  to  the  various 
parts  of  the  gland. 

The  lymphatics  are  known  to  be  very  plentiful,  and  in  intimate 
relation  to  the  blood  vessels. 

Method  of  Obtaining  Bile. — For  most  practical  purposes  the 
bile  from  the  gall  bladder  of  recently  killed  animals  is  sufficient. 
The  bile  pigments  and  cholesterin  may  be  conveniently  ob- 
tained from  the  gall  stones  so  often  found  in  the  human  gall 
bladder. 

In  order  to  investigate  the  composition  of  the  bile  as  it  comes 
from  the  ducts,  before  it  has  been  modified  by  its  sojourn  in  the 
gall  bladder,  it  is  necessary  to  make  a  biliary  fistula,  commu- 
nicating either  with  the  gall  bladder  or  with  the  bile  duct.  In 
this  way  the  rate,  pressure,  and  other  points  concerning  the  mode 
of  secretion  may  be  determined. 


COMPOSITION    OF   BILE.  175 

Composition  of  Bile. — The  bile  of  man  and  carnivorous  animals 
is  of  a  deep  orange-red  color,  turning  to  greenish-brown  by 
decomposition  of  its  coloring  matter.  In  herbivorous  animals  it 
has  some  shade  of  green  when  quite  fresh,  but  turns  to  a  muddy 
brown  after  a  time.  It  is  transparent,  and  more  or  less  viscid 
according  to  the  length  of  time  it  has  remained  in  the  gall  blad- 
der. It  has  a  strong,  bitter  taste,  a  peculiar  aromatic  odor,  and 
after  remaining  for  some  time  in  the  gall  bladder  it  has  an 
alkaline  reaction.  Its  specific  gravity  is  about  1005  when  taken 
from  the  bile  ducts  directly,  but  it  may  rise  to  1030  after  pro- 
longed stay  in  the  gall  bladder,  owing  to  the  addition  of  mucus 
and  the  absorption  of  some  of  its  fluid. 

The  following  table  gives  approximately  the  proportions  of  the 
chief  constituents  of  bile : — 

Water 85.0  per  cent. 

Bile  salts 10.0 

Coloring  matter  and  mucus  3.0 

Fats 1.0 

Cholesterin 0.3 

Inorganic  salts 0.7 

100.0 

Bile  contains  no  structural  elements  nor  any  trace  of  albu- 
minous bodies. 

1.  The  bile  acids  are  two  compound  acids,  glycocholic  and 
taurocholic,  which  exist  in  the  bile  in  combination  with  sodium. 
The  amount  of  each  varies  in  different  animals  and  at  different 
times  in  the  same  animal.  The  bile  of  the  dog  and  other  car- 
nivora  contains  only  taurocholate  of  soda.  In  the  ox  the 
glycocholate  of  soda  is  greatly  in  excess.  In  man  both  are 
present,  the  proportion  being  variable,  but  the  glycocholate 
greatly  preponderates. 

To  separate  these  acids,  bile  is  evaporated  to  one-fourth  its 
volume,  rubbed  to  a  paste  with  animal  charcoal  to  remove  the 
pigments,  and  carefully  dried  at  100°  C.  The  black  cake  is 
extracted  with  absolute  alcohol,  which  dissolves  the  bile  salts. 
From  the  strong  alcoholic  solution  after  partial  evaporation  the 
bile  salts  can  be  precipitated  by  ether.  They  first  appear  as  an 


176  MANUAL   OF   PHYSIOLOGY. 

emulsion,  and  then  form  glistening  crystals  which  are  soluble  in 
water  or  alcohol,  but  insoluble  in  ether. 

From  the  solution  of  the  two  salts  the  glycocholic  acid  may 
be  precipitated  by  neutral  lead  acetate,  as  lead  glycocholate, 
from  which  the  lead  may  be  removed  by  sulphuretted  hydrogen, 
and  the  acid  precipitated  from  its  alcoholic  solution  by  the  addi- 
tion of  water.  The  taurocholic  acid  may  be  obtained  subse- 
quently by  treating  with  basic  lead  acetate. 

Glycocholic  acid,  when  boiled  with  weak  acids,  alkalies,  or 
baryta  water,  takes  up  an  atom  of  water,  and  splits  into  cholic 
acid  and  glycin  (amido-acetic  acid).  (See  p.  74.) 

Taurocholic  acid,  under  similar  treatment,  splits  into  cholic 
acid  and  taurin  (amido-ethyl-sulphonic  acid).  (See  p.  73.) 

Cholic  acid  occurs  free  in  the  intestines,  the  bile  salts  being 
split  up  in  digestion,  and  taurocholic  and  glycocholic  acids 
decomposed. 

The  non-nitrogenous  cholic  acid  is  in  a  great  measure  elimi- 
nated with  the  fteces,  while  the  taurin  and  glycin  are  reabsorbed 
into  the  blood  with  some  of  the  other  constituents  of  the  bile,  and 
are  again  probably  utilized  in  the  economy. 

No  traces  of  these  bile  acids  can  be  detected  in  the  blood,  and 
there  is  no  accumulation  of  them  in  the  body  after  the  removal 
of  the  liver;  hence,  it  has  been  concluded  that  they  are  manu- 
factured in  the  liver. 

2.  The  greater  proportion  of  the  mucus  contained  in  the  bile  is 
produced  in  the  gall  bladder,  and  there  added  to  the  bile.   Some 
mucus  comes  from  the  mucous  glands  in  the  bile  ducts,  but,  unless 
the  bile  has  remained  in  the  gall  bladder,  there  is  but  an  insig- 
nificant amount  of  mucus  present,  as  is  seen  when  a  fistula  is 
made  from  the  hepatic  duct.   The  mucus  passes  in  an  unchanged 
state  through  the  intestine,  and  is  evacuated  with  the  fseces. 

3.  The  bile  pigment  of  man  and  carnivora  is  chiefly  the  red- 
dish form  called  bilirubin.     It  is  insoluble  in  water  but  soluble 
in  chloroform.     It  can  be  obtained  in  rhombic  crystals,  and  is 
easily  converted  by  oxidation  into  a  green  pigment,  biliverdin, 
which  is  the  principal  coloring  matter  in  the  bile  of  many  ani- 
mals, and  is  not  soluble  in  chloroform,  but  readily  so  in  alcohol. 


BILE   CONSTITUENTS.  177 

Bilirubin  is  supposed  to  be  identical  with  hsematoidin,  a  deeply 
colored  material  found  by  Virchow  in  old  extravasations  of 
blood  within  the  body,  and  hence  the  bile  pigment  is  said  to  be 
derived  from  the  coloring  matter  of  the  blood.  Probably  the 
haemoglobin  of  some  red  corpuscles  which  have  been  broken  up 
in  the  spleen  is  converted  into  bile  pigment  by  the  liver. 

Under  the  influence  of  decomposition  bilirubin  undergoes  a 
change,  taking  up  water  and  forming  hydro-bilirubin ;  this 
occurs  in  the  intestine,  and  the  bilirubin  is  thus  eliminated  as 
the  coloring  matter  of  the  faeces  (stercobilin),  which  is  probably 
identical  with  the  urobilin  of  the  urine. 

4.  Fatty  matters,  the  principal  of  which  are  lecithin,  palmitin, 
stearin,  olein,  and  their  soda  soaps. 

5.  Cholesterin  (C26H44O)  is  an  alcohol,  and  crystallizes  in  clear 
rhombic  plates,  insoluble  in  water  but  held  in  solution  by  the 
presence  of  the  bile  salts.     It  can  be  obtained  from  gall  stones, 
the  pale  variety  of  which  are  almost  entirely  composed  of  it. 
The  cholesterin  leaves  the  intestines  with  the  faeces. 

6.  Among    the   inorganic   salts  are    sodium    and    potassium 
chloride,  calcium  phosphate,  some  magnesia,  and  a  considerable 
quantity  of  iron. 

Tests  for  Bile. — The  most  important  constituents  of  the  bile,  viz., 
the  bile  acids  and  pigment,  may  be  detected  by  appropriate  tests, 
which  are  in  common  practical  use  : — 

1.  Pettenkofer's  test  for  the  bile  acids. — To  a  fluid  containing 
either  or  both  bile  acids,  or  any  solution  of  cholic  add,  add 
some  cane  sugar,  and  then  slowly,  drop  by  drop,  strong  sul- 
phuric acid.  The  solution  turns  to  a  cherry-red  and  then 
changes  to  purple.  As  other  substances,  such  as  albuminous 
bodies,  give  under  this  treatment  a  similar  color,  in  order  to 
make  the  reaction  a  trustworthy  test  for  bile  salts,  the  two 
characteristic  absorption  bands  given  by  the  spectroscope 
should  also  be  observed. 

The  following  is  said  to  be  a  characteristic  method :  Kinse, 
out  a  porcelain  capsule  successively  with  the  fluid  to  be  tested 
with  weak  sulphuric  acid,  and  with  a  weak  solution  of  sugar, 
then  heat  to  70°  C.,  when  the  capsule  turns  purple. 


178  MANUAL   OF    PHYSIOLOGY. 

2.  Gmelin's  test  for  the  bile  pigments  depends  upon  the  fact  that 
during  the  stages  of  oxidation  the  bilirubin  undergoes  a  series 
of  changes  in  color  which  follow  the  sequence  of  the  familiar 
solar  spectrum.  Place  a  few  drops  of  the  fluid  to  be  tested  on 
a  white  surface  (capsule  or  plate),  and  allow  a  drop  of  nitric 
acid,  yellow  with  nitrous  acid  fumes  which  make  it  more 
oxidizing,  to  run  into  it ;  as  they  mingle  together  the  rain- 
bow-like play  of  color  appears.  This,  when  watched,  will 
be  found  to  consist  of  a  series  of  changes  to  green,  blue, 
violet,  red  and  yellow. 

This  can  also  be  observed  by  allowing  the  acid  to  trickle 
gently  down  the  side  of  a  test  tube  fixed  in  an  inclined 
position  so  that  it  cannot  be  shaken  :  the  play  of  color  can 
then  be  seen  starting  from  the  point  of  junction  of  the  two 
fluids. 

METHOD  OF   SECRETION  OF  BILE. 

The  secretion  of  the  liver  varies  less  in  the  amount  formed  at 
different  times  than  that  of  other  digestive  glands.  Although 
the  changes  in  the  rate  of  its  secretion  are  not  so  marked,  they 
follow  the  same  general  rule  as  those  of  other  glands  connected 
with  digestion,  i.  e.,  after  food  is  taken  there  is  a  sudden  rise, 
then  a  gradual  fall,  followed  by  a  second  rise  in  the  rate  of 
secretion.  This  is  well  seen  in  the  case  of  the  pancreas.  Want 
of  food  is  said  to  check  the  secretion  of  bile,  but  only  does  so  in 
a  slight  degree,  for  the  more  important  work  of  the  liver  is  con- 
tinuous, as  is  the  activity  of  all  glands  whose  duty  it  is  to  elimi- 
nate noxious  substances  or  otherwise  influence  the  composition 
of  the  blood.  At  the  end  of  a  period  of  fasting,  the  gall  bladder 
is  always  found  greatly  distended,  because  the  secretion  has  con- 
tinued to  flow  into  that  receptacle,  and  there  has  been  no  call 
for  its  discharge  into  the  duodenum. 

The  amount  of  bile  produced  by  dogs  is  much  influenced  by 
their  diet.  It  is  very  great  when  meat  alone  is  consumed,  less 
with  vegetable,  and  very  small  with  a  diet  of  pure  fat.  As  a 
general  rule,  bile  is  more  abundantly  produced  in  herbivorous 
than  in  carnivorous  animals. 

The  rate  of  secretion  is  much  influenced  by  the  amount  of 


BILE   SECRETION.  179 

blood  flowing  through  the  organ,  which  probably  explains  the 
increase  during  digestion.  Ligature  of  the  portal  vein  causes 
arrest  of  the  secretion,  and  death.  After  ligature  of  the  hepatic 
artery  the  secretion  continues,  but  soon  diminishes  from  mal- 
nutrition of  the  tissue  of  the  liver,  which  ultimately  causes  death 
if  the  entire  vessel  be  tied. 

These  variations  in  the  rate  of  secretion  may  depend  on  direct 
nervous  influence,  but  no  special  secretory  nerve  mechanism  has 
been  discovered  for  the  liver,  and  it  is  quite  possible  that  the 
changes  in  the  activity  of  the  gland  which  accompany  the  dif- 
ferent periods  of  digestion  may  be  accounted  for  by  changes  in 
the  intestinal  blood  supply,  which  give  rise  to  corresponding 
differences  in  the  amount  of  blood  flowing  through  the  portal 
vein. 

The  force  with  which  the  bile  is  secreted  is  very  small.  That 
is  to  say,  the  pressure  in  the  ducts  never  exceeds  that  of  the 
blood  (as  is  the  case  in  the  salivary  glands) ;  but,  on  the  con- 
trary, when  a  pressure  of  about  16  mm.  (.63  in.)  mercury  is 
attained,  the  evacuation  of  the  bile  ceases,  and  with  a  little  in- 
crease of  opposing  force  the  fluid  in  the  manometer  retreats  and 
finds  its  way  into  the  blood.  The  low  pressure  which  can  be 
reached  in  the  gall  ducts  does  not  imply  any  want  of  secretory 
power  on  the  part  of  the  liver  cells,  but  merely  that  there  exists 
a  great  facility  of  communication  between  the  duct  radicles  and 
the  blood  vessels,  probably  through  the  medium  of  the  lym- 
phatics. This  is  made  obvious  by  experiment,  by  which  it  can 
be  shown  that  with  a  comparatively  low  pressure  (200  mm.  — 
nearly  8  in.  of  water  for  a  guinea-pig)  fluid  can  be  forced  into 
the  circulation  from  the  bile  ducts. 

This  is  seen  also  in  stoppage  of  the  bile  ducts  in  the  human 
subject,  when  some  of  the  bile  constituents  continue  to  be  formed, 
and  pass  into  the  blood,  where  their  presence  is  demonstrated  by 
the  yellow  color  characteristic  of  jaundice.  The  ready  evacua- 
tion of  the  bile  is  a  matter  of  great  importance  for  health,  the 
least  check  to  its  free  exit  causing  the  secretion  to  be  forced 
into  the  circulating  blood  instead  of  into  the  gall  passages. 
Under  normal  circumstances,  the  large  receptacle  of  the  gall 


180  MANUAL   OF   PHYSIOLOGY. 

bladder  being  always  ready  to  receive  the  bile,  ensures  its  easy 
exit  from  the  ducts,  but  the  forces  which  cause  its  flow  are 
extremely  weak.  The  smooth  muscle  in  the  walls  of  the  duct 
seem  rather  for  the  purpose  of  regulating  than  aiding  the  flow. 

When  food  from  the  stomach  begins  to  flow  into  the  duodenum, 
the  muscular  coat  of  the  gall  bladder  contracts  and  sends  a  flow 
of  bile  into  the  intestine,  which  action  is  doubtless  brought  about 
by  a  reflex  nerve  impulse,  for  it  is  only  when  this  part  is  stimu- 
lated that  the  bile  flows  freely  from  the  bladder.  The  acid  gastric 
contents  seem  to  be  the  most  efficacious  stimulus, 

In  the  human  subject  the  quantity  of  bile  secreted  has  been 
found  to  be  about  600  c.c.  (21  oz.)  per  diem  in  cases  where  there 
were  biliary  fistulse.  This  would  equal  about  13  grms.  per  kilo 
of  the  body  weight. 

In  the  guinea-pig  and  rabbit,  it  has  been  estimated  to  be 
about  150  grms.  per  kilo  of  the  body  weight. 

FUNCTIONS  OF  THE  BILE. 

1.  Neutralizing  and  Precipitating  Acid  Peptones. — When   the 
acid  contents  of  the  stomach  are  poured  into  the  duodenum  and 
meet  with  a  gush  of  alkaline  bile,  a  copious  cheesy  precipitate 
is  formed  which  clings  to  the  wall  of  the  intestine.    This  precipi- 
tate consists  partly  of  acid  albumin  (parapeptone)  and  peptones 
thrown  down  by  the  strong  solution  of  bile  salts,  and  partly  of 
bile  acids,  the  salts  of  which  have  been  decomposed  by  the 
hydrochloric  acid  of  the  gastric  juice.     With  the  bile  acids  the 
pepsin  is  mechanically  carried   down.     Thus,  immediately  on 
their  entrance  into  the  duodenum  the  peptic  digestion  of  the 
gastric  contents  is  suddenly  stopped  not  only  by  the  precipita- 
tion of  the  soluble  peptones  and  the  shrinking  of  the  swollen 
parapeptone,  but  also  by  the  removal  of  the  pepsin  itself  from 
the  fluid,  and  the  neutralization  of  the  gastric  fluid   by  the 
alkaline  bile. 

By  thus  checking  the  action  of  the  gastric  ferment  the  bile 
prepares  the  chyme  for  the  action  of  the  pancreatic  juice. 

2.  As  a  Stimulant,  the  bile  is  of  considerable  use,  for  it  excites 
the  muscles  of  the  intestine  to  increased  action,  and  thereby  aids 


FUNCTIONS   OF    THE    BILE.  181 

in  absorption  and  promotes  the  forward  movement  of  food,  and 
more  particularly  of  those  insoluble  materials  which  have  to  be 
evacuated  per  anum.  This  stimulation  may  amount  to  mild 
purging. 

3.  Moistening  and  Lubricating. — The  bile  adds  to  the  ingesta  an 
abundant  supply  of  fluid  and  mucus,  much  of  which  passes  along 
the  intestine  to  moisten  and  lubricate  the  faeces  and  facilitate 
their  evacuation.     In  cases  of  jaundice,  or  when  the  bile    is 
removed  by  a  fistula,  the  faeces  are  hard  and  friable,  and  with 
difficulty  expelled,  owing  to  the  deficient  fluid  and  mucus,  as  well 
as  to  the  weaker  peristaltic  movements. 

4.  As  an  Aid  to  Absorption. — The  bile  having  some  soap  in 
solution  has  a  close  relationship  to  both   watery  and  oily  fluids, 
and  possibly  on  this  account,  as  well  as  owing  to  a  peculiar 
power  possessed  by  the  bile  salts,  a  membrane  saturated  with  bile 
allows  an  emulsion  of  fat  to  pass  through  it  much  more  readily 
than  if  the  same  membrane  were  kept  moistened  with  water. 
This  can  be  seen  experimentally  with  filter  paper. 

5.  As  Excrement. — Although  much  of  the  bile  is  reabsorbed  from 
the  intestinal  tract  into  the  blood,  and  again  used  in  the  economy, 
some  of  its  constituents  pass  off  with  the  faeces,  and  are  no  doubt 
simply  excrementitious  matters  that  must  be  got  rid  of.   Thus  all 
the  cholesterin,  mucus,  and  coloring  matter  are  normally  elimi- 
nated, and  a  considerable  quantity  of  the  bile  acids  are  split  up, 
the  cholic  acid  being  found  in  the  faeces. 

6.  Emuhificaiion  of  Fats. — The  bile  has  some  share  in  forming 
an  emulsion,  but  far  less  than  the  secretion  of  the  pancreas ; 
however,  the  mixed  secretions  are  probably  more  efficacious  than 
either  separately,  from  the  presence  of  the  free  fatty  acids,  which 
form  soaps  and  aid  in  forming  the  emulsion. 

7.  As  an  Antiseptic,  bile  has  been  said  to  have  some  function  to 
perform.     Possibly  it  restricts  the  formation  of  certain  of  the 
bye  products,  such  as  the  indol  resulting  from  pancreatic  diges- 
tion ;  but  it  is  certainly  not  antiseptic,  since  bacteria  abound  and 
thrive  in  it  and  in  the  duodenum. 


182 


MANUAL   OP   PHYSIOLOGY. 


CHAPTER  XL 
FUNCTIONS  OF  THE  INTESTINAL  MUCOUS  MEMBRANE. 

Two  distinct  varieties  of  gland  are  found  in  the  small  intestine. 
Those  known  as  Brunner's  glands  are  localized  to  the  submucosa 
of  the  duodenum ;  they  are  insignificant  in  number  when  com- 
pared with  the  sound  variety,  called  Lieberkuhn's  glands,  which 
are  distributed  over  the  entire  intestinal  tract  and  are  closely  set 
in  the  mucous  membrane. 


FIG.  76. 


PIG.  77. 


Portion  of  the  Wall  of  the  Small  Intestine  laid  open    Drawing   of  transverse  section  of  the 
to  show  the  valvulse  conniventes.    (Brinton.)  duodenum  showing  Brunner's  Glands 

(B^  opening  into  Lieberkuhn's  follicles 
(L),  (v)  villi,  (M)  muscular  coats. 

Brunner's  glands  form,  in  some  animals,  a  dense  layer  in  the 
submucous  tissue  of  the  beginning  of  the  duodenum  ;  they  are 
small,  branched  saccular  glands  resembling  mucous  glands  in 
structure.  Owing  to  their  small  size  the  secretion  cannot  be 
obtained  in  sufficient  quantity  to  make  satisfactory  experiments 
in  respect  to  its  properties.  It  is  said  to  dissolve  albumin  and  to 
have  a  diastatic  fermentative  action,  so  that  probably  the  secre- 
tion is  analogous  to  that  of  the  pancreas,  as  Brunner  originally 


STRUCTURE   OF   THE   SMALL   INTESTINES. 


183 


supposed.  The  quantity  of  fluid  secreted  by  these  glands  is  so 
small  that  its  existence  is  not  taken  into  account  in  speaking  of 
the  intestinal  juice,  by  which  is  meant  the  fluid  poured  out  by 
the  innumerable  short  tubes  or  follicles  of  the  intestine. 

These  Lieberkuhn's  follicles  belong  to  a  very  simple  form  of 

FIG.  78. 


...a 


Section  of  the  Mucous  Membrane  of  small  intestine,  showing  Lieberkuhn's  follicles  (a) 
with  their  irregular  epithelium  and  the  villi  (6)  passing  out  of  view ;  (c)  Muscularis 
mucosse;  (d)  Submucous  tissue.  (Cadiat.) 


gland,  each  one  being  a  single  straight  cavity  in  the  mucous 
membrane  hardly  deep  enough  to  deserve  the  name  of  a  tube. 
In  the  small  intestine  they  are  set  as  closely  as  the  villi  permit. 
In  the  large  intestine,  where  the  villi  are  absent,  they  are  more 
closely  set  and  are  also  deeper  (Fig.  78).  They  are  bounded  by 


184 


MANUAL   OF   PHYSIOLOGY. 


a  thin  basement  membrane 
which  is  embraced  by  a 
close  capillary  network  of 
blood  vessels,  and  are 
lined  by  a  single  layer  of 
cylindrical  or  spherical 
epithelial  cells. 

The  epithelial  covering 
of  the  processes  known  as 
villi,  which  are  studded  all 
over  the  mucous  mem- 
brane  of  the  small  intes- 
tine,  produce  some  mucus. 


2  Method    of    Obtaining 

I  Intestinal  Secretion. — 

a"  Considerable  difficulty  has 

|  been   found    in   obtaining 

•|,  the  proper  intestinal  juice 

•2  free  from  admixture  with 

S3 

|  the  secretions  of  the  liver 

H  and   pancreas   which    are 

1  carried   along  and  mixed 

£  with  it.     A  short  portion 

5  of  the  small  intestine  has 

•2  been  successfully  isolated 

|  from  the  rest  without  in- 

|  juring  the  mesentery  or  its 

§  blood  vessels.     One  of  the 

8  extremities  of  the  isolated 

|  portion   was    closed,    and 

§*  the  other  was  retained  by 

5  sutures  at  an  opening  in 

is  the  abdominal  wall.     The 

g  cut  ends  of  the  remainder 

£  of  the   intestine   were    at 
the  same  time  united,  so  that  the   continuity  of  the  alimentary 


FUNCTIONS   OF   THE   INTESTINAL   JUICE.  185 

tract  was  preserved.  Thus,  a  limited  piece  of  gut  formed  a  cul- 
de-sac  from  which  the  fluid  could  be  collected  through  a  fistulous 
opening. 

Characters  of  the  Secretion,— The  liquid  obtained  from  such 
a  fistula  is  thin,  opalescent  and  yellowish,  with  a  strong  alkaline 
reaction  and  a  specific  gravity  of  1011.  It  contains  some  pro- 
teid  and  other  organic  material,  a  ferment  and  inorganic  salts  in 
which  sodium  carbonate  preponderates. 

Mode  of  Secretion.— The  secretion  flows  slowly  from  such  a 
fistula,  but  the  amount  increases  during  digestion,  showing  that 
the  secretion  of  the  intestine  is  under  the  control  of  some  nerve 
centre  which  can  call  the  entire  tract  into  action  when  one  part 
is  stimulated.  The  local  stimulation  of  the  mucous  membrane 
makes  it  red,  and  causes  it  to  pour  out  a  more  abundant  secre- 
tion. Beyond  this  little  is  known  of  the  nervous  mechanism  or 
the  local  cell  changes  which  accompany  the  formation  of  the 
secretion. 

Functions  of  the  Intestinal  Juice, — All  the  properties  of  the 
secretion  of  the  pancreas  have  been  accorded  to  the  intestinal 
juice.  It  is  said  to  have  a  ferment,  capable  of  being  extracted 
with  glycerine,  which  can  convert  cane  sugar  and  starch  into 
grape  sugar,  and  bring  about  lactic  fermentation.  It  dissolves 
fibrin  very  slowly,  and  still  less  easily  other  proteids.  It  is  also 
said  to  emulsify  fats. 

The  observations  as  to  its  digestive  properties  are  discordant, 
for  experiments  have  given  opposite  results  in  different  animals, 
and  in  the  hands  of  different  persons  even  in  the  same  animal. 
From  the  foregoing  account  of  the  intestinal  secretions  it  may 
be  seen  that  the  changes  which  the  various  kinds  of  food  undergo 
on  their  way  through  this  part  of  the  alimentary  tract  are 
numerous;  a  short  review  may  therefore  be  useful. 

When  the  acid  gastric  chyme  escapes  into  the  duodenum  a 

flow  of  bile  takes  place  from  the  gall  bladder,  and  at  the  same 

time  the  secretions  of  the  pancreas,  Brunner's  glands,  and  Lie- 

berkuhn's  follicles  are  poured  copiously  into  the  intestine.     The 

16 


186  MANUAL   OF   PHYSIOLOGY. 

bile  meeting  with  the  turbid  fluid  chyme  causes  it  to  change  to  a 
soft,  cheesy,  granular  mass,  the  appearance  of  which  depends 
chiefly  on  the  precipitation  of  the  peptones  and  shrinking  of  the 
parapeptone.  The  pepsin  is  rendered  powerless,  both  it  and  the 
bile  acids  being  carried  down  with  the  precipitate.  Gastric 
digestion  is  thus  arrested  and  the  onward  flow  of  the  fluid 
chyme  checked.  As  the  alkaline  pancreatic  and  intestinal  juices 
meet  this  semi-fluid  cheesy  mass  the  conversion  of  starch  into 
sugar  proceeds  rapidly,  even  the  raw  starch  granules  being 
changed.  The  small  oil  globules  come  in  contact  with  the  alka- 
line mixture  of  bile  and  pancreatic  juice.  The  pancreatic  fer- 
ment steapsin  splits  up  some  of  the  fat  separating  the  fatty  acid 
from  the  glycerine  radicle.  Some  of  the  soda  of  the  bile  salt  is 
substituted  for  the  latter,  and  uniting  with  the  fatty  acid  forms  a 
soap.  In  such  a  mixture  as  this — an  alkaline  fluid  with  proteid 
and  soap  in  solution — a  fine  emulsion  is  readily  formed,  as  can 
be  seen  by  adding  sodium  carbonate  to  some  rancid  oil.  The 
free  acid  (the  cause  of  rancidity  in  the  oil)  unites  with  some  soda 
to  form  a  soap  which  in  the  alkaline  mixture  enables  the  oil  to 
be  converted  into  an  emulsion  by  even  slight  agitation,  so  that 
the  pancreas,  by  setting  the  fatty  acid  free,  and  the  bile  possibly 
by  contributing  some  soda,  aid  one  another  in  giving  rise  to  a 
definite  but  small  amount  of  soap. 

The  precipitated  parapeptone  and  peptone  and  the  finely  divided 
proteid  are  presented  to  the  pancreatic  juice  in  a  form  which  it 
can  easily  attack,  and  thus  the  conversion  of  proteid  into  pep- 
tones in  the  small  intestine  goes  on  rapidly. 

How  far  the  peculiar  action  of  trypsin  on  proteids,  converting 
them  further  into  leucin  and  ty rosin,  goes  on  in  normal  digestion 
is  not  known,  but  it  is  probable  that  the  production  of  these 
bodies  is  increased  with  the  over-abundant  ingestion  of  proteid 
or  a  purely  meat  diet,  and  is  then  useful  as  a  means  of  prevent- 
ing the  injurious  effects  of  too  great  proteid  absorption. 

The  gastric  chyme  is  therefore  completely  changed  in  the 
duodenum,  and  in  the  other  parts  of  the  small  intestines  we  find 
in  its  stead  a  thin  creamy  fluid  which  clings  to  the  mucous  mem- 
brane, coats  over  its  folds  (valvulse  conniventes)  and  surrounds 


FUNCTIONS   OF   THE   LARGE   INTESTINE.  187 

the  long  villi  of  the  jejunum,  etc.  This  intestinal  chyme  is  the 
form  in  which  the  food  is  presented  to  the  mucous  membrane  for 
absorption.  It  resembles  somewhat,  by  its  whiteness,  the  fluid 
called  chyle  which  flows  in  the  lacteals,  and  formerly  was  con- 
sidered to  be  identical  with  it.  This  creamy  lining  is  the  chief 
material  found  in  the  upper  part  of  the  small  intestine,  the 
coarser  parts  of  the  food  being  hurried  onward  by  peristaltic 
action  to  the  large  intestine. 

In  the  large  intestine  the  secretion  of  the  long,  closely-set  Lie- 
berkiihn's  follicles  is  the  only  one  of  importance.  Its  reaction 
and  that  of  the  mucous  membrane  is  alkaline,  but  the  contents 
of  the  colon  are  acid,  owing  to  certain  fermentative  changes 
which  go  on  in  this  part  of  the  intestine. 

Of  the  changes  brought  about  in  the  large  intestine  by  the 
agency  of  the  digestive  juices  we  know  but  little.  Judging  from 
the  large  size  of  the  caecum  and  colon  in  herbivorous  animals,  we 
are  prompted  to  conclude  that  vegetable  substances,  possibly  cel- 
lulose, may  be  dissolved  here,  but  we  do  not  know  how  this  is 
accomplished. 

Although  devoid  of  villi,  the  large  intestine  can  certainly 
absorb  readily  such  materials  as  are  in  solution.  As  the  in- 
soluble materials  pass  along  the  small  intestines  the  supply  of 
fluid  is  kept  up  to  about  the  same  standard,  the  absorption  and 
secretion  being  nearly  equal ;  but  in  the  large  intestine,  the  absorp- 
tion of  the  fluid  so  much  exceeds  the  secretion  that  the  undigested 
materials  are  gradually  deprived  of  their  fluid,  and  are  con- 
verted into  soft  solid  masses  which  pass  on  to  be  added  to  the 
faeces. 

Owing  to  its  absorbent  power  the  large  intestine  forms  a  ready 
channel  by  which  materials  can  be  introduced  into  the  system 
in  cases  in  which  the  stomach  is  too  irritable  to  retain  food. 

The  quantity  of  faeces  evacuated  in  the  day  depends  upon  the 
kind  of  diet,  being  greater  with  a  vegetable  than  meat  diet, 
averaging  about  150  grammes  a  day  (60-250  grms.).  This 
amount  may  be  greatly  increased  by  partaking  largely  of  indiges- 
tible forms  of  food.  The  more  rapid  the  passage  of  the  ingesta 
through  the  intestine  the  greater  is  the  amount  of  fluid  remain- 


188  MANUAL   OF   PHYSIOLOGY. 

ing  with  the  faeces,  so  that  any  stimulant  to  the  intestinal  move- 
ments reduces  the  consistence  of  the  faeces  and  facilitates  the 
evacuation.  The  fetor  depends  in  a  great  measure  on  the  pre- 
sence of  indol,  which  is  an  outcome  of  pancreatic  digestion,  and 
also  upon  the  presence  of  certain  volatile  fatty  acids.  The  color 
depends  upon  the  amount  of  the  bile  pigment  and  the  degree  of 
change  the  latter  has  undergone. 

The  faeces  are  composed  of  (1)  the  undigested  parts  of  the 
food,  and  (2)  the  useless  or  injurious  parts  of  the  secretions  of 
the  various  glands.  In  the  first  category  we  find  perfectly  in- 
digestible stuffs,  such  as  yellow  elastic  tissue,  horny  structure, 
portions  of  hairs  from  animal  food,  and  cellulose,  woody  fibre 
and  spiral  vessels  from  plants,  and  also  masses  of  digestible  sub- 
stances which  have  been  swallowed  in  too  large  pieces  to  be 
thoroughly  acted  on  by  the  secretions.  All  forms  of  food  may 
thus  appear  in  the  faeces,  but  vegetable  substances  are  most  con- 
spicuous. 

In  the  second  category  we  find  a  variable  quantity  of  mucus 
and  the  decomposed  coloring  matter  of  the  bile,  together  with 
some  cholic  acid,  cholesterin,  etc. 

A  few  inorganic  substances  are  found,  mainly  those  which  dif- 
fuse with  difficulty,  as  calcium  salts  and  ammonio-magnesium 
phosphate. 

Putrefactive  Fermentations  in  the  Intestine. — With  the  air 
and  saliva  which  are  swallowed  mixed  with  the  food,  large  num- 
bers of  the  lower  organisms  existing  in  them  are  introduced  into 
the  alimentary  canal. 

The  effect  of  these  organisms  is  to  produce  certain  fermentative 
changes  quite  distinct  from  the  action  of  the  special  ferments  of 
the  digestive  fluids. 

This  is  proved  by  the  composition  of  the  gases  found  in  the 
intestine.  Atmospheric  air  only  is  introduced  from  without, 
and  this  is  not  found  in  any  part  of  the  alimentary  tract,  the 
oxygen  soon  being  absorbed  and  the  nitrogen  left,  while  a  quan- 
tity of  carbonic  anhydride  and  hydrogen  from  the  fermentation 


FERMENTATIONS   IN   THE   INTESTINE.  189 

of  the  sugar  are  set  free,  lactic  and  butyric  acids  being  produced 
at  the  same  time. 

Indol  and  skatol  are  also  formed  by  putrefactive  fermentation 
of  the  leucin  and  tyrosin. 

It  is  in  the  large  intestine  that  putrefactive  fermentations  have 
greatest  effect,  the  acid  reaction  being  caused  by  the  various  acids 
thus  produced. 

With  regard  to  the  interesting  question,  why  the  digestive 
juices  do  not  dissolve  the  tissues  of  the  organs  in  which  they  are 
contained,  we  cannot  speak  positively.  We  can  no  longer  say 
that  the  "  vital  principle  "  has  a  protective  influence,  for  we  know 
that  the  fact  of  a  tissue  being  alive  is  not  sufficient  to  ward  off 
the  digestive  action  of  the  alimentary  juices.  The  limb  of  a 
living  frog  is  digested  when  introduced  through  a  fistula  into  the 
stomach  of  a  dog ;  and  when  the  intestinal  juice  trickles  from  a 
fistula  the  neighboring  skin,  the  snout,  and  the  tongue  of  the 
animal  soon  become  eaten  away,  owing  to  its  licking  the  fluid, 
which  rapidly  digests  these  parts  so  as  to  destroy  the  skin  and 
even  expose  the  blood  vessels. 

We  can,  however,  modify  John  Hunter's  statement  that  the 
resisting  power  was  associated  with  the  life  of  the  structures,  by 
saying  that  it  is  not  the  property  of  an  abstract  "  vital  principle," 
but  a  special  resisting  power  dependent  upon  the  specific  charac- 
ter of  the  vital  processes  of  those  textures  which  manufacture  and 
are  habitually  exposed  to  the  influence  of  the  juices. 


190  MANUAL   OF   PHYSIOLOGY. 


CHAPTER  XII. 
ABSORPTION. 

The  nutritive  materials  must  be  distributed  to  the  textures  and 
organs  in  order  that  the  food  stuffs,  when  altered  by  the  various 
processes  described  under  digestion,  may  be  of  any  use  to  the 
economy.  For  this  purpose  they  must  pass  through  the  lining 
membrane  of  the  alimentary  canal  and  gain  access  to  the  blood, 
which  is  the  common  mode  of  intercommunication  between  the 
various  parts  of  the  body. 

The  nutrient  part  of  the  food  has  then  to  be  absorbed  out  of 
the  alimentary  canal  by  the  surrounding  tissues,  and  mixed  with 
the  general  circulating  fluids,  lymph  and  blood. 

But  the  blood  is  separated  from  the  intestinal  contents  by 
barriers,  which,  as  far  at  least  as  the  blood  is  concerned,  are  im- 
passable, although  it  exerts  considerable  pressure,  and  thereby 
tends  to  escape  from  the  blood  vessels. 

The  question  then  arises,  How  does  the  elaborated  chyme 
make  its  way  through  this  barrier,  which  is  sufficient  to  prevent 
the  flow  of  blood  into  the  intestinal  tract  ? 

The  general  answer  is  easily  given,  viz.:  the  blood  cannot 
pass  through  an  animal  membrane.  But  this  is  not  a  satisfactory 
solution  of  the  question,  for  under  certain  abnormal  circum- 
stances, the  blood  does  pass  through  the  wall  of  the  vessels,  and 
normally  the  plasma  escapes  from  the  capillaries  into  the  tissues, 
in  order  to  nourish  them.  We  must  further  remember,  in  con- 
sidering this  point,  that  the  wall  of  the  vessels  and  the  membrane 
lining  the  intestine  are  both  made  up  of  living  cells  which  are 
endowed  with  a  capability,  coincident  with  their  lives,  of  con- 
trolling any  passage  through  or  between  them.  Some  of  these 
cells,  which  we  might  call  secreting  agents,  do  allow,  or  rather 
cause,  a  passage  of  fluid  from  the  blood  to  the  intestinal  cavity, 
and,  as  we  shall  presently  see,  others  of  them  induce  a  passage  of 


ABSORPTION. 


191 


the  nutritious  materials  from  the  intestinal  canal  into  the  sur- 
rounding tissues. 

In  order  clearly  to  understand  the  method  by  which  absorp- 
tion is  accomplished,  it  is  necessary  to  have  some  idea  of  the 
absorbent  system  generally ;  it  may  be  well,  therefore,  at  this 

FIG.  80. 


Diagram  showing  the  Course  of  the  Main  Trunks  of  the  Absorbent  System.  The  lym- 
phatics oflower  extremities  (D)  meet  the  lacteals  of  intestines  (LAC)  at  the  receptacu- 
lum  chyli  (R.  c.),  where  the  thoracic  duct  begins.  The  superficial  vessels  are  shown 
in  the  diagram  on  the  right  arm  and  leg  (s),  and  the  deeper  ones  on  the  arm  to  the 
left  (D).  The  glands  are  here  and  there  shown  in  groups.  The  small  right  duct  opens 
into  the  veins  on  the  right  side.  The  thoracic  duct  opens  into  the  union  of  the  great 
veins  of  the  left  side  of  the  neck  (T). 


192  MANUAL   OF    PHYSIOLOGY. 

place  to  give  a  brief  account  of  the  construction  of  the  special 
apparatus  which  carries  on  this  function.  Although  the  absorb- 
ent vessels  form  one  continuous  system,  they  may  be  conveni- 
ently divided  into  two  departments,  namely,  interstitial  and  sur- 
face absorption.  A  certain  modification  of  the  latter,  called  the 
lacteal  system,  occurs  in  the  alimentary  canal,  and  is  described 
under  intestinal  absorption. 

I.  INTERSTITIAL  ABSORPTION. 

The  blood  flowing  through  the  body  in  the  delicate  capillary 
vessels  yields  to  the  various  tissues  a  kind  of  irrigation  stream  of 
plasma,  which  leaving  the  capillaries  permeates  every  tissue  and 
saturates  them  with  nutrient  fluid.  The  surplus  of  this  irrigation 

FIG.  81. 


Tendon  of  Mouse's  Tail  treated  with  nitrate  of  silver,  showing  clefts  or  cell  spaces  around 
the  bundles  of  fibrils  as  white  patches.  These  interstices  may  be  called  the  smallest 
lymph  channels  or  spaces.  (SchiLfer.) 

stream  is  collected  and  carried  back  to  the  blood  current  by  a 
special  set  of  fine  flattened  vessels  with  slender  walls,  called  the 
lymph  vascular  system,  which  acts  as  the  drainage  of  the  tissues, 
and  pours  its  contents  into  the  veins. 

When  the  nutrient  fluid  escapes  from  the  capillaries,  it  lies  in 
the  interstices  between  the  tissue  elements,  and  here  bathes  the 
cells  which  commonly  occupy  these  lymph  spaces.  (Figs.  81  and 
86.) 

Communicating  freely  with  the  interstices  of  the  tissues  are 
irregular  anastomosing  flattened  channels,  which  convey  the 
lymph  or  any  fluid  forced  between  the  tissues  into  vessels  with 
definite  boundaries.  These  vessels,  which  are  lined  with  charac- 
teristic endothelium,  form  a  more  or  less  dense  network  of  lym- 


LYMPHATIC    SYSTEM. 


193 


phatic  capillaries,  from  which  spring  the   tributaries    of   the 
lymph  vessels.     (Figs.  82  and  83.) 

The  lymphatic  vessels  are  throughout  slender,  thin-walled 
channels  with  frequent  anastomoses  and  close-set  valves,  usually 
in  pairs.  They  lie  imbedded  in  the  connective  tissue,  and  when 
empty  are  difficult  to  see,  owing  to  the  extreme  thinness  of  their 
coats.  They  converge  toward  a  central  vessel  called  the  tho- 

FIG.  82. 


Lymph  Channels  from  the  thoracic  side  of  the  central  tendon  of  the  diaphragm  of  the 
rabbit,  treated  with  silver  nitrate.  The  fine  lines  indicate  the  boundaries  of  the 
endotheliuin  cells  lining  the  lymph  channels.  The  dark  part  shows  the  islets  between 
the  lymphatic  network.  (Klein.) 

racic  duct,  which,  passing  from  the  abdominal  cavity  through  the 
thorax,  reaches  the  left  side  of  the  neck,  and  opens  into  the 
angle  of  junction  of  the  two  great  veins  from  the  head  and 
upper  extremity.  (Fig.  80.)  On  the  right  side  a  smaller  trunk, 
conveying  the  lymph  from  the  right  arm  and  that  side  of  the 
head,  chest  and  neck,  opens  into  the  corresponding  venous 
trunks. 
17 


194 


MANUAL   OF   PHYSIOLOGY. 


The  thoracic  duct  is  much  larger  than  any  of  the  numerous 
tributaries  which  enter  it  at  close  intervals  from  all  directions. 

Its  lower  extremity  or  point  of  origin  is  an  irregular  dilatation 
called  the  receptaculum  chyli,  because  the  lymphatic  vessels  from 
the  stomach  and  intestines,  or  lacteals  as  they  are  called,  pour 
their  contents  into  it.  The  chyle  from  the  intestines  thus  flows 
into  the  same  main  channel  as  the  lymph  which  is  derived  from 

FIG.  83. 


Diagram  of  a  Lymphatic  Gland,  showing  (a  1)  afferent  and  (e  I)  efferent  lymphatic  ves- 
sels ;  (c)  Cortical  substance ;  (M)  Medullary  substance ;  (c)  Fibrous  coat  sending  tra- 
beculse  (t  r)  into  the  substance  of  the  gland,  where  they  branch,  and  in  the  medullary 
_part  form  a  reticulum ;  the_trabeculse  are  surrounded  by  the  lymph  path  or  sinus  (I  s), 
which  separates  them  from  the  adenoid  tissue  (I  h),  (Sharpey.) 

the  drainage  of  the  tissues  and  organs  of  the  lower  extremity, 
the  trunk  and  left  side  of  the  head,  neck  and  arm,  and  the  two 
fluids  are  mixed  in  the  receptaculum  chyli,  and  other  parts  of 
the  thoracic  duct. 

LYMPHATIC  GLANDS,  ETC. 

Along  the  course  of  the  lymphatic  vessels  numerous  small 
bodies  called  lymphatic  glands  or  follicles  are  found,  which  are 


STRUCTURE   OF    LYMPHATIC   GLANDS. 


195 


composed  of  a  delicate  trelliswork  of  adenoid  tissue,  packed  with 
nucleated  protoplasmic  cells,  called  lymph  corpuscles,  the  combi- 
nation making  what  is  known  as  lymphoid  tissue.  (Fig.  83  (I  h~) 
and  85  (a).')  These  masses  of  cells  and  their  delicate  supporting 
reticulum  are  enclosed  in  a  fibrous  case  or  capsule  from  which 
branching  trabeculse  pass  into  the  gland  and  separate  the  por- 


FIG.  84. 


Lymphatic  Network  from  between  the  Muscle  Coats  of  the  Intestinal  Wall,  with  fine 
vessels  and  many  valves,  causing  the  walls  to  bulge.     (Cadiat.) 

tions  of  lyraphoid  tissue  from  one  another.  The  lymph  channels 
enter  and  pour  their  contents  through  the  convex  side  of  the 
capsule.  The  lymph  then  flows  through  irregular  p&ths,  which 
lie  between  the  lymph  follicles  next  to  the  capsule  and  trabeculse, 
and  lead  to  the  concavity  of  the  gland  from  which  the  efferent 
vessel  escapes. 


196 


MANUAL   OF   PHYSIOLOGY. 

FIG.  85. 


Section  through  the  central  or  medullary  part  of  a  Lymphatic  Gland,  showing  adenoid 
tissue  (a)  containing  capillaries  (6)  and  a  fibrous  trabecula  (c)  cut  across  showing  a 
central  artery.  (Cadiat.)  t 

FIG.  86. 


Clefts  in  the  Corneal  Tissue  of  a  Frog  treated  with  nitrate  of  silver,  which  leaves  the 
spaces  clear  and  stains  the  intermediate  structure.  These  clefts  (a)  and  their  pro- 
cesses (ft)  form  the  lymph  canalicular  system,  and  at  the  same  time  are  the  spaces  in 
which  the  corneal  corpuscles  reside.  (Klein.) 


STRUCTURE   OF   LYMPHATIC   GLANDS.  197 

These  lymph  glands  occur  in  groups  in  the  flexures  of  the  limbs, 
the  recesses  of  the  neck,  and  the  thoracic  and  abdominal  cavities, 
a  large  number  being  placed  in  the  mesentery,  in  the  course  of 
the  intestinal  lacteals. 

In  the  submucous  tissue  of  the  intestine  this  lymphoid  tissue 
is  widely  diffused,  and  here  and  there  arranged  in  small  follicles, 
which  doubtless  have  a  function  similar  to  that  of  the  lymph 
glands  found  elsewhere. 

LYMPHATIC  VESSELS. 

There  are  various  modes  of  origin  of  the  lymphatic  vessels 
which  are  more  or  less  characteristic  of  the  different  parts  in 
which  they  occur. 

FIG.  87. 


Endotheliuni  from  serous  surface  without  stomata  (nitrate  of  silver). 

In  the  connective  and  allied  tissues  there  are  variously-formed 
fissures  or  clefts,  which  can  be  filled  with  fluid  forced  into  the 
tissues  by  puncturing  the  skin  with  the  nozzle  of  a  fine  syringe, 
such  as  is  used  for  hypodermic  injection. 

These  fissures  contain  the  protoplasmic  units  of  the  tissue,  and 
transmit  the  ordinary  transudation  stream  for  nourishing  the 
tissues.  They  freely  communicate  with  one  another,  and  lead 
into  the  beginnings  of  the  network  of  lymphatic  capillaries. 

The  lymph  capillaries  run  midway  between  the  blood  capil- 
laries, and  are  made  up  of  a  single  layer  of  nucleated  endothe- 
lial  cells,  which  can  be  brought  to  light  with  silver  staining. 


198 


MANUAL   OF   PHYSIOLOGY. 


Ill  some  tissues,  such  as  that  of  the  central  nervous  system,  liver 
and  bone,  the  lymph  vessels  commence  as  channels  encircling 
the  blood  vessels,  or  perivascular  lymph  spaces.  Here  the  lymph 
channels  form  a  kind  of  sheath  for  the  minute  blood  vessels,  and 
pass  along  to  the  connective  tissue  of  the  adventitia  of  the 
larger. 

The  lymph  vessels  may  also  be  said  to  commence  on  the  sur- 
face of  serous  membranes  which  are  intimately  connected  with 
the  lymphatic  system,  and  may  indeed  be  regarded  as  nothing 


FIG.  88. 


Endothelium  from  serous  surface  with  stomata  surrounded  with  granular 
protoplasmic  cells. 


more  than  inordinately  developed  lymph  spaces.  In  most  parts 
of  the  endothelial  surface  of  serous  cavities  are  a  number  of  so- 
called  stomata,  or  small  apertures  surrounded  by  a  few  cells, 
which  differ  from  the  ordinary  endothelial  cells  in  many  respects, 
and  probably  have  to  control  the  passage  of  the  fluid  from  the 
serous  cavity  into  the  lymph  vessels.  These  stomata  are  found 
to  be  placed  at  the  commencement  of  the  dense  network  of 
lymph  capillaries,  which  lies  in  the  subserous  tissue. 


INTESTINAL   ABSORPTION. 


199 


FIG.  89. 


Diagram  of  relation  of  the 
epithelium  to  the  lacteal 
radical  in  villus.  The  pro- 
toplasmic epithelial  cells 
supposed  to  be  connected  to 
the  absorbent  vessel  by  ade- 
noid tissue.  (After  Funke.) 


II.  INTESTINAL  ABSORPTION. 

The  intestinal  absorbents  form  a  special  department  of  the 
lymphatic  system  aiding  nutrition.  On 
account  of  the  white  chyle  seen  as  a 
milky  fluid  through  their  transparent 
walls,  they  have  been  called  lacteals. 
Their  functions  are  to  take  up  the  nutri- 
ent fluid  from  the  intestinal  cavity,  and 
to  drain  the  tissue  in  which  they  lie.  In 
order  to  fulfill  these  functions,  they  are 
arranged  in  a  particular  way,  especially 
adapted  to  the  peculiar  construction  of 
the  mucous  membrane  lining  this  part  of 
the  alimentary  tract,  which  must  be 
briefly  described  before  the  mechanism  of 
absorption  can  be  understood. 

The  most  striking  characteristic  of  the 
lining  membrane  of  the  small  intestine  is 
the  existence  of  villi,  which  are  only 
found  in  this  part  of  the  alimentary  tract.  They  consist  of  nip- 
ple-shaped processes,  projecting  into  the  intestinal  cavity,  so 
closely  set  that  they  have  the  appearance  of  the  pile  of  velvet ; 
and  being  just  visible  to  the  naked  eye,  they  give  the  mucous 
membrane,  when  washed  and  held  under  water,  a  peculiar  vel- 
vety look.  By  means  of  these  villi,  and  also  of  the  ring-like  folds 
of  mucous  membrane  in  the  upper  part  of  the  small  intestine, 
the  extent  of  surface  over  which  the  chyme  has  to  travel  is  greatly 
increased. 

The  surface  of  the  villi  is  covered  over  with  a  simple  layer  of 
columnar  epithelial  cells  in  continuity  with  the  epithelium  lining 
the  rest  of  the  intestinal  tract.  The  free  surface  of  these  cells  is 
marked  by  a  clear  striated  margin  composed  of  a  row  of  minute 
rods  closely  packed  together,  while  the  deep-seated  end  of  the 
cells  is  branched,  and  appears  to  be  prolonged  into  the  substance 
of  the  villus  and  in  some  way  to  be  connected  with  the  support- 
ing retiform  tissue.  Some  of  the  cells  are  seen  to  swell  upon  the 
addition  of  certain  reagents,  owing  to  their  containing  mucus, 


200 


MANUAL   OF   PHYSIOLOGY. 


which  gives  them  a  peculiar  goblet  shape ;  hence  they  are  called 
goblet  cells.  These  occur  at  intervals,  and  some  observers  con- 
sider that  they  form  a  distinct  variety,  differing  from  the  neigh- 
boring cells  just  as  the  border  cells  of  the  stomach  glands  differ 
from  the  central  cells.  (Fig.  79,  p.  183.) 

The  body  of  the  villus  is  composed  of  a  very  delicate  kind  of 

FIG.  90. 


Section  of  Intestine  of  a  Dog  in  which  the  blood  vessels  (c)  and  the  lacteals  (d)  have  been 
injected.  The  blind  ending  or  simple  loop  of  the  black  lacteal  is  seen  to  be  surrounded 
by  the  capillary  network  of  the  blood  vessels.  (Cadiat.) 

connective  tissue,  forming  a  slender  frame  in  which  a  little  cage 
like  network  of  blood  vessels  surrounds  a  central  lacteal  radicle. 
The  interstices  of  this  connective  tissue  are  filled  with  pale  pro- 
toplasmic cells,  like  those  formed  in  the  lymph.  Under  the  base- 
ment membrane  forming  the  foundation  of  the  epithelium  are 
some  unstriated  muscle  cells  which  embrace  the  villus  and  are 


LYMPH   FOLLICLES   OF   SMALL   INTESTINE.  201 

FIG.  91. 


Diagram  of  Section  of  the  Mucous  Membrane  of  the  Intestine,  showing  the  position  of 
the  lymph  follicles  (a).  (Cadiat.) 


Section  of  Single  Lymph  follicle  of  the  Small  Intestine,  showing  (a)  follicle  covered  with 
epithelium  (6),  which  has  fallen  from  the  villi  (c)  \  (d)  LieberkUhn's  follicles;  (e)  Mus- 
cularis  mucosse.  (Cadiat.) 


202 


MANUAL   OF   PHYSIOLOGY. 


able  to  squeeze  it  and  empty  the  vessels  that  lie  within  it.  The 
lacteal  radicles  which  lie  in  the  villi  are  sometimes  double,  and 
have  a  communication  with  the  lymph  spaces  of  the  connective 
tissue.  They  frequently  branch  as  they  pass  down  from  the  villi 
to  reach  the  dense  network  of  lacteal  vessels  which  lies  beneath 
the  mucous  membrane.  (Fig.  93.) 

FIG.  93. 


Section  through  the  Intestinal  Wall  in  the  neighborhood  of  the  grouped  lymph  follicles 
(/)  (Peyer's  patch),  showing  the  upper  narrow  (6)  and  the  deep,  wide  (c)  lymphatic 
plexuses. 


At  irregular  intervals  throughout  the  submucous  tissue  are 
found  masses  of  lymphoid  tissue  similar  to  those  seen  in  packets 
within  a  lymph  gland  or  in  other  lymph  follicles.  These  are 
either  isolated  (solitary  glands)  or  collected  into  groups  (agmin- 
ated  glands  or  Peyer's  patches).  Though  called  glands  by 
anatomists,  it  should  be  borne  in  mind  that  they  are  in  no  way 


MECHANISM    OF    ABSORPTION.  203 

connected  with  the  secretion  of  any  of  the  intestinal  juices,  but 
belong  to  the  absorbing  arrangements  of  the  intestine.  Around 
these  solitary  and  grouped  lymph  follicles  are  spaces  and  net- 
works from  which  the  lacteal  vessels  arise  (Fig.  93). 

MECHANISM  OF  ABSORPTION. 

Formerly  absorption  was  supposed  to  take  place  by  means  of 
the  blood  vessels  alone.  After  the  discovery  of  lymph  and  chyle 
vessels  by  Caspar  Aselli  the  belief  in  the  direct  absorption  by 
the  blood  vessels  was  abandoned,  and  all  the  work  of  absorption 
was  attributed  to  the  lymphatics.  Now,  however,  ample  evidence 
exists  to  show  that  substances  capable  of  absorption  can  make 
their  way  into  the  blood  vessels  of  any  part  not  protected  by  an 
impermeable  covering  like  the  horny  layer  of  the  skin,  and  thus 
be  carried  directly  to  the  general  circulation.  The  share  taken 
by  the  blood  vessels  in  interstitial  absorption  in  the  tissues  is  not 
denned,  and  when  no  impediment  to  the  lymph  flow  exists  is 
probably  insignificant. 

In  the  absorption  from  the  alimentary  tract  the  blood  vessels 
appear  to  take  a  considerable  part. 

How  far  the  tissue  interspaces  and  the  local  lymph  channels, 
many  of  which  surround  the  blood  vessels,  aid  in  the  passage  of 
substances  into  the  blood  currents  is  not  known ;  but  they  pro- 
bably have  some  such  effect,  for  the  experiments  showing  direct 
absorption  by  the  blood  vessels  leave  the  local  lymph  channels 
in  operation,  and  at  the  same  time  the  normal  flow  of  lymph 
toward  the  thoracic  duct  is  more  or  less  hindered. 

We  can  easily  imagine  that  a  surface  covered  by  a  single 
layer  of  epithelial  cells,  with  numerous  blood  vessels  and  a  good 
supply  of  absorbents  beneath  them,  is  capable  of  absorbing 
materials  in  solution  ;  and  we  know  that  large  quantities  of  fluids 
and  solutions  of  various  materials  are  absorbed  from  the  stomach 
and  large  intestine,  partly,  no  doubt,  by  means  of  the  lacteals  or 
lymphatics,  and  partly  by  the  minute  blood  vessels  themselves. 

The  small  intestine,  however,  seems  to  be  the  part  of  the  ali- 
mentary tract  which  is  especially  adapted  for  taking  up  the 
materials  elaborated  from  the  food. 


204  MANUAL   OF   PHYSIOLOGY. 

In  the  upper  part  of  the  small  intestine  the  valvulse  con- 
niventes  are  most  marked,  and  the  villi  are  long  and  set  closely 
together.  It  is  here  we  find  the  thickest  layer  of  creamy  chyme 
covering  the  mucous  membrane,  but  seldom  any  masses  of  un- 
digested food.  All  these  points  tend  to  show  that  the  upper 
part  of  the  small  intestine  is  the  part  specially  adapted  for 
absorption.  The  chyme  which  clings  to  the  mucous  membrane 
contains  all  the  substances  destined  to  pass  into  the  economy. 
Into  this  mixture  the  villi  dip,  so  that  each  villus  is  bathed  in 
chyme.  From  what  has  been  said  of  the  construction  of  the  villi, 
it  is  obvious  that  such  an  arrangement  is  well  adapted  to  the 
absorption  of  the  nutrient  material,  which  is  in  the  closest  prox- 
imity to  the  lacteals  and  blood  vessels. 

The  various  food  stuffs  in  the  chyme  differ  in  the  degree  of 
readiness  with  which  they  are  absorbed.  Hence  the  facility  of 
absorption  of  its  principal  ingredients  must  be  examined  sepa- 
rately in  detail. 

Water  can  be  absorbed  from  the  intestinal  tract  in  almost  un- 
limited quantity,  but  not  solutions  of  salts.  The  amount  of  the 
solution  of  any  salt  capable  of  absorption  seems  to  depend  on  its 
endosmotic  equivalent.  The  lower  the  endosmotic  equivalent  the 
more  readily  the  solution  passes  into  the  blood  vessels.  In  those 
cases  where  the  equivalent  is  very  high,  such  as  magnesium  sul- 
phate, there  is  a  tendency  of  the  fluid  to  pass  o.ut  from  the  blood 
vessels  into  the  intestinal  cavity ;  this  has  been  supposed  to  explain 
the  watery  stools  caused  by  this  and  such  like  saline  purgatives. 

Among  the  carbohydrates  we  need  only  take  into  account  the 
sugars,  for  starch  unchanged  is  but  little,  if  at  all,  absorbed. 
Only  a  certain  quantity  of  sugar  can  be  taken  up  by  the  intes- 
tinal absorbents,  since  some  is  found  in  the  fseces  when  the 
amount  taken  with  the  food  exceeds  a  certain  quantity.  Some 
of  the  sugar  in  the  intestine  undergoes  fermentation,  by  which  it 
is  converted  into  lactic  and  butyric  acid.  The  quantity  of  sugar 
absorbed  as  lactic  and  butyric  acid  has  not  been  determined, 
but  the  amount  found  in  the  portal  vessels  or  lacteals  does  not 
appear  to  correspond  with  that  which  disappears  from  the  cavity 
of  the  intestine. 


ABSORPTION   OF   SPECIAL   MATERIALS.  205 

Ordinary  proteids,  being  colloids,  can  only  pass  with  difficulty 
through  an  animal  membrane,  hence  it  is  supposed  that  they 
must  be  changed  during  digestion  into  peptones  before  they  can 
be  absorbed.  Their  absorption  takes  place  readily  in  the  stomach, 
and  is  completed  in  the  small  intestine,  as  only  a  small  quantity 
of  albuminous  substances  is  found  in  the  large  intestine  even  after 
an  excessive  meat  diet.  The  more  concentrated  the  solutions  of 
peptones  the  more  rapidly  are  they  absorbed,  and  the  rate  of 
absorption  is  greatest  at  first,  and  then  by  degrees  diminishes. 
The  presence  of  free  alkali  is  said  to  facilitate  the  absorption  of 
peptones.  It  is  a  curious  fact  that  neither  in  the  lacteals  nor  in 
the  portal  blood  can  any  quantity  of  peptones  be  found,  even 
during  active  proteid  digestion ;  so  that  it  is  impossible  to  trace 
out  their  course  as  peptones,  or  to  say  by  which  set  of  absorbent 
channels  they  reach  the  blood.  If  we  assume  that  all  proteids 
must  be  absorbed  as  diffusible  peptones,  we  are  forced  to  conclude 
that  during  their  passage  from  the  intestinal  cavity  they  must  be 
reconverted  into  ordinary  proteids.  But  we  know  that  soluble 
forms  of  albumin  are  to  some  extent  diffusible  (when  a  solution 
of  salt  is  used)  through  a  dead  animal  membrane.  But  even 
were  they  quite  indiffusible,  this  fact  would  not  preclude  the 
possibility  of  their  passing  through  the  intestinal  wall,  which  is  a 
living  structure  not  restricted  by  such  physical  difficulties  as  are 
met  with  in  diffusion  through  an  inanimate  membrane.  When 
we  know  that  solid  particles  of  fat  can  enter  the  lacteals  (an 
event  which  we  cannot  explain  physically),  we  can  have  no  dif- 
ficulty in  believing  that  an  insoluble  solution  of  albumin  may 
also  be  admitted.  We  may  then  conclude  that  it  is  not  only 
possible,  but  even  probable,  that  a  good  deal  of  proteid  is  absorbed 
as  ordinary  soluble  albumin.  A  certain  limit  to  proteid  absorp- 
tion exists,  so  that  if  an  amount  of  albuminous  material  above 
the  maximum  that  can  be  absorbed  be  eaten,  the  albumins  are 
either  converted  into  leucin  and  tyrosin,  or  thrown  off  with  the 


In  the  absorption  of  water,  watery  solutions  of  salts,  sugars, 
and  peptones  by  the  lacteals,  there  are  no  great  physical  difficul- 
ties to  be  got  over  ;  so  that  we  are  in  the  habit  of  speaking  con- 


206  MANUAL   OF   PHYSIOLOGY. 

j 

fidently  about  the  mechanism  of  their  absorption,  although  in 
all  probability  many  circumstances  connected  with  the  life  of  the 
epithelial  cells,  etc.,  of  which  we  are  ignorant,  cooperate  in  bring- 
ing about  the  results  which  seem  to  us  so  simple  to  explain  in  our 
own  fashion. 

It  is  not  so  with  the  fatty  food  stuffs.  A  small  quantity  of 
these  may,  no  doubt,  be  split  up  into  soluble  glycerine  and  fatty 
acids,  which  are  at  once  changed  into  soluble  soaps,  and  in  this 
condition  are  capable  of  simple  osmotic  transmission  into  the 
blood  vessels  or  lacteals.  The  greater  portion  of  fat  enters  the 
lacteals  as  fat  in  the  condition  of  fine  emulsion,  i.  e.,  composed  of 
solid  particles.  This  process  is  difficult  to  reconcile  with  our 
physical  experiences ;  for,  however  finely  divided  it  may  be,  fat 
emulsified  does  not  pass  through  an  animal  membrane  more 
freely  than  ordinary  fluid  fat. 

The  fat  emulsion  is  chiefly  taken  up  by  the  villi  of  the  small 
intestines,  for  in  the  stomach  it  exists  only  in  large  fluid  masses 
or  globules,  and  the  amount  of  fat  found  in  the  large  intestine  is 
small,  unless  used  as  food  in  great  excess.  This  can  also  be  seen 
in  examining  the  absorbent  vessels  after  a  fatty  meal,  when  those 
which  carry  materials  from  the  stomach  and  large  intestine  are 
clear  and  transparent,  while  those  coming  from  the  small  intes- 
tines are  filled  with  the  white  milky  fluid  which  gives  them  their 
special  name  of  lacteals.  There  is  a  limit  to  the  absorbent  capa- 
city of  the  intestine  for  fatty  matters,  for  when  a  great  excess  of 
fat  is  eaten  it  appears  with  the  excrement,  sometimes  giving  rise 
to  adipose  diarrhoea,  thus  showing  that  the  amount  has  exceeded 
this  limit. 

The  important  question  remains,  How  does  the  fat  emulsion 
make  its  way  through  the  intestinal  mucous  membrane  ?  That 
it  really  does  so  there  can  be  no  shadow  of  doubt ;  for  it  disap- 
pears from  the  intestinal  cavity,  and  can  be  detected  in  the  chyle 
with  the  aid  of  the  microscope  more  easily  than  any  other  of  the 
intestinal  contents  absorbed. 

It  has  been  shown  that  while  a  membrane  moistened  with  water 
acts  as  a  complete  barrier  to  a  fat  emulsion,  and  only  after  pro- 
longed exposure  under  high  pressure  allows  traces  of  fat  to  pass, 


ABSORPTION   OF   SPECIAL   MATERIALS.      U 

the  same  membrane  when  saturated  with  bile  will  without  pres- 
sure permit  the  passage  of  a  considerable  amount  of  oil.  It  has, 
therefore,  been  suggested  that  the  epithelial  cells  of  the  mucous 
membrane  are  more  or  less  moistened  with  bile,  and  the  particles 
of  fat  in  the  emulsion  are  also  coated,  as  it  were,  with  a  film  of 
bile  or  soap.  Thus  they  are  enabled  to  pass  into  the  epithelial 
cells,  in  which  they  can  be  detected  during  digestion.  The  bile  or 
soapy  coating  of  the  fat  particles  may,  no  doubt,  aid  in  their 
transit  through  the  various  obstacles  met  on  their  way  to  the 
lacteal  radicles,  but  the  course  taken  by  the  fat  particles  can 
hardly  be  explained  in  this  way.  Many  circumstances  force  us 
to  believe  that  the  activity  of  the  protoplasm  of  the  epithelial 
or  some  special  wandering  cells  forms  a  necessary  factor  in  the 
case. 

By  means  of  osmic  acid,  which  renders  the  fat  granules  black, 
they  may  be  demonstrated  to  occur  in  the  following  situations 
during  the  active  digestion  of  fat.  1.  In  many  of  the  epithelial 
cells  lining  the  villi,  etc.  2.  In  lymph  cells  lying  in  close  rela- 
tion to  the  epithelium  and  others  in  the  lymphoid  tissue  of  the 
villi.  3.  Between  the  epithelial  cells ;  possibly  held  here  by  pro- 
cesses from  the  amoeboid  lymph  cells.  The  fat  particles  are  then 
either  taken  up  by  the  epithelial  cells  from  the  cavity  of  the 
intestine,  and  handed  over  to  the  subjacent  lymph  cells,  or 
seized  by  the  protoplasmic  processes  of  the  lymph  cells  which 
pass  between  the  epithelial  cells  to  reach  the  surface. 

When  the  fat  is  once  lodged  in  the  protoplasm  of  the  cells, 
these  amoeboid  elements  convey  it  through  the  delicate  connec- 
tive tissue  of  the  villi  to  the  lacteal  radicle.  Other  forces,  such 
as  the  contraction  of  the  villi,  may  aid  in  their  further  movement 
to  the  central  lacteal  space  of  the  villus. 

The  exact  utility  of  the  marginal  bands  ot  rods  or  pores  which 
characterize  the  surface  of  the  intestinal  epithelium  is  not  known, 
though  it  has  been  supposed  to  be  connected  with  the  absorption 
of  fats. 

We  may  conclude,  then,  that  the  passage  through  the  intes- 
tinal wall  of  some  of  the  materials  taken  as  food  may  possibly  be 
accomplished  by  mere  physical  processes,  but  it  is  probable  that 


208  MANUAL   OF    PHYSIOLOGY. 

the  vital  activity  of  the  epithelial  cells  controls  the  absorption  of 
all  food  stuffs.  The  passage  of  the  fat  can  only  be  explained  by 
the  aid  of  the  direct  activity  of  cells  which  by  amoeboid  move- 
ment take  up  the  fine  particles  and  pass  them  on  to  the  inter- 
stices of  the  connective  tissue  of  the  villi. 

LYMPH  AND  CHYLE. 

As  these  two  fluids  are  generally  mixed  in  the  thoracic  duct, 
whence  the  lymph  is  commonly  obtained  for  examination,  we  may 
discuss  them  together,  though  the  lymph  might  more  properly  be 
considered  with  the  distribution  of  the  nutrient  materials  to  the 
tissues. 

The  fluids  coming  from  the  tissue  drainage,  from  the  lym- 
phatic glands  and  from  the  lacteals  of  the  alimentary  tract,  when 
mingled  in  the  thoracic  duct,  form  an  opaque  mixture  which 
holds  a  considerable  quantity  of  proteid  in  solution,  and  contains 
a  number  of  morphological  elements,  viz. :  (1),  protoplasmic 
cells  similar  to  those  found  in  the  lymph  follicles,  and  in  most 
essential  points  identical  with  the  pale  cells  found  in  the  blood  ; 
(2),  some  red  blood  corpuscles  which  gave  the  fluid  in  the  tho- 
racic duct  a  pinkish  color ;  (3),  a  quantity  of  very  finely  divided 
fat,  which  varies  in  proportion  to  the  amount  of  fat  recently 
digested ;  (4),  other  minute  particles  of  unknown  function  and 
origin. 

When  removed  from  the  body  and  allowed  to  stand,  the  lymph 
becomes  converted  into  a  soft  jelly.  This  coagulation,  no  doubt, 
depends  upon  the  chemical  changes  in  the  lymph  which  give 
rise  to  fibrin,  the  formation  of  which  will  be  discussed  more  fully 
in  a  future  chapter.  The  amount  of  fibrin  formed  in  the  lymph 
is  very  small,  and,  therefore,  the  clot  is  very  soft,  and  shrinks 
considerably. 

The  lymph  of  the  thoracic  duct  contains  three  forms  of  pro- 
teid :  (1),  serum  albumin,  which  can  be  coagulated  by  heat ;  (2), 
alkali  albumin  precipitated  by  neutralization  ;  and  (3),  globulin. 
It  also  contains  soap  in  solution,  cholesterin,  grape  sugar,  urea, 
leucin,  and  some  salts,  particularly  sodium  chloride,  and  the  sul- 
phates and  phosphates  of  the  alkalies. 


LYMPH   CORPUSCLES.  209 

The  quantity  of  chyle  which  can  be  obtained  from  the  lacteals 
is  small,  and,  therefore,  its  thorough  investigation  is  difficult. 
The  fluid  from  the  lacteals  differs  from  the  mixed  lymph  in 
appearance  and  constitution  only  during  digestion,  and  then 
chiefly  in  containing  a  greater  amount  of  fat  and  solids  derived 
from  the  intestinal  cavity. 

On  their  way  to  enter  into  the  blood  current  both  the  lymph 
and  chyle  undergo  changes.  Before  passing  through  the  lym- 
phatic glands  the  fluid  contains  much  fewer  lymph  corpuscles 
than  after  it  has  traversed  the  glands :  from  this  fact,  and  from 
the  structure  of  the  lymph  glands,  we  may  conclude  that  they 
are  the  chief  sources  of  these  white  cells.  The  chyle  of  the  lac- 
teal vessel  of  the  mesentery  contains  particles  of  fat  which  greatly 
exceed  in  size  those  found  in  the  thoracic  duct,  so  we  may  infer 
that  the  fat  emulsion  undergoes  a  further  subdivision  or  modifi- 
cation on  its  way  through  the  glands. 

Lymph  which  has  been  collected  from  the  lymph  channels  of 
the  extremities  is  an  almost  clear,  colorless  fluid,  rich  in  the 
waste  products  of  tissue  change,  but  containing  less  albumin 
than  that  coming  from  the  main  trunk,  and  no  fat.  After 
long  fasting  the  lymph  from  the  thoracic  duct  has  the  same 
characters. 

Lymph  contains  a  considerable  quantity  of  carbonic  acid  gas, 
about  50  vol.  per  cent.,  some  of  which  is  readily  removed  by  the 
air  pump,  and  is  therefore  said  to  be  absorbed  by  the  fluid, 
while  some  can  only  be  removed  by  the  addition  of  acids,  and 
therefore  is  considered  to  be  in  chemical  combination.  Only 
mere  traces  of  oxygen  have  been  found  in  the  lymph. 

The  quantity  of  chyle  and  lymph  poured  into  the  blood  varies 
so  much  that  any  estimation  of  the  amount  entering  in  a  given 
time  is  unreliable. 

The  following  circumstances  upon  which  the  variations  may 
depend  are  instructive  : — 

1.  The  ingestion  of  liquid  .and  solid  food  causes  a  great  in- 
crease in  the  amount  of  chyle.     This  is  obvious  from 
the  change  in  the  state  of  the  lacteal  vessels,  which,  from 
18 


210  MANUAL    OF    PHYSIOLOGY. 

being  transparent  and  almost  empty,  become  widely 
distended  and  white. 

2.  The  activity  of  any  organ  causes  an  increase  of  lymph  to 

flow  from  it. 

3.  Impediment  to  the  return  of  the  venous  blood  from  any 

part  increases  the  irrigation,  and  hence  the  lymph. 

4.  Increase  of  the  amount  or  the  pressure  of  the  blood  flow- 

ing through  any  part  augments  the  lymph  flow. 

5.  The  administration  of  curare  increases   the  amount  of 

lymph. 

The  history  of  the  structural  elements  or  lymph  corpuscles, 
which  exist  in  such  numbers  in  the  large  lymph  channels, 
requires  some  further  discussion,  as  these  cells  are  composed  of 
active  protoplasm  destined  for  some  important  function,  and 
must  be  produced  by  some  vital  process. 

The  origin  of  the  lymph  corpuscle  is  not  restricted  to  any  one 
part  of  the  body  or  to  any  special  organ.  It  has  been  already 
said  that  the  lymphatic  glands  are  the  most  important  source  of 
these  cells,  because  the  follicular  tissue  is  filled  with  them,  and 
the  lymph  contains  a  much  larger  number  after  it  has  passed 
through  some  lymph  glands.  In  the  lymphoid  tissue  of  the 
spleen  and  the  intestinal  mucous  membrane  they  are  very  nu- 
merous, and,  no  doubt,  many  have  their  origin  in  the  follicular 
tissue  of  that  organ  and  intestine.  They  are  said  also  to  be 
formed  in  the  red  marrow  of  the  bones.  Although  their  number 
is  relatively  small,  lymphatic  cells  occur  even  in  those  lymph 
channels  that  are  unconnected  with  a  lymphatic  gland,  and 
these  cells,  no  doubt,  come  from  the  blood,  which  contains  many 
cell  elements,  identical  with  the  lymph  cells  found  in  the  lym- 
phatic duct.  These  cells,  when  they  arrive  at  the  minute  blood 
vessels,  sometimes  leave  the  vessels  and  creep  by  amoeboid  move- 
ments into  the  interstices  of  the  tissue  with  the  irrigation  stream. 
They  may  permanently  abide  in  the  tissue,  or  be  washed  back 
into  the  larger  lymph  channels  with  the  surplus  stream  of  lymph. 
When  the  abnormal  increase  of  activity  in  a  tissue  known  as 
inflammation  occurs,  this  escape  of  the  white  cells  from  the 


MOVEMENT  OF  THE  LYMPH.  211 

blood  takes  place  with  great  rapidity,  and  the  stages  in  the  pro- 
cess can  be  watched  under  the  microscope. 

Still  another  source  of  the  lymph  cells  may  be  from  prolifer- 
ation of  the  cells  which  lie  in  the  tissues.  The  fixed  tissue  cells 
are  said  to  be  capable  of  producing  elements  identical  with 
lymph  cells,  which  by  division  possibly  multiply  and  produce 
their  like,  and  may  be  carried  along  by  the  lymph  stream  as 
lymph  cells. 

The  enormous  number  of  cells  which  accumulate  as  pus  when 
an  abscess  forms  are  structurally  identical  with  lymph  cells,  and 
probably  arise  from  these  combined  sources,  viz.,  escape  from  the 
blood  vessels  and  proliferation  of  the  tissue  cells. 

The  lymph  cells,  therefore,  whether  they  have  their  origin  in 
a  lymph  gland,  spleen,  or  connective  tissue,  perform  a  kind  of 
circuit,  going  with  the  lymph  into  the  blood,  and  are  distributed 
with  the  latter  to  the  tissues,  whence  they  may  be  once  more 
carried  into  the  lymph  stream. 

MOVEMENT  OP  THE  LYMPH. 

In  some  of  the  lower  animals  small  muscular  sacs  occur  in  the 
course  of  the  main  lymph  channels,  which  pump  the  lymph  into 
the  great  veins  by  contracting  rhythmically,  much  in  the  same 
way  as  the  heart. 

In  man  and  the  higher  animals  no  such  lymph  hearts  have 
been  found ;  the  onward  movement  of  the  fluid  depends  chiefly  on 
the  pressure  under  which  the  irrigation  stream  leaves  the  •  blood 
vessels.  The  fluid  in  the  blood  vessels,  as  we  shall  presently  see, 
is  under  considerable  pressure,  which  causes  the  plasma  to  leave 
the  capillaries.  Hence,  if  a  lymphatic  trunk  be  tied,  its  tribu- 
taries are  filled  with  lymph  until  a  considerable  pressure  (8-10 
mm.,  soda  solution)  is  developed  in  their  radicles. 

While  the  pressure  exerted  on  the  small  tributaries  of  the 
lymph  channel  may  become  considerable,  that  in  the  thoracic 
duct  is  invariably  very  low,  for  the  following  reasons :  The  blood 
in  the  large  veins  into  which  the  duct  opens  is  under  less  pres- 
sure than  in  any  other  part  of  the  vascular  system,  owing  to  the 
thoracic  suction,  or  negative  pressure  in  the  thorax,  caused  by 


212  MANUAL   OF   PHYSIOLOGY. 

the  elastic  traction  of  the  lungs.  In  fact,  the  pressure  in  the 
large  veins,  e.g.,  brachial,  etc.,  varies  from  0  to  — 4  mm.  Hg., 
and  that  in  the  venae  cavse  is  always  negative,  except  in  sudden 
or  forced  expiration,  and  varies,  according  to  the  period  of  the 
respiratory  rhythm,  from  —  5  mm.,  in  inspiration,  to  —  2  mm.  in 
expiration. 

FIG.  94. 


Diagram  showing  the  Course  of  the  Main  Trunks  of  the  Absorbent  System.  The  lym- 
phatics of  lower  extremities  (D)  meet  the  lacteals  of  intestines  (LAC)  at  the  recep- 
taculum  chyli  (R.  c.),  where  the  thoracic  duct  begins.  The  superficial  vessels  are 
shown  in  the  diagram  on  the  right  arm  and  leg  (s),  and  the  deeper  ones  on  the  arm  to 
the  left  (D).  The  glands  are  here  and  there  shown  in  groups.  The  small  right  duct 
opens  into  the  veins  on  the  right  side.  The  thoracic  duct  opens  into  the  union  of  the 
great  veins  of  the  leftside  of  the  neck  (T). 


MOVEMENT  OF  THE  LYMPH.  213 

The  fact  that  the  lymph  at  the  origin  of  the  small  channels  is 
at  a  pressure  of  8  to  10  mm.  of  water,  while  at  the  entrance  to 
the  vein  it  is  nil,  would  be  sufficient  to  explain  the  movement, 
even  if  there  were  no  other  force  aiding  it. 

It  must  be  remembered  that  every  lymph  vessel  is  furnished 
with  closely  set  valves,  which  prevent  the  fluid  it  contains  from 
being  forced  backward,  so  that  any  accidental  local  pressure  ex- 
ercised on  the  exterior  of  a  lymph  channel  helps  the  fluid  onward 
to  the  veins.  Along  their  entire  extent  these  vessels  are  subject 
to  certain  forces  which  must  materially  aid  the  flow  of  the  lymph 
stream.  The  first  of  these  is  the  pressure  exerted  on  the  small 
vessels  by  the  movement  of  the  muscles  in  the  neighborhood. 
The  second  is  the  unequal  distribution  of  atmospheric  pressure, 
which  has  full  force  on  the  peripheral  channels,  but  is  kept  off 
the  thoracic  duct  and  its  termination,  as  already  mentioned,  by 
the  rigidity  of  the  thoracic  wall,  which,  together  with  the  ten- 
dency of  the  elastic  lungs  to  shrink,  causes  a  permanent  negative 
pressure  in  the  thoracic  cavity  through  which  the  duct  passes. 
And,  lastly,  the  thin-walled  lymphatics  are  everywhere  surrounded 
with  very  elastic  textures  enclosed  in  an  elastic  skin  which  exert 
an  amount  of  pressure  sufficient  to  empty  and  press  together  the 
walls  of  the  vessels  after  death,  and  therefore  during  life  must 
have  considerable  influence  upon  the  fluid  they  contain. 

The  movements  of  the  chyle  depend  on  the  same  forces,  with 
the  addition  of  the  power  used  in  the  contraction  of  the  villi, 
which  pump  the  chyle  from  the  lacteal  radicles  into  the  network 
of  valved  vessels  in  the  submucous  tissue. 

The  commencements  of  the  thoracic  duct  and  the  lacteals  are 
placed  in  the  abdominal  cavity,  and  therefore  are  constantly 
under  the  influence  of  the  positive  pressure  exerted  by  the  abdom- 
inal wall  on  the  contained  viscera.  The  rest  of  the  duct  is  in 
the  thorax,  where  the  pressure  is  habitually  negative.  Certain 
variations  coincident  with  inspiration  and  expiration  take  place 
in  both  these  cavities,  and  must  aid  the  onward  flow  of  fluid  in 
a  vessel  containing  valves  so  closely  set. 


214  MANUAL  OF   PHYSIOLOGY. 


CHAPTER  XIII. 
THE   CONSTITUTION  OF  THE   BLOOD. 

In  all  animals,  except  those  which  form  the  lowest  class  (Pro- 
tozoa), the  distribution  of  the  nutritious  materials  to  the  various 
parts  of  the  body,  as  well  as  the  collection  of  the  effete  matters 
prior  to  their  expulsion,  is  carried  on  by  the  medium  of  a  fluid 
which  circulates  through  the  different  parts  of  the  body.  This 
fluid  is  the  blood. 

In  vertebrate  animals  the  blood  passes  through  a  closed  sys- 
tem of  elastic  pipes  and  is  kept  in  constant  motion  by  the  action 
of  a  muscular  pump.  It  is  first  forced  through  strong,  branching 
canals  called  arteries,  whose  walls  gradually  become  thinner  as 
the  branches  get  smaller,  and  end  in  a  network  of  delicate  chan- 
nels (capillaries),  through  which  it  slowly  trickles  into  the  wide, 
soft-walled  veins  by  means  of  which  it  flows  gently  back  again 
to  the  heart.  In  its  course  it  receives  the  nutritive  materials 
from  the  stomach  and  intestines  after  digestion,  the  specially 
elaborated  substances  from  the  liver,  spleen  and  lymph  glands, 
and  the  oxygen  absorbed  from  the  air  in  the  lungs.  In  short,  it 
contains  and  bears  to  their  destination  all  the  materials  required 
for  the  chemical  changes  of  the  economy.  While  passing  through 
the  capillary  networks  of  the  various  tissues,  it  takes  up  the 
waste  materials  resulting  from  the  tissue  changes  and  bears  them 
to  their  proper  point  of  exit  from  the  body ;  at  the  same  time  the 
nutriment  oozes  through  the  delicate  vessel  walls  and  is  diffused 
in  the  tissues. 

GENERAL  CHARACTERISTICS  OF  THE  BLOOD. 

The  blood  of  vertebrate  animals  is  a  bright  scarlet  color  when 
exposed  to  the  oxygen  of  the  air,  but  when  not  in  contact  with 
oxygen  it  becomes  a  dark  purplish  red. 

The  blood  is  remarkably  opaque,  as  may  be  seen  by  placing  a 
thin  layer  on  a  piece  of  glass  over  the  page  of  a  book.  This 


ABSOLUTE    AMOUNT   OF    BLOOD.  215 

opacity  depends  on  the  fact  that  the  blood,  as  will  presently  be 
seen,  is  not  a  red  fluid,  but  owes  its  color  to  the  presence  of  solid 
red  particles  or  corpuscles  which  float  in  a  clear,  pale  fluid.  The 
blood  has  a  peculiar  smell  (halitus)  distinct  in  different  animals 
and  man,  dependent  on  certain  volatile  fatty  acids.  Its  specific 
gravity  varies  from  1045  to  1075,  the  average  being  1055.  The 
solid  parts  (corpuscles)  are  heavier  (sp.  gr.  1105)  than  the  liquor 
sanguinis  (1027). 

When  first  shed  the  blood  has  a  slippery  feel,  which  it  soon 
loses,  becoming  viscous  as  it  passes  into  the  first  stage  of  coagula- 
tion. 

AMOUNT  OF  BLOOD  IN  THE  BODY. 

The  total  amount  of  blood  has  been  estimated  to  be  from  T^  to 
j1^  of  the  body  weight  for  an  adult  man,  and  somewhat  less  for  a 
newborn  child. 

Much  difficulty  has  been  found  in  arriving  at  an  accurate 
estimation  of  the  amount  of  blood  in  the  body.  In  the  first  place, 
all  the  blood  cannot  be  made  to  flow  out  of  the  vessels  of  an  animal 
when  it  is  killed.  Secondly,  the  quantity  and  quality  of  blood 
are  constantly  varying  with  the  capacity  of  the  blood  vessels. 
Thirdly,  when  slowly  withdrawn  from  the  body  during  life  it  is 
rapidly  replaced  by  more  fluid  passing  into  the  blood  vessels. 
This  explains  the  enormous  quantity  of  blood  occasionally  reported 
to  be  shed  in  cases  of  bleeding  to  death.  In  these  cases,  as  quickly 
as  the  blood  is  lost,  fluid  is  absorbed  by  the  fine  vessels  to  re- 
place it,  so  that  if  the  bleeding  be  gradual  the  standard  quantity 
is  still  kept  up  in  the  vessels.  Thus  the  very  sudden  loss  of  a 
comparatively  small  quantity  of  blood  may  cause  death,  whereas, 
if  the  bleeding  go  on  sufficiently  slowly  and  gradually,  as  much 
or  even  more  in  quantity  than  normally  exists  in  the  entire  body 
may  escape  without  fatal  result.  Of  course  much  of  this  is  fluid 
which  has  recently  entered  the  vessels  to  replace  the  blood 
already  lost. 

Weber's  Method. — The  percentage  of  solid  matters  in  the 
blood  is  first  carefully  estimated.  The  absolute  quantity  of 
solids  in  all  the  blood  drawn  is  then  ascertained  and  added  to 
the  solids  obtained  by  washing  out  the  blood  vessels.  Here  an 


216  MANUAL   OF   PHYSIOLOGY. 

error  arises  from  the  fact  that,  in  washing  out  the  blood  vessels, 
much  solid  matter  besides  that  belonging  to  the  blood  is  taken 
from  the  tissues,  and  thus  an  excess  is  found. 

Valentine's  Method. — A  small  measured  quantity  of  blood  is 
drawn  from  a  vein  and  its  percentage  of  solids  accurately  esti- 
mated; a  known  quantity  of  water  is  then  injected  into  the 
vessels.  When  some  time  has  been  allowed  for  proper  distribu- 
tion of  the  water,  a  sample  of  the  diluted  blood  is  taken  and  its 
solids  estimated.  The  difference  in  solid  contents  of  the  two 
samples  shows  the  degree  of  dilution  caused  by  a  known  quan- 
tity of  water  introduced  into  blood  of  ascertained  strength,  and 
thus  the  amount  of  the  diluted  fluid  (the  blood)  may  be  calculated 
and  added  to  the  amount  of  the  first  sample  to  make  the  absolute 
quantity. 

This  method  cannot  give  accurate  results,  because  in  the  time 
necessary  for  the  distribution  and  mixture  of  the  water  with  the 
circulating  blood  much  of  the  former  is  excreted  by  the  kidneys 
and  skin,  and  the  second  sample  of  blood  is  more  concentrated 
than  should  result  from  such  dilution. 

WelcJcer's  Method  depends  upon  the  estimation  of  the  coloring 
matter  of  the  blood.  He  connected  the  carotid  with  a  small  T 
piece,  and  allowed  the  animal  to  bleed  into  a  bottle  in  which  the 
blood  could  be  defibrinated  by  shaking  with  pieces  of  glass.  One 
cubic  centimetre  of  this  defibrinated  blood  was  carefully  measured 
off  and  saturated  with  carbon  monoxide  (CO),  which  gives  a 
permanent  and  equally  bright  red  color.  It  was  diluted  to  500 
.cc.  with  distilled  water  and  kept  as  a  standard  color  solution. 
The  blood  vessels  of  the  animal  were  then  washed  out  with  .6 
per  cent,  solution  of  sodium  chloride  until  the  solution  flowing 
from  the  jugular  vein  was  colorless.  The  tissues  of  the  animals 
were  chopped  up,  steeped  in  water  and  pressed.  The  washings 
of  the  vessels  and  the  infusion  from  the  tissues  were  added 
together  and  diluted  until  they  had  the  same  color  intensity  as  a 
layer  of  the  standard  solution  of  the  same  thickness.  Every  500 
cc.  of  these  diluted  washings  corresponds  to  1  cc.  of  blood. 

By  this  method  the  following  estimates  have  been  made  of  the 
relation  of  the  blood  to  the  body  weight: — . 


PHYSICAL    CONSTRUCTION    OF   THE    BLOOD.  217 

Mouse T2  ~rV 

G-uinea  pig , 

Rabbit 

Dog ..     „ 

Cat 2T 

Bird TV-TV 

Frog J3-Jo 

Only  approximate  estimates  of  the  distribution  of  blood  in  the 
body  during  life  can  be  made,  since  there  can  be  no  accurate 
method  of  investigation,  and  the  amount  varies  considerably, 
according  as  the  organ  or  part  is  in  a  state  of  rest  or  activity. 
It  is  supposed  that  a  quarter  of  the  entire  amount  is  habitually 
flowing  through  each  of  the  following  regions : — 

1.  The  heart,  great  vessels  and  lungs. 

2;  The  skeletal  muscles. 

3*.  The  liver. 

4.  Skin  and  other  tissues. 

PHYSICAL  CONSTRUCTION  OF  THE  BLOOD. 
As  already  stated,  the  blood  is  not  really  a  red  fluid.     It  is 
seen  with  the  microscope  to  be  made  up  of  a  clear  fluid  called 


Human  Blood  after  death  of  the  elements.  The  red  corpuscles  are  seen  in  different  posi- 
tions showing  their  shape,  some  also  are  seen  in  rolls.  Only  one  white  cell  (w)  is  seen, 
misshapen  and  entangled  in  fibrin  threads. 

plasma  or  liquor  sanguinis,  which  contains  an  immense  number 
of  little  disc-shaped  bodies — red  corpuscles — and  a  few  colorless 
protoplasmic  cells— white  corpuscles — so  that  the  living  blood 
may  be  physically  tabulated,  giving  approximately  an  estimation 
of  the  relative  amounts,  thus : — 


T  Plasma  or  Liquor  Sanguinis 


Blood 

(  Solid  or  Corpuscle 


19 


218 


MANUAL   OF   PHYSIOLOGY. 


PLASMA. 


The  fluid  part  of  the  blood,  plasma  or  liquor  sanguinis,  is  of  a 
pale  straw  color,  when  pure  and  free  from  the  coloring  matter  of 
the  corpuscles,  and  of  slightly  less  density  (p.  215). 

Unless  special  precautions  have  been  taken,  the  plasma  is 
altered  when  removed  from  the  blood  vessels  and  coagulation  of 
the  blood  takes  place,  so  that  plasma  does  not  come  under  ob- 
servation, except  when  suitable  methods  are  employed  to  separate 
it  from  the  corpuscles.  It  was  first  separated  from  the  corpuscles 


FIG.  96. 


600 


Reticulum  of  Fibrin  Threads  after  staining  has  made  them  visible.    The  network  (b) 
appears  to  start  from  granular  centres  (a).    (Ranvier.) 

by  the  filtration  of  frog's  blood  to  which  strong  syrup  had  been 
added  to  delay  coagulation  and  destroy  the  flexibility  of  the  cor- 
puscles, so  that  they  were  rapidly  caught  in  the  meshes  of  the 
filter  and  the  clear  plasma  passed  through. 

To  obtain  mammalian  plasma  free  from  corpuscles  it  is  neces- 
sary to  use  some  other  method,  as  the  small  elastic  corpuscles 
easily  run  through  the  meshes  of  the  thickest  filter  paper. 

The  blood  of  the  horse  is  chosen  because  it  coagulates  more 
slowly  than  that  of  most  mammals,  and  delay  in  the  coagula- 


BLOOD   PLASMA.  219 

tion  or  postponement  of  the  change  in  the  plasma  is  the  chief 
object  to  be  obtained.  To  encourage  this  delay  the  blood  is 
drawn  from  a  vein  into  a  cylinder  surrounded  with  a  freezing  mix- 
ture. The  cold,  however,  must  not  be  so  intense  as  to  absolutely 
freeze  the  blood,  for  the  wished  for  subsidence  of  corpuscles  could 
not  go  on  if  the  blood  become  solid.  It  is  then  left  quite  motion- 
less for  twenty-four  hours,  after  which  time  it  will  be  found  that 
the  heavy  corpuscles  have  fallen  and  left  a  clear  supernatant 
fluid,  which  is  plasma  containing  some  white  cells.  This  can  be 
removed  with  a  cool  pipette  and  passed  through  an  ice-cold  filter 
to  remove  the  cells,  then  tolerably  pure  plasma  is  obtained  which 
soon  coagulates  at  the  ordinary  temperature. 

Another  method  of  checking  coagulation  consists  in  letting  the 
blood  flow  into  a  25  per  cent,  solution  of  magnesium  sulphate 
(about  three  volumes  of  blood  to  one  of  the  solution).  This, 
if  left  in  a  cool  place,  will  not  coagulate,  and  the  corpuscles  will 
separate  by  subsidence  from  the  plasma  and  salt  solution,  which 
form  an  upper  layer  of  clear  fluid.  If  the  salt  be  removed  by 
dialysis  or  weakened  by  dilution  with  water,  coagulation  com- 
mences. 

The  coagulation  of  plasma  can  be  seen  with  the  microscope  to 
depend  upon  the  appearance  of  a  close  feltwork  of  exquisitely 
delicate,  finely  granular,  elastic  fibrils,  which  pervade  the  entire 
fluid  and  cause  it  to  set  into  a  soft  jelly.  The  substance  forming 
the  meshes  is  called  fibrin. 

Some  time  after  the  plasma  has  gelatinized,  the  threads  of  fibrin 
break  away  from  their  attachment  to  the  vessel  in  which  the 
coagulum  is  contained,  and  owing  to  their  elasticity  the  general 
mass  of  fibrin  contracts,  squeezing  out  of  its  meshes  clear  drops 
of  fluid  termed  serum. 

The  fibrin  clot  gradually  shrinks  to  small  size  and  floats  in  the 
abundant  fluid  serum. 

The  separation  of  the  serum  is  accelerated  by  agitation  of  the 
soft  clot ;  and  if  brisk  agitation,  such  as  whipping,  be  kept  up 
for  a  few  minutes  in  recently  drawn  blood,  the  plasma  does  not 
form  a  jelly,  but  the  fibrin  firmly  adheres  to  the  stirring  rods  and 
at  once  contracts  around  them. 


220  MANUAL   OF   PHYSIOLOGY. 

CHEMICAL  COMPOSITION  OF  PLASMA. 

On  account  of  the  rapid  spontaneous  formation  of  fibrin  and 
serum  when  the  plasma  is  removed  from  the  body  and  allowed 
to  die,  the  exact  chemical  condition  of  the  liquor  sanguinis  dur- 
ing life  cannot  be  investigated,  the  separation  occurring  before 
the  simplest  chemical  method  can  be  carried  out. 

We  have  no  reason  to  suppose  that  fibrin  exists  normally  in 
the  blood,  but  it  would  appear  that  this  substance  is  only  formed 
at  the  moment  of  coagulation,  its  appearance  being  one  of  the 
most  obvious  of  many  changes  which  take  place  at  the  time  of 
the  death  of  blood  plasma. 

The  chemical  changes  comprehended  under  the  term  coagulation 
occurring  when  plasma  is  deprived  of  its  means  of  vitality,  and 
ending  in  the  production  of  fibrin  and  serum,  are  naturally  of 
the  first  importance  in  studying  the  chemical  relationships  of 
living  plasma.  They  can  best  be  followed  out  in  the  coagulation 
of  plasma  when  separated  from  the  corpuscles,  for  (although  the 
stages  in  the  coagulation  of  blood  are  the  same,  the  appearance 
of  an  insoluble  albumin — fibrin — being  the  one  essential  in  either 
case),  the  corpuscles  complicate  the  process  and  modify  the  appear- 
ance of  the  clot. 

Not  only  is  the  fibrin  not  present  as  such  in  the  living  plasma, 
but  it  requires  for  its  production  the  presence  of  other  substances 
which  either  do  not  exist  in  the  living  plasma,  or  are  there  so 
chemically  associated  as  not  to  bring  about  the  change  which 
occurs  when  the  plasma  dies. 

The  reasons  for  believing  this  are  the  following :  Fluids 
which  sometimes  collect  by  a  slow  process  in  the  serous  cavities 
of  the  body,  e.  g.,  hydrocele  fluid,  pleural  effusion,  etc.,  if  kept 
quite  clean  do  not  generally  undergo  spontaneous  coagulation.  If 
to  one  of  these  some  serum  or  recently  washed  blood  clot  be  added, 
coagulation  takes  place  just  as  in  plasma  (Buchanan).  That  is 
to  say,  we  have  here  two  fluids,  neither  of  which  coagulates  when 
left  to  itself,  but  which  do  if  mixed  together.  From  each  of 
these  fluids  a  substance  can  be  precipitated  by  passing  a  stream 
of  carbon  dioxide  (CO2)  through  the  fluids.  Both  precipitates 
readily  redissolve  in  weak  saline  solutions. 


COMPOSITION    OP    BLOOD    PLASMA.  221 

The  solution  prepared  from  the  hydrocele  fluid  causes  blood 
serum  to  coagulate ;  that  prepared  from  the  blood  serum  causes 
the  hydrocele  fluid  to  coagulate  ;  and  when  mixed  together  the 
mixture  of  the  two  solutions  coagulates ;  while  the  serum  and 
hydrocele  fluid  from  which  the  substances  have  been  removed 
no  longer  have  the  power  of  exciting  coagulation  in  each  other 
or  in  like  fluids.  Here,  then,  are  two  materials  ;  one,  obtained 
in  considerable  quantity  from  serum  after  coagulation,  is  called 
paraglobulin  (Schmidt)  or  serum-  globulin  (Hammarsten) ;  the 
other,  occurring  in  serous  fluids,  is  named  fibrinogen.  Both 
these  substances  are  present  in  the  dying  plasma  of  the  blood 
prior  to  coagulation.  They  can  be  obtained  both  together  from 
the  plasma  if  the  plasma  be  treated  with  sodium  chloride  to  satu- 
ration after  either  of  the  precautions  already  mentioned — viz., 
the  application  of  cold,  or  the  addition  of  neutral  salt — has  been 
taken  to  prevent  the  formation  of  fibrin.  This  precipitates  a 
substance  which  readily  dissolves  if  water  be  added  to  weaken 
the  salt  solution,  and  after  some  time  the  solution  undergoes 
spontaneous  coagulation,  while  the  plasma  from  which  it  has 
been  made  has  lost  that  power.  This  plasmin  (Denis)  no  doubt 
is  made  up  of  different  globulins,  chiefly  serum  globulin,  and 
fibrinogen,  and  contains  in  itself  all  the  necessary  "  factors  "  of 
fibrin  formation,  but  is  not  at  all  identical  with  fibrin,  since  it 
readily  dissolves  in  weak  saline  solutions,  like  the  class  of  proteids 
called  globulins,  while  fibrin  is  quite  insoluble  in  such  solutions. 

In  plasma  removed  from  its  normal  relationships,  both  serum 
globulin  and  fibrinogen  exist;  but  the  former  in  far  greater 
quantity  than  the  latter,  since  the  serum,  after  the  blood  clot  is 
formed,  contains  no  more  fibrinogen,  while  the  serum  globulin 
makes  up  nearly  half  the  remaining  solids. 

In  preparing  fibrinogen  and  serum  globulin  Schmidt  found 
that  the  more  carefully  he  operated,  the  weaker  and  more 
uncertain  their  action  as  fibrin  factors  became;  and,  finally,  he 
made  solutions  which,  when  added  together,  did  not  produce 
coagulation,  but  which,  when  added  to  less  pure  solutions,  gave 
good,  firm  clots.  From  this  he  suspected  that  a  third  agent 
which  acted  as  a  ferment  was  necessary  to  put  into  operation  the 


222  MANUAL   OF   PHYSIOLOGY. 

fibrin-producing  properties  of  the  other  two  factors.  He  further 
succeeded  in  preparing  the  third  agent,  to  which  he  gave  the 
name  of  fibrin  ferment.  By  treating  serum  with  strong  alcohol 
the  proteids  are  precipitated ;  the  ferment  is  carried  down  with 
them,  and  extracted  with  water.  This  extract,  added  to  the 
mixture  of  the  pure  fibrin  factors,  which  previously  did  not  clot, 
caused  rapid  coagulation,  but  not  when  added  to  either  of  them 
singly. 

This  material  is  influenced  by  those  circumstances  which  affect 
the  activity  of  ferments  in  general :  it  has  a  minimum,  0°  C., 
optimum,  38°  C.,  and  maximum,  80°  C.,  temperature  of  activity, 
with  various  gradations  of  rapidity  of  action  between  each,  and  is 
destroyed  if  heated  above  80°  C.  An  active  solution  having  the 
properties  of  the  ferment  can  be  extracted  from  whipped  fibrin 
preserved  in  alcohol  by  an  8  per  cent,  solution  of  common  salt 
(Gamgee). 

Hammarsten  thinks  that  the  serum  globulin  is  not  indis- 
pensable to  the  formation  of  fibrin,  because  (1)  a  solution  of 
fibrinogen  may  be  made  to  coagulate  without  its  presence ;  (2) 
the  fibrinoplastic  property  of  serum  globulin  is  shared  by  casein 
and  calcic  chloride ;  (3)  and  is  absent  from  pure  serum  globulin, 
(4)  such  as  is  present  in  hydrocele  fluid  which  does  not  coagulate 
on  the  addition  of  fibrin  ferment. 

The  source  of  fibrin  is  still  a  question  of  much  difficulty,  and 
will  be  further  discussed  with  the  question  of  blood  coagulation 
within  and  without  the  vessels,  after  the  morphological  elements 
have  been  described. 

FIBRIN. 

Fibrin  may  be  procured  either  from  plasma  or  blood  by 
whipping,  and  then  washing  the  insoluble  product  with  water. 
When  fresh  it  has  a  pale  yellow  or  whitish  color,  a  filamentous 
structure,  and  is  singularly  elastic.  It  is  not  soluble  in  water, 
weak  saline  solution,  or  ether.  Alcohol  makes  it  shrink  by 
removing  its  water.  When  quite  dry  it  is  brittle  and  hard,  and 
can  be  reduced  to  a  powder.  It  swells  in  1  per  cent,  hydrochloric 
acid,  and  if  warmed  is  converted  into  acid  albumin  and  dissolved. 

The  amount  formed  varies  very  much  even  in  the  blood  drawn 


PREPARATION    OF   SERUM.  223 

from  the  same  animal  at  the  same  time,  but  is  always  very  small 
compared  with  the  size  of  the  blood  clot.  It  never  reaches  as 
much  as  1  per  cent.,  commonly  varying  from  0.1  per  cent,  to  0.3 
per  cent,  of  the  entire  mass  of  blood. 

SERUM. 

This  name  is  given  to  the  clear  fluid  which  oozes  out  of  the 
clot  of  plasma.  It  only  differs  from  the  latter  in  its  chemical 
composition  in  so  far  that  fibrin  is  separated  from  it.  Though 
chemically  this  is  a  slight  difference,  it  signifies  the  change  from 
a  complex  living  body  (blood  plasma)  into  a  solution  of  dead 
albumins,  etc. 

Serum  is  a  clear,  straw-colored,  alkaline  fluid  of  1028-1030  sp. 
gr.,  holding  in  solution  different  organic  substances  and  some 
inorganic  salts.  After  a  full  meal  the  serum  is  said  to  be  slightly 
milky,  from  the  presence  of  finely  divided  fat. 

It  contains  about  9  per  cent,  of  solid  matters,  of  which  a  large 
proportion,  7  per  cent.,  are  proteids.  Of  these  the  most  abundant 
is  (1)  serum  albumin  (about  4  per  cent,  in  man),  a  solution  of 
which  becomes  opaque  at  60°  C.,  and  coagulates  at  a  heat  of  73° 
-75°  C.  The  proteid  next  in  importance  is  (2)  serum  globulin 
or  paraglobulin  (about  3  per  cent,  in  man),  which  has  already 
been  mentioned.  It  may  be  precipitated  imperfectly  by  CO2,  or 
completely  by  magnesium  sulphate.  (3)  Serum  casein  has  been 
obtained  from  serum  by  careful  neutralization  with  acetic  acid 
after  the  removal  of  the  serum  globulin  by  CO2.  This  is  said  to 
be  serum  globulin  which  has  failed  to  come  down  with  the  CO2. 
(4)  Neutral  fats  in  a  state  of  fine  subdivision  are  present  in  a 
variable  quantity:  also  (5)  lecithin;  (6)  traces  of  sugar;  (7) 
various  products  of  tissue  change — kreatin,  urea,  etc. ;  and  (8) 
inorganic  salts,  viz.,  sodium  chloride,  about  5  per  cent.,  and 
sodium  carbonate,  which  probably  existed  in  the  blood  as  sodium 
hydric  carbonate.  There  is  also  a  small  quantity  of  potassium 
chloride.  But  it  should  be  remembered  that  about  ten  times 
more  sodium  than  potassium  salts  exist  in  the  serum,  and  pro- 
bably in  the  blood  plasma. 


224 


MANUAL   OF   PHYSIOLOGY. 


CHAPTER  XIV. 

BLOOD  CORPUSCLES. 

The  relative  number  of  red  discs  to  the  colorless  cells  is  said 
to  be,  on  the  average,  350  to  1.     This  is  true  of  the  blood  drawn 

from  the  fine  vessels  by  punc- 
FlG- '97.  ture.      While  in  the  vessels 

the  blood  must  contain  a 
greater  proportion  of  the 
colorless  cells,  for  by  the  or- 
dinary method  of  obtaining 
blood  for  examination,  they 

Human  Blood  after  death  of   the  elements,     do  not   flow  OUt  of  the 


The  red  corpuscles  are  seen  in  different  posi-  .                      ..... 

tions  showing  their  shape,  some  also  in  rolls,  tured  Capillaries  as  readily  as 

Only  one  white  cell  (w)  is  seen,  misshapen  .              ,      , .                   ,                        ~ 

and  entangled  in  fibrin  threads.  the    red    dlSCS,    and    many    01 

them  are  said  to  become  dis- 
integrated very  shortly  after  they  are  removed  from  the  circula- 
tion. Although  the  number  of  red  discs  normally  alters  but 
little,  on  account  of  the  constant  changes  occurring  in  the  num- 
ber of  the  white  cells,  the  proportion  of  white  to  red  varies  much. 
It  has  been  found  to  differ  according  to  the  observer,  the  situa- 
tion, and  other  circumstances,  as  shown  in  the  following  table, 
which  gives  the  number  of  red  corpuscles  to  one  colorless  cell : — 

Observer's  estimate  of  normal  proportion  : —  Bed.  White. 

Welcker 335-1 

Moleschott 357-1 

In  various  parts  of  the  circulation  : — 

Splenic  vein 60-1 

Splenic  artery 2260-1 

Hepatic  vein 170-1 

Portal  vein 740-1 

According  to  age  or  sex  : — 

Girls 405-1 

Boys 226-1 

Adult 334-1 

Old  age 381-1 


COLORLESS   BLOOD   CORPUSCLES.  225 

According  to  general  conditions  : —  Red.  White. 

When  fasting 716-1 

After  meal 347-1 

During  pregnancy 281-1 

In  a  disease  of  the  spleen  and  lymphatic  glands  called  Leuco- 
cytheraia  there  may  appear  to  be  nearly  as  many  white  cells  as 
red  discs.  Here,  however,  the  red  discs  are  deficient,  while  the 
colorless  cells  are  multiplied. 

THE  COLORLESS  CORPUSCLES. 

The  colorless  cells  of  the  blood,  commonly  called  the  white 
corpuscles,  differ  in  no  essential  respect  from  the  pale  round  cells 
which  are  found  in  most  of  the  tissues  of  the  body.  They  exist 
in  large  numbers  in  that  fluid,  namely,  the  lymph,  which  is 
drained  back  from  the  tissues  into  the  blood,  and  occupy  a  great 
part  of  the  lymphatic  glands  and  spleen.  They  are  often  spoken 
of  as  lymphoid  cells,  leucocytes,  indifferent,  or  formative  cells, 
on  account  of  their  being  so  widely  distributed  throughout  the 
tissues. 

When  fresh  blood  is  examined  with  the  microscope  these  cells 
can  generally  be  seen  adhering  to  the  glass  slide  or  cover  glass 
and  lying  singly,  apart  from  the  groups  of  red  discs.  They  can 
be  recognized  by  their  absence  of  marked  color,  finely  granular 
structure,  spherical  shape,  and  the  nuclei  which  may  often  be 
recognized  near  the  centre  of  the  cell.  Though  not  always  visible 
in  fresh  preparations,  the  nuclei  can  be  brought  to  light  by  the 
action  of  many  reagents — e.  g.,  acetic  acid.  If  examined  while 
being  moved  by  the  blood  current  in  the  capillary  vessels,  they 
are  seen  to  pass  slowly  along  in  contact  with  the  vessel  wall,  while 
the  red  corpuscles  rush  rapidly  past  them  down  the  centre  of  the 
channel  (Fig.  98).  This  may  partly  be  due  to  their  peculiar 
adhesiveness,  which  also  causes  them  to  stick  to  the  glass  slide, 
while  the  red  discs  are  washed  away  when  a  gentle  stream  of 
saline  solution  is  allowed  to  flow  under  the  cover  glass.  These 
cells  show  all  the  manifestations  of  activity  characteristic  of 
independent  living  beings.  If  kept  in  a  medium  suitable  to 
them,  and  at  the  temperature  of  the  body,  they  will  soon  be  seen 


226 


MANUAL   OF    PHYSIOLOGY. 


to  alter  their  appearance ;  their  outline  becomes  faint,  they  are 
no  longer  spherical,  but  very  irregular  in  shape,  and  constantly 

change  their  form  by  send- 

FlG-  98-  ing  out  and  retracting  pro- 

cesses, by  means  of  which 
they  change  their  position, 
so  that  they  may  be  said 
to  perform  locomotion. 
These  movements  are  ren- 
dered more  active  by  a 
slight  increase  of  tempera- 
ture, and  are  checked  by 
cold.  For  continued  obser- 
vation, about  38°  C.  is  the 
best  temperature  for  mam- 
malian blood.  The  blood 
of  the  frog  is  generally 
used  to  see  the  motion  of 
the  white  corpuscles,  as 
warming  is  unnecessary  in 
the  case  of  cold-blooded 
animals.  They  respond  to 
many  other  influences, 
such  as  electricity,  etc., 
even  for  a  considerable  time  after  removal  from  the  body. 

No  doubt  they  continually  absorb  fluid  nutriment  from  the 
surrounding  medium,  as  is  shown  by  the  effect  of  poisons  on 
them  ;  and,  by  the  repeated  contractions  and  relaxations  of  parts 
of  their  substance  in  the  form  of  pseudopodia,  they  appear  to 
take  into  the  inner  parts  of  the  protoplasm  solid  particles,  which 
after  some  time  are  ejected  after  the  manner  of  the  small  unicel- 
lular animals  known  as  amcebse  (p.  91). 

While  in  motion  in  the  circulation  none  of  these  amoeboid 
movements  appear  to  take  place,  but  when  an  arrest  of  the  flow 
of  blood  in  the  capillaries  occurs,  they  not  only  change  their 
form,  but  also  their  position  ;  and  if  there  be  no  onward  flow  of 
blood  for  some  little  time,  they  creep  out  of  the  capillaries,  pass- 


Vessels  of  the  Frog's  Web. 

(o)  Trunk  of  vein,  and  (b  b)  its  tributaries  passing 
across  the  capillary  network.  The  dark  spots 
are  pigment  cells. 


COLORLESS   CORPUSCLES.  227 

ing  through  the  delicate  vessel  walls.  This  emigration  of  the 
blood  cells  is  possibly  a  common  event  when  a  tissue  is  in  need 
of  textural  repair.  When  excessive,  it  forms  one  of  the  most 
striking  items  of  the  series  of  events  occurring  in  inflammation. 

These  cells  differ  much  in  size ;  generally  they  are  somewhat 
larger  than  the  red  discs.  Nothing  like  a  cell  wall  can  be  seen 
to  surround  them,  and  from  the  movements  above  described  it 
would  appear  certain  that  they  are  free  masses  of  active  proto- 
plasm. 

The  number  of  white  cells  that  can  be  collected  is  too  small  to 
allow  of  accurate  chemical  analysis,  but  there  is  no  reason  to 
suppose  that  they  differ  from  other  forms  of  protoplasm. 

ORIGIN  OF  THE  COLORLESS  BLOOD  CELLS. 

Since  such  an  ordinary  circumstance  as  a  hearty  meal  can 
materially  influence  the  numbers  of  the  white  corpuscles,  it  would 
appear  that  they  must  be  usually  undergoing  rapid  variations  in 
their  number — probably  by  their  being  constantly  used  up  and 
periodically  replaced  by  new  ones.  The  places  in  which  they 
occur  in  greatest  number  are  the  lymphatic  glands,  the  spleen, 
and  the  lymph  follicular  tissue  in  the  intestinal  tract. 

There  is  no  doubt  that  the  lymph  contains  a  much  larger  pro- 
portion of  these  cells  after  it  has  passed  through  the  lymph 
glands,  and  the  blood  coming  from  the  spleen  contains  an  exces- 
sive proportion  of  them. 

It  is  then  not  unreasonable  to  suppose  that  many  of  the  white 
cells  found  in  the  blood  have  their  origin  in  these  organs. 

They  may  also  be  developed  from  similar  cells  in  any  tissue, 
but  their  reproduction  by  division,  other  than  that  which  probably 
occurs  in  the  lymph  follicles  where  it  cannot  be  seen,  is  a  cir- 
cumstance of  the  greatest  rarity,  and  few  observers  have  been 
fortunate  enough  to  witness  the  phenomenon. 

The  destiny  of  the  white  blood  cells  is  probably  manifold. 
From  the  readiness  with  which  they  escape  from  the  capillaries 
and  wander  by  their  amoeboid  movement  through  the  neighbor- 
ing tissues  to  reach  any  point  of  injury,  it  would  appear  that  they 
take  an  active  part  in  the  repair  of  a  tissue  whose  vitality  has  in 


228  MANUAL   OF    PHYSIOLOGY. 

any  way  suffered.  During  the  growth  of  all  tissues  these  cells 
seem  to  contribute  active  agents  to  their  formation  ;  thus  in  the 
formation  of  bone  it  has  been  stated  that  escaped  blood  cells 
or  their  immediate  offspring  help  to  lay  down  the  calcareous 
material,  and  some  even  settle  themselves  as  permanent  inhab- 
itants of  the  lacunae. 

Further,  they  are  in  all  probability  the  means  of  renewing  the 
red  discs.  Their  protoplasm  either  takes  up  the  coloring  matter 
from  its  surroundings,  or  forms  it  within  itself  from  suitable 
ingredients.  Certain  it  is  that  cells  are  found  which  are  recogni- 
zable as  white  blood  cells,  which  have  more  or  less  of  the  red 
coloring  matter  imbedded  in  their  substance.  As  this  increases, 
the  cell  gradually  loses  its  distinctive  characters  and  assumes 
those  of  a  red  corpuscle.  Such  elements,  it  will  be  seen,  are 
common  in  the  spleen  and  the  blood  leading  from  it. 

THE   COLORED  CORPUSCLES. 

The  red  discs  were  discovered  in  the  human  blood  by  Leuwen- 
hoek,  about  1673.  They  give  the  red  color  which  characterizes 
the  blood  of  all  vertebrated  animals  (except  the  amphioxus),  but 
are  not  found  in  the  blood  of  the  invertebrata,  which  only  con- 
tains colorless  cells.  When  the  blood  of  the  invertebrates  has  a 
color  it  owes  it  to  the  fluid,  not  to  the  corpuscles.  The  indivi- 
dual discs  when  viewed  singly  under  the  microscope  appear  to  be 
pale  orange,  but  when  in  masses  the  red  becomes  apparent. 

The  shape  of  the  corpuscles  differs  in  different  classes  of 
animals.  In  man  and  all  other  mammalia  they  are  discs,  concave 
on  each  side  and  rounded  off  at  the  margin.  The  only  class  of 
mammals  which  forms  an  exception  to  this  rule  is  the  camelidse, 
whose  red  corpuscles  are  elliptical  in  shape,  like  those  of  non- 
mammalian  vertebrates. 

The  corpuscles  of  birds,  amphibia  and  fish  are  flattened,  ellip- 
tical plates,  slightly  convex  on  each  side,  and  containing  a 
distinct  oval  nucleus  in  their  centre. 

The  size  of  the  corpuscles  varies  greatly  in  different  classes  of 
animals,  but  is  strikingly  constant  in  the  same  class.  A  glance 
at  the  following  diagram,  in  which  the  corpuscles  are  drawn  to 


COLORED   CORPUSCLES. 


229 


scale,  will  give  an  idea  of  their  relative  sizes,  in  examples  of  the 
different  classes  of  animals,  and  will  make  the  following  points 
more  rapidly  obvious  than  mere  description. 

The  size  of  the  animal  has  no  general  relation  to  the  size  of  the 
corpuscles.     The  human  red  discs  are  of  a  fair  average  size  when 


FIG.  99. 


Diagram  of  the  relative  sizes  of  red  corpuscles  of  different  animals.    The  measurements 
below  are  in  fractions  of  a  millimetre. 

1.  Amphiuma AX  A  6.  Man T£s 

2.  Proteus A  X  A  7.  Dog T^ 

3.  Frog AXA  8.  Horse r|r 

4.  Pigeon 355X155  9.  Goat 5£3 

5.  Elephant TJ3  10.  Musk  Deer T£3 

compared  with  those  of  other  mammals,  and  therefore  man's 
blood  cannot  be  distinguished  from  that  of  the  other  mammalia. 
The  mammalian  corpuscles  are,  on  the  whole,  small  when  com- 
pared with  those  of  the  other  vertebrates.     The  batrachians  are 


230  MANUAL   OF   PHYSIOLOGY. 

distinguished  by  the  great  size  of  the  corpuscles.  Those  of  the 
Amphiuma  tridactylum  are  visible  to  the  naked  eye. 

The  following  measurements  are  given  by  Welcker  for  the 
human  discs : — 

Diameter    .     .0077  of  a  millimetre  (7.7//*)  =  ¥^Vo  of  an  inch. 
Thickness   .     .0019  of  a  millimetre  (1.9//)    =  T2io o  of  an  inch- 
Volume  .000000077  of  a  cubic  millimetre. 
Surface    .     .000128  of  a  square  millimetre. 

The  last  measurement  would  give  a  surface  of  about  2816 
square  metres  for  the  corpuscles  of  an  adult.  A  surface  of  11 
square  metres  is  exposed  every  second  in  the  lungs  for  the 
absorption  of  oxygen. 

When  circulating  in  the  vessels,  or  immediately  after  removal, 
the  red  corpuscles  are  very  soft  and  elastic,  being  bent  and 
altered  in  shape  by  the  slightest  pressure,  and  easily  stretched  to 
twice  their  diameter.  But  the  moment  pressure  or  traction  is 
removed,  they  return  to  their  normal  biconcave  disc  shape  if  the 
medium  in  which  they  lie  continue  of  the  normal  density. 

Changes  take  place  in  the  blood  shortly  after  it  is  removed 
from  the  body,  which  seem  to  be  associated  with  the  loss  of  func- 
tion (death)  of  the  red  discs,  as  shown  by  their  rapid  destruction 
if  reintroduced  into  the  circulation. 

The  changes  are  checked  by  cold  and  facilitated  by  heat,  a 
temperature  above  that  of  the  body  causing  them  to  take  place 
almost  immediately.  Associated  with  the  loss  of  function  of  the 
discs  is  observed  a  change  accompanied  by  an  apparent  increase 
of  adhesiveness,  which  causes  them  to  stick  together,  commonly 
adhering  by  their  flat  surfaces,  so  as  to  form  into  rolls,  like  so 
many  coins  placed  side  by  side.  That  this  adhesion  is  not  a 
mere  physical  process,  independent  of  the  chemical  properties 
of  the  corpuscles  themselves,  seems  proved  by  the  following 
facts :  (1)  It  does  not  occur  immediately  when  the  blood  is 
drawn,  and  disappears  after  a  few  hours  without  the  addition  of 
reagents;  (2)  while  the  blood  is  in  the  living  vessels  under  nor- 
mal conditions  there  is  no  adhesion,  but  this  soon  appears  when 

*  The  Greek  letter  n  is  used  by  histologists  to  denote  TTJVjj  of  a  millimetre,  which  is 
taken  as  a  convenient  unit  of  measurement. 


COLORED    CORPUSCLES.  231 

any  standstill  in  the  circulation  takes  place — as  in  inflammation  ; 
(3)  it  does  not  occur  when  saline  solutions  are  added  to  the 
blood.  It  seems  to  be  dependent  upon  a  peculiar  property  of 
the  discs,  which  only  exists  for  a  time  coincident  with  the 
changes  that  accompany  the  death  of  the  blood  and  the  appear- 
ance of  fibrin. 

The  shape  of  the  discs  changes  when  the  density  of  the  medium 
in  Which  they  are  suspended  is  altered.  When  the  density  is 
reduced,  as  by  the  addition  of  water,  they  swell  and  become 
spherical,  and  break  up  the  rouleaux;  the  coloring  matter  at  the 
same  time  becoming  dissolved  in  the  medium.  (Fig.  100.) 
When  the  density  is  increased  by  slight  evaporation,  or  the 
addition  of  salt  solution  about  1  per  cent.,  they  cease  to  be  con- 

FIG.  100.  FIG.  101. 


Microscopic  appearance  of  the  blood  after  Showing  effect  of  evaporation, 

the  addition  of  distilled  water.     Red  Six  Red  Corpuscles  crenated. 

Corpuscles  become    colorless  or    pale,  (vv)  White  cell  changing  shape, 

separate  and  spherical.  The  white  are 
seen  to  be  swollen,  round  and  granular 
with  clear  nuclei. 

cave,  and  become  crenated  or  spiked  like  the  green  fruit  of  the 
horse-chestnut.  (Fig.  101.)  The  addition  of  strong  syrup  causes 
the  corpuscles  to  shrivel  and  assume  a  great  variety  of  peculiar 
bent  or  distorted  forms.  (Fig.  102.)  Elevation  of  temperature 
or  repeated  electric  shocks  causes  a  peculiar  change  in  shape, 
but  since  the  change  is  associated  with  the  death  of  the  element, 
it  cannot  be  attributed  to  vital  activity  comparable  with  that 
seen  in  the  white  cells. 

The  discs  show  no  signs  of  structure  under  the  microscope : 
they  are  perfectly  homogeneous,  transparent  bodies,  of  a  pale 
orange  color,  all  efforts  to  demonstrate  the  limiting  membranes 
formerly  supposed  to  surround  them  having  failed.  Their 


232  MANUAL   OF   PHYSIOLOGY. 

behavior  when  certain  reagents  are  added  to  the  blood  shows 
that  the  corpuscles  have  two  constituents  :  (1)  the  coloring  mat- 
ter, Oxyhcemoglobin ;  and  (-2)  the  Stroma.  The  coloring  matter 
may  be  removed  from  the  corpuscle,  as  above  stated,  by  water, 
and  leaves  a  perfectly  colorless  transparent  foundation  or  ground- 
work, which  appears  to  be  in  some  way  porous,  so  as  to  hold  the 
coloring  matter  in  its  interstices.  The  effect  on  the  naked-eye 
appearance  of  the  blood  produced  by  the  removal  of  the  color- 
ing matter  from  the  stroma  is  to  alter  the  color  and  increase  the 
transparency  of  the  fluid.  The  oxy haemoglobin  now  forms  a 
transparent,  dark-red,  lakey  solution,  and  the  corpuscles,  being 
quite  colorless,  are  practically  invisible.  This  transparency  of 
the  fluid  does  not  depend  on  any  change  in  the  oxyhsemoglobin, 

FIG.  102.  FIG.  103. 

o  a?  .'  ^ 

^®  ^6  °  S&felftf 

Red  Corpuscles    shriveled  by  Blood  Corpuscle  after  the  addi- 

the  addition  of  strong  syrup.  tion  of  tannic  acid.    (%  $ ) 

(V?)  White  Corpuscle. 

but  merely  on  its  being  dissolved  out  of  the  discs,  which  become 
transparent  and  can  no  longer  reflect  the  light.  This  process, 
which  is  commonly  spoken  of  as  rendering  the  blood  "  lakey," 
maybe  brought  about  by  the  following  means:  (1)  The  addi- 
tion of  about  \  its  bulk  of  distilled  water,  to  wash  the  coloring 
matter  out  of  the  stroma,  which  may  then  be  rendered  visible  by 
a  weak  solution  of  iodine.  (2)  By  the  addition  of  chloroform, 
ether,  or  alkalies.  (3)  By  passing  repeated  strong  induction 
shocks  through  the  blood.  (4)  By  rapidly  freezing  and  thaw- 
ing the  blood  several  times. 

All  these  processes  produce  the  same  effect ;  viz.,  the  red  mat- 
ter leaves  the  stroma  and  passes  into  solution  without  producing 
a  marked  change  in  either,  as  if  the  solution  depended  upon  the 
destruction  of  some  vital  relationship  between  the  stroma  and 


METHOD   OF   COUNTING   CORPUSCLES. 


233 


the  oxyhsemoglobin  which  prevented  the  diffusion  of  the  latter 
in  the  living  blood.  • 

Solutions  of  urea,  bile,  acids  and  heat  of  about  60°  C.  seem  to 
destroy  the  discs,  and  thus  remove  the  coloring  matter.  Car- 
bolic, boracic  and  tannic  acids  cause  the  coloring  matter  to  coag- 
ulate and  localize  itself  either  at  the  centre  or  margin  of  the 
corpuscle.  (Fig.  103.) 

The  number  of  discs  in  the  blood  of  man  is  enormous,  namely, 
in  a  cubic  millimetre  of  blood,  about  5  millions  for  males  and  4i 


FIG.  104. 


Malassez1  Apparatus  for  the  Enumeration  of  Blood  Corpuscles. 
A,  Measuring  and  mixing  pipette.  B,  Flattened  and  calibrated  capillary  tube. 

millions  for  females,  or  about  250,000  millions  for  one  pound  of 
blood.  The  number  varies  much,  not  only  in  disease,  but  also  as 
a  result  of  the  many  physiological  processes,  such  as  changes  in 
the  amount  of  plasma,  brought  about  by  pressure  differences,  etc. 
In  order  to  count  the  corpuscles  the  following  method  is  em- 
ployed :  The  blood  is  diluted  with  artificial  plasma  to  100  or 
1000  times  its  volume,  and  the  corpuscles  in  a  portion  of  the 
mixture  carefully  measured  off  by  a  capillary  tube,  and  counted. 
This  operation  requires  great  care  and  delicate  apparatus.  One 
20 


234 


MANUAL   OF   PHYSIOLOGY. 


of  the  best-known  methods  is  that  of  Malassez,  the  details  of 
which  are  as  follows  : — 

Blood  is  drawn  into  the  capillary  tube  of  a  specially  prepared 
delicate  pipette  (Fig.  104,  A)  up  to  a  mark  which  indicates  y^- 
part  of  the  capacity  of  the  pipette.  This  known  quantity  of 
blood  is  then  washed  into  the  bulb  of  the  pipette  by  drawing  up 
artificial  serum  to  fill  the  bulb,  where  the  fluids  are  mixed  by 
shaking  about  a  glass  bead  contained  in  its  cavity.  Some  of  this 

FIG.  105. 


The  appearance  presented  by  the  Capillary  Tube  of  Malassez' Apparatus  when  filled  with 
diluted  blood  and  examined  under  a  microscope  magnifying  100  diameters,  provided 
with  an  eye-piece  micrometer. 

mixture  is  then  allowed  to  pass  into  a  flattened  capillary  tube 
of  known  capacity  fixed  on  a  slide,  and  the  number  of  corpuscles 
in  a  given  length  of  this  tube  is  carefully  counted  at  two  or  three 
places.  The  important  question,  how  much  oxyhsemoglobin 
exists  in  a  given  sample  of  blood,  can  be  determined  by  diluting 
some  of  it  until  the  color  equals  that  of  a  standard  solution  of 
known  strength. 


OXYH^EMOGLOBIN.  235 

CHEMISTRY  OF  THE  COLORING  MATTER  OF  THE 
OXYH^EMOGLOBIN. 


Of  the  chemical  constituents  found  in  the  blood  corpuscles,  the 
coloring  matter  is  by  far  the  most  important.  To  it  alone  the 
blood  owes  one  of  its  most  important  functions  —  the  respiratory. 

Oxyhcemoglobin  is  a  chemical  compound  of  great  complexity, 
of  which  the  percentage  composition  is  given  as  — 

Carbon  .............................................  53.85 

Hydrogen  .........................................     7.32 

Nitrogen  ...........................................  16.17 

Oxygen  ............................................  21.84 

Sulphur  ............................................  39 

Iron  ..................................................  43 

Its  rational  formula  is  unknown,  but  the  following  has  been 
proposed  as  approximate,  Ceoo  H^  N154  Fe  S3  Om.  It  may  be 
regarded  as  a  form  of  globulin,  associated  with  a  colored  mate- 
rial containing  iron,  called  hsematin.  Its  chief  peculiarities  are 
(1)  that,  although  it  contains  a  colloid  substance,  it  crystallizes 
more  or  less  readily  in  all  vertebrates  when  removed  from  the 
stroma  of  the  corpuscles  ;  (2)  the  considerable  amount  of  iron  it 
contains  (0.4  per  cent.)  ;  (3)  the  remarkable  manner  in  which  it 
is  combined  with  oxygen  to  form  an  unstable  compound  ;  and 
(4)  the  ease  with  which  it  yields  its  oxygen  to  the  tissues  and 
takes  it  from  the  air. 

The  readiness  with  which  the  oxyhcemoglobin  crystals  are  formed 
varies  much  in  different  animals  and  under  different  circum- 
stances, as  may  be  seen  from  the  following  list  :  — 

Most  readily  —  rat,  guinea  pig,  mouse. 
Readily  —  cat,  dog,  horse,  man,  ape,  rabbit. 
With  difficulty  —  sheep,  cow,  pig. 
Not  at  all—  frog. 

The  presence  of  oxygen  causes  the  crystals  to  form  more 
rapidly,  so  that  a  stream  of  oxygen  passed  through  a  strong 
solution  of  haemoglobin  causes  small  crystals  of  oxy  haemoglobin 
to  form. 

The  crystals  always  belong  to  the  rhombic  system,  being  most 


» 


236  MANUAL   OF   PHYSIOLOGY. 

frequently  plates  (man,  etc.)  and  prisms  (cat),  and  rarely 
tetrahedra  (guinea  pig)  and  hexagonal  plates  (squirrel). 

FIG.  ice.  The  color  of  the  crystals 

and  their  solution  varies  ac- 
cording to  the  light  by  which 
they  are  looked  at.  By  re- 
flected light  they  are  bluish 
red  or  greenish  in  color,  and 
by  direct  light,  scarlet. 

The  preparation    of   oxy~ 
haemoglobin  crystals    is    ac- 

Crystals  of  Hemoglobin  from  different  animals,      COinplished  by    first   Separat- 
showing  the  variety  in  form  of  crystals.  jng  the  coloring  matter  from 

the  corpuscles  by  freezing,  or  the  addition  of  water  or  ether,  and 
then  rendering  it  less  soluble  by  evaporation,  cold,  and  the  addi- 
tion of  alcohol. 

For  microscopic  observation  it  generally  suffices  to  kill  a  rat 
with  ether,  and  expose  a  drop  of  the  blood  diluted  with  distilled 
water  on  a  slide  until  half  dried,  and  then  cover.  Crystals 
appear  in  the  fluid  as  it  becomes  concentrated. 

The  combinations  which  haemoglobin  enters  into  are  numerous 
and  throw  much  light  upon  the  function  of  the  corpuscles. 

As  already  stated,  the  coloring  matter,  when  exposed  to  the 
air,  combines  with  oxygen  to  form  a  loose  chemical  compound 
called  oxyhsemoglobin.  This  is  the  condition  in  which  the 
coloring  matter  of  the  blood  is  generally  found.  Although  so 
prone  to  combine  with  oxygen,  the  oxyhsemoglobin  very  readily 
parts  with  some  of  it.  In  the  circulation  it  is  always  united  with 
oxygen,  normally  leaving  the  lungs  in  a  state  of  saturation.  On 
its  way  through  the  capillaries  of  the  tissues,  some  of  it  parts 
with  a  little  of  its  oxygen,  becoming  partially  reduced  (hsemo- 
globin),  but  even  the  most  venous  blood  always  contains  some 
oxyhsemoglobin. 

The  oxygen  can  be  removed  by  reducing  the  pressure  under  an 
air  pump,  or  by  exposing  the  solution  to  a  mixture  of  nitrogen 
and  hydrogen.  Various  reducing  agents  rob  the  oxyhsemoglobin 
of  its  oxygen ;  and  if  blood  or  a  solution  of  oxyhsemoglobin  be 


SPECTRA   OF   HAEMOGLOBINS. 
W 


237 


238  MANUAL   OF   PHYSIOLOGY. 

sealed  in  a  glass  tube  so  as  to  exclude  the  air,  the  loose  oxygen 
is  taken  up  by  some  of  the  other  constituents  of  the  blood,  and 
the  oxyhaemoglobin  becomes  gradually  reduced  to  haemoglobin, 
after  which  it  undergoes  no  further  change  or  decomposition. 
The  reduction  in  the  sealed  tube  depends  on  the  putrefactive 
changes  in  the  proteids,  and  may  be  prevented  by  careful  aseptic 
precautions.  If  the  reduced  haemoglobin  be  shaken  for  a  few 
moments  with  air,  the  bright  color  characteristic  of  oxyhsemo- 
globin  soon  reappears,  and  if  the  reducing  agent  be  not  injurious 
to  the  blood,  the  reduction  and  reoxidation  may  be  repeated 
several  times,  the  haemoglobin  going  through  the  changes  which 
take  place  in  it  during  normal  respiration. 

The  union  of  oxygen  with  haemoglobin  solutions  is  not  mere 
absorption  of  the  oxygen  by  the  liquid,  but  a  definite  chemical 
combination.  This  is  proved  by  the  following  facts:  (1)  When 
the  pressure  is  removed,  the  oxygen  does  not  come  away  from  the 
solution  in  accordance  with  the  law  which  governs  the  escape  of 
absorbed  gas,  but  all  comes  off  suddenly  when  the  pressure  is 
lowered  to  about  T^  of  an  atmosphere  (vide  p.  244).  (2)  The 
two  substances  give  a  different  absorption  band  when  examined 
with  the  spectroscope.  The  reduced  haemoglobin  gives  one  wide 
diffuse  band,  which  lies  between  the  D  and  E  lines  of  the  solar 
spectrum,  and  much  of  the  violet  end  is  cut  off.  The  single  band, 
which  is  characteristic  of  reduced  haemoglobin,  is  replaced  by  two 
when  the  haemoglobin  combines  with  oxygen — one  broad  band 
in  the  green  near  E,  and  a  narrow  one,  more  clearly  defined,  in 
the  yellow  close  to  D  line ;  both  bands  lie  between  D  and  E. 
With  strong  solutions  the  spectrum  is  darkened  at  either  extrem- 
ity, and  the  two  bands  become  wider  and  tend  to  fuse  into  one. 
(3)  Further,  the  oxygen  may  be  replaced  by  other  substances 
which  unite  with  the  haemoglobin.  One  of  the  most  important  of 
these  is  carbonic  oxide,  which  forms  a  much  more  stable  com- 
pound \vith  haemoglobin  than  oxygen.  It  is  of  a  bright  cherry- 
red  color  and  has  two  absorption  bands  in  the  spectrum  very  like 
those  of  oxyhaemoglobin  ;  that  in  the  yellow  is,  however,  removed 
a  greater  distance  from  the  D  line  toward  the  violet  end. 

It  is  this  compound  which  is  formed  in  poisoning  with  carbonic 


DECOMPOSITION    OF    HAEMOGLOBIN.  239 

oxide.  The  CO  occupying  the  place  of  the  oxygen,  destroys  the 
function  of  the  blood  corpuscles.  CO-haemoglobin  may  be  dis- 
tinguished from  O-haemoglobin  by  not  being  reduced  by  reagents 
greedy  of  oxygen,  and  by  the  bright  red  color  which  persists  when 
10  per  cent,  solution  of  caustic  soda  is  added,  and  the  mixture 
heated.  O-haemoglobin  gives  a  muddy-brown  color  under  the 
same  treatment. 

METH^MOGLOBIN. 

When  a  solution  of  oxyhaemoglobin  is  exposed  to  the  atmos- 
phere for  a  few  days  its  color  changes  to  a  dingy  brown,  and  it 
takes  up  more  oxygen  than  it  previously  contained.  The  new 
product  is  called  methcemoglobin.  The  oxygen  is  more  firmly 
combined  than  in  the  oxyhaemoglobin,  so  that  it  cannot  be 
removed  by  passing  other  gases  (CO,  etc.)  through  the  liquid, 
or  by  exhaustion  with  an  air  pump.  Methaemoglobin  gives  an 
absorption  spectrum  which  diifers  from  that  of  oxyhaemoglobin 
in  having  only  a  single  band,  and  from  that  of  reduced  haemo- 
globin  in  that  the  single  band  is  placed  more  to  the  red  side  of 
the  spectrum,  i.  e.,  between  the  lines  C  and  D.  This  substance 
can  also  be  formed  by  the  addition  of  potassium  permanganate 
or  alkaline  nitrites  to  haemoglobin.  A  solution  of  rnethsernoglo- 
bin,  though  unaltered  when  placed  in  vacuo,  may  be  reduced  to 
haemoglobin  by  ammonium  sulphide.  It  then  regains  its  red 
color,  shows  the  spectrum  of  reduced  haemoglobin,  and  when 
shaken  with  air  reforms  oxyhsemoglobin. 

DECOMPOSITION  OF   HAEMOGLOBIN. 

Haemoglobin  may  easily  be  broken  up  into  two  constituents — 
namely,  (a)  a  colorless  substance  which  is  nearly  related  to  the 
class  of  proteids  called  globulin,  and  (&)  a  blackish-red  amorphous 
material  called  hcematin,  which  contains  all  the  iron  of  the 
haemoglobin. 

This  change  is  brought  about  by  whatever  causes  the  coagula- 
tion of  albumin,  such  as  the  addition  of  acids,  strong  alkalies,  and 
heaf  above  70°  C. 


240  MANUAL   OF   PHYSIOLOGY. 

H^IMATIN,  ETC. 

Hsematin  is  a  secondary  product,  being  the  result  of  the  oxida- 
tion of  a  substance  called  hcemochromogen,  which  is  the  first  out- 
come of  the  decomposition  of  the  haemoglobin  by  acids  or  strong 
alkalies.  Hsemochromogen  or  reduced  hcematin,  as  it  may  be 
called,  can  be  obtained  from  hsematin  by  acting  on  that  body 
with  ammonium  sulphide,  but  it  can  only  be  preserved  in  an 
atmosphere  of  hydrogen  or  nitrogen,  as  it  immediately  takes  up 
oxygen  to  form  hsematin  on  exposure  to  the  air.  The  formula 
C68  H70  N8  Fe2  OIQ  has  been  given  for  hsematin.  It  dissolves  in 
weak  alkaline  and  acid  solutions,  but  not  in  water  or  in  alcohol. 

Hsematin  is  readily  prepared  by  mixing  acetic  acid  with  a 
strong  solution  of  haemoglobin,  which  becomes  a  dark-brown 
color.  The  dark  hsematin  can  be  removed  by  ether.  But  if  the 
acid  used  be  strong,  the  solution  of  hsematin  is  found  to  be  free 
from  iron.  This  iron-free  hsematin  is  called  hsematoporphyrin 
or  hcematoin.  If  now  the  acid  hsematin  solution  be  saturated 
\,ith  ammonia,  the  iron  again  becomes  united  with  the  hsematoin, 
forming  alkali-hcematin. 

H.EMIN. 

Hsematin  unites  with  hydrochloric  acid  to  form  a  crystalliz- 
able  body  called  hcemin  or  hydrochlorate  of 
FIG.  IDS.  hsematin  (Teichmann's  crystals). 

If  blood  or  dry  hsematin  be  mixed  with 
a  small  quantity  of  common  salt,  a  drop  of 
glacial  acetic  acid  added,  and  the  mixture 
boiled,  small  characteristic  crystals  appear, 
which  have  been  found  to  be  produced  by 
Crystals.  the  union  of  two  molecules  of  hydrochloric 

acid  with  the  hsematin. 

The  formation  of  these  crystals  is  very  easily  accomplished 
with  a  small  quantity  of  old  dried  blood ;  therefore  this  sub- 
stance becomes,  in  medico-legal  inquiries,  an  important  test  for 
blood  stains. 

Crystals  of  a  substance  called  hcematoidin  are  formed  in  old 
blood  clots  retained  in  the  body.  It'does  not  contain  any  iron, 


DEVELOPMENT   OF   THE   RED   DISCS.  241 

and  has  the  chemical  formula  C32  H36  N4  O6.  It  is  probably 
identical  with  bilirubin,  one  of  the  coloring  matters  found  in 
bile. 

GLOBIN. 

This  name  has  been  given  by  Preyer  to  the  proteid  part  of  the 
haemoglobin,  on  account  of  its  slightly  differing  from  globulin, 
though  it  resembles  it  in  being  precipitated  by  the  weakest  acids, 
even  carbon  dioxide,  and  it  leaves  no  ash  on  ignition. 

CHEMISTRY  OF  THE  STROMA. 

The  stroma  forms  only  about  10  per  cent,  of  the  solid  parts 
of  the  corpuscles,  the  rest  being  haemoglobin.  The  proteid 
basis  of  the  stroma  is  probably  made  up  of  a  globulin,  also  con- 
taining lecithin,  cholesterin  and  fats  in  minute  proportions. 
There  is  little  more  than  one-half  per  cent,  of  inorganic  salts  in 
the  red  blood  corpuscles,  of  which  more  than  half  consists  of 
potassium  phosphate  and  chloride. 

DEVELOPMENT  OF  THE  RED  DISCS. 

In  the  early  days  of  the  embryo  the  blood  vessels  and  corpus- 
cles appear  to  be  formed  at  the  same  time  from  the  middle  layer 
of  the  blastoderm  (mesoblast).  They  first  consist  of  round, 
nucleated,  colorless  cells,  which  subsequently  become  colored, 
gradually  lose  their  nucleus,  and  assume  the  characteristic  shape 
of  the  red  corpuscles,  the  rest  of  the  original  mass  of  protoplasm 
remaining  as  a  rudimentary  blood  vessel. 

In  the  later  stages  of  embryonic  life  the  red  corpuscles  are  said 
to  be  formed  in .  the  liver,  possibly  out  of  protoplasmic  elements 
which  are  made  in  the  spleen  and  thence  carried  to  the  liver  by 
the  portal  circulation. 

In  the  connective  tissue  of  rapidly  growing  animals — tadpole 
(Kolliker),  rabbit  (Ranvier),  rat  (Schafer) — certain  cells  can  be 
seen  connected  in  the  form  of  a  capillary  network,  and  within 
the  protoplasm  of  these  cells  red  coloring  matter  is  developed, 
and  the  particles  of  color  can  soon  be  recognized  as  character- 
istic blood  corpuscles,  arranged  in  rows  within  the  newly-formed 
networks.  Thus  isolated,  small  networks  of  capillaries,  consist- 
21 


242  MANUAL   OF   PHYSIOLOGY. 

ing  of  a  few  meshes  filled  with  blood  corpuscles,  are  formed  inde- 
pendently of  the  general  circulation. 

These  corpuscles  and  their  haemoglobin  are  manufactured  by 
isolated  protoplasmic  elements  in  the  connective  tissue,  and  sub- 
sequently added  to  the  general  mass  of  blood  by  the  growth  of 
the  network  bringing  it  into  continuity  with  the  neighboring 
vessels. 

In  the  adult  the  formation  of  red  blood  corpuscles  is  much 
less  active,  but  never  ceases  to  take  place  in  health,  for  the  cor- 
puscles must  be  renewed  as  they  become  worn  out  and  incapable 
of  performing  their  function.  This  reproduction  can  go  on  with 
considerable  rapidity,  as  we  see  after  severe  hemorrhage,  when 
the  normal  richness  in  hsemoglobin  and  corpuscles  is  soon 
regained.  Their  formation  is,  however,  probably  confined  to  a 
few  special  organs — spleen,  liver,  red  medulla  of  bones — where 
transitional  forms  are  found  in  such  numbers  as  to  point  to  the 
probability  of  the  red  corpuscles  being  the  offspring  of  the  color- 
less cells,  whose  protoplasm  either  manufactures  anew  or  collects 
the  necessary  hsemoglobin,  and  then  loses  its  nucleus  and  ordi- 
nary cellular  characters. 

We  can  only  guess  at  the  fate  of  the  discs,  but  there  are  many 
things  which  point  to  the  spleen  as  the  organ  in  which  they  are 
destroyed.  In  the  spleen  an  enormous  number  of  protoplasmic 
elements  are  produced,  and  the  blood  comes  into  relationship 
with  the  nascent  cells  in  a  way  unknown  in  any  other  part  of 
the  body.  Further,  various  unusual  elements,  some  like  altered 
red  corpuscles,  others  like  white  cells  enveloping  hsemoglobin, 
are  found  in  this  organ. 

The  blood  corpuscles  on  coming  to  the  spleen  are  possibly 
submitted  to  a  kind  of  preliminary  test  of  general  fitness,  some 
elements  of  the  spleen  pulp  having  the  faculty  of  examining 
their  condition  and  deciding  upon  their  fate.  Many,  no  doubt, 
pass  the  trial  without  any  change,  being  found  in  good  working 
order.  Others  that  are  found  totally  unfit  are  broken  up,  and 
their  effete  hsemoglobin  carried  to  the  liver  to  be  eliminated  as 
bile  pigment.  Some  possibly  undergo  a  form  of  repair  ;  a  white 
cell  taking  charge  of  a  weakly  disc  renews  its  stroma,  adds  to  its 


THE  GASES  OF  THE  BLOOD.  243 

haemoglobin,  and  carries  it  through  the  final  proof  in  the  liver, 
where  it  is  chemically  refreshed  before  going  to  the  lungs  for 
the  load  of  oxygen  which  it  has  to  carry  to  the  systemic  capil- 
laries. 

THE  GASES  OF  THE  BLOOD. 

These  are  present  in  two  conditions :  (1)  dissolved  in  accord- 
ance with  well-established  physical  laws,*  and  (2)  chemically 
combined.  But  since  those  present  in  the  latter  state  are  but 
loosely  combined  they  may  be  separated  by  the  same  means  as 
the  former,  and  thus  the  oxygen,  carbon  dioxide,  and  nitrogen 
can  all  be  removed  by  reducing  the  pressure  with  the  air  pump. 
For  this  purpose  a  mercurial  pump  must  be  used,  by  means  of 
which  a  practically  perfect  vacuum  can  be  formed  and  all  the 
gases  obtained  in  a  manner  which  facilitates  further  analysis. 
Together  they  are  found  to  measure  about  60  volumes  for  every 
100  volumes  of  blood. 

Oxygen. — The  amount  of  oxygen  in  the  blood  is  found  to  vary 
much  with  circumstances.  In  arterial  blood  the  quantity  is 
much  more  constant,  and  always  exceeds  that  in  venous  blood. 
It  is  estimated  (at  0°C.  and  760  mm.  pressure)  that  every  100 
volumes  of  arterial  blood  yield  20  volumes  of  oxygen,  while  the 
volume  of  oxygen  in  venous  blood  varies  from  8  to  12  per  cent. 

The  oxygen  which  comes  off  in  the  Torricellian  vacuum  exists 
in  the  blood  in  two  distinct  states :  (1)  a  very  small  quantity 
simply  absorbed, — about  as  much  as  water  absorbs  under  atmos- 
pheric pressure ;  (2)  chemically  combined,  in  which  state  nearly 

*  1.  At  the  same  temperature  the  volume  of  a  gas  varies  inversely  with 
the  pressure,  so  that  with  twice  the  pressure  a  given  volume  of  a  gas  is 
twice  the  weight. 

2.  A  given  liquid  absorbs  the  same  volume  of  a  given  gas,  to  which  it  is 
exposed,  independent  of  the  pressure  exercised  by  that  gas. 

3.  Therefore  the  amount  by  weight  of  gas  absorbed  by  a  liquid,  at  a 
given  temperature,  depends  directly  on  the  pressure,  being  nil  in  vacuo. 

4.  The  weight  of  a  given  volume  of  a  gas  decreases  and  the  coefficient 
of  absorption  of  a  liquid  diminishes,  as  the  temperature  increases. 

5.  Therefore  the  amount  of  gas  absorbed  is  in  inverse  proportion  to  the 
temperature,  being  practically  nil  at  boiling  point. 


244  MANUAL   OF    PHYSIOLOGY. 

all  the  oxygen  exists,  and  forms  with  the  haemoglobin  the  loose 
combination  called  oxy haemoglobin.  This  oxygen  therefore  does 
not  follow  the  laws  of  absorption  by  leaving  the  blood  in 
proportion  as  the  pressure  is  reduced,  but  when  a  certain  point 
of  reduction  of  pressure  (20-30  mm.  mercury,  according  to  the 
temperature)  is  reached,  the  oxygen  comes  off  almost  completely. 

Carbon  Dioxide  (CO2). — The  amount  of  carbon  dioxide  also 
varies  more  in  venous  than  in  arterial  blood,  for  under  certain 
circumstances  (suffocation)  it  may  rise  to  over  60  volumes  per 
cent.,  although  ordinary  venous  blood  on  an  average  contains 
only  46  volumes  in  every  100  of  blood.  On  the  other  hand,  the 
amount  of  this  gas  in  arterial  blood  varies  little  from  39  volumes 
per  cent. 

Nearly  all  the  carbon  dioxide  exists  in  the  plasma,  where  some 
of  it  appears  to  be  chemically  combined  with  soda  salts. 

Nitrogen. — The  amount  ojf  nitrogen  does  not  vary  much,  being 
in  both  venous  and  arterial  blood  about  1.5  volume  per  cent., 
and  it  would  appear  to  be  simply  absorbed. 

For  further  details  about  the  gases  of  arterial  and  venous 
blood,  see  Respiration. 


COAGULATION   OF   THE   BLOOD.  245 


CHAPTER  XV. 
COAGULATION  OF  THE  BLOOD. 

In  speaking  of  the  chemical  relationship  of  the  plasma  (see  p. 
222),  the  formation  of  fibrin  has  been  mentioned  as  the  essential 
item  in  coagulation,  and  the  relation  of  fibrin  to  its  probable 
precursors  has  been  discussed.  If  the  points  there  explained  be 
borne  in  mind,  and  the  presence  of  the  corpuscles  be  taken  into 
account,  the  various  characteristics  of  the  clot  which  forms  when 
blood  is  shed  into  a  vessel  can  be  easily  understood,  and  should 
require  no  further  description. 

The  great  importance  of  the  coagulation  of  the  blood  in 
arresting  bleeding,  and  in  certain  pathological  processes,  makes 
it  expedient,  however,  to  consider  more  closely  the  steps  of  the 
process  and  to  inquire  into  the  various  circumstances  which 
facilitate  its  occurrence  after  the  blood  is  shed,  as  well  as  in  the 
living  vessels. 

COAGULATION  OF  SHED  BLOOD. 

Before  the  formation  of  a  perfect  clot,  blood  may  be  seen  to 
pass  through  three  stages:  1,  viscous;  2,  gelatinous;  3,  contrac- 
tion of  clot  and  separation  of  serum. 

The  first  stage  is  commonly  very  short,  and  in  thin  layers  of 
blood  passes  immediately  into  the  second.  In  cold  weather  con- 
siderable quantities  of  blood,  if  contained  in  deep  vessels,  take  a 
much  longer  time  to  stiffen,  so  that  the  first  stage  may  occupy 
from  one  minute  to  some  hours. 

The  second  stage,  when  the  mass  has  been  turned  into  a  firm 
jelly,  may  be  arrived  at  within  the  varying  limits  just  named, 
and  occupies  a  corresponding  period  :  only  a  few  minutes  if  the 
mass  be  small,  spread  out  or  shaken,  but  many  hours  if  a  large 
quantity  be  kept  motionless  and  cool. 

The  third  stage  therefore  begins  sometimes  as  soon  as  ten  to 
fifteen  minutes,  but  generally  after  some  hours.  Clear  drops  of 


246 


MANUAL   OF   PHYSIOLOGY. 


serum  appear  about  the  clot.  After  several  hours  this  contracts 
until  it  forms  but  a  comparatively  small  mass  floating  in  the 
serum.  If  the  jelly-like  clot  be  disturbed,  the  serous  fluid  makes 
its  appearance  much  sooner  than  the  time  just  stated. 

During  the  formation  of  the  clot  under  ordinary  circumstances 
the  corpuscles  are  entangled  in  the  meshwork  of  fibrin,  so  that 
the  gelatinous  mass  has  throughout  a  dark-red  color. 

If  the  coagulation  takes  place  slowly — as  it  does  in  very  cold 
weather,  in  horses'  blood,  or  in  human  blood  if  removed  from  a 

FIG.  109. 


Reticulum  of  Fibrin  Threads  after  staining  has  made  them  visible.    The  network  (b) 
appears  to  start  from  granular  centres  (a).    (Ranvier.) 

person  during  fever — then  the  heavier  red  corpuscles  have  time 
to  subside  to  the  lower  layers  of  the  clotting  plasma,  while  the 
white  cells  are  caught  in  the  meshes  of  the  fibrin  and  remain  in 
the  upper  layer  of  the  clot,  which  then  has  the  pale  color  famil- 
iar to  the  physician  in  the  old  days  of  bleeding  as  the  "  bufFy 
coat,"  or  crusta  phlogistica.  This  buffy  coat  contains  a  greater 
proportion  of  the  elastic  fibrin  and  soft  white  cells  than  the 
rest  of  the  clot,  and  encloses  but  few  red  corpuscles,  therefore  the 


CIRCUMSTANCES   INFLUENCING   COAGULATION.  247 

fibrin  can  contract  more  completely  in  this  upper  layer  than  in 
the  deeper  part  of  the  clot  which  includes  the  red  corpuscles. 
The  effect  of  this  is,  that  the  upper  surface  becomes  concave,  and 
a  "  cupped  "  clot  is  formed.  The  contraction  of  the  clot  proceeds 
for  days,  and  in  order  to  see  the  characters  described  above,  the 
blood  should  be  kept  in  a  cool  place  and  perfectly  motionless. 

The  contraction  of  the  fibrin  and  separation  of  the  serum  can 
be  made  to  take  place  much  more  quickly  by  gentle  agitation 
causing  the  ends  of  the  fibrin  threads  to  separate  from  the  sides 
of  the  vessel,  but  by  thus  disturbing  the  clot  during  its  forma- 
tion, the  corpuscles  are  displaced  and  escape  into  the  serum, 
which  is  then  stained  and  cannot  be  seen  in  its  clear,  transparent 
state. 

If  brisk  agitation  with  a  glass  rod  —  or  better  a  bundle  of 
twigs  —  be  commenced  the  moment  the  blood  is  drawn,  the  fibrin 
is  formed  more  rapidly  ;  but  the  corpuscles  are  not  entangled  in 
its  meshes,  for  as  quickly  as  the  elastic  threads  are  formed  they 
adhere  to  and  are  removed  by  the  rod  or  twigs.  Thus  the  fibrin 
is  formed  very  rapidly,  and  the  ordinary  stages  in  the  formation 
of  a  blood  clot,  consisting  of  fibrin  and  the  corpuscles,  do  not 
occur,  for  the  fibrin  is  separated  from  the  corpuscles  as  quickly 
as  it  is  formed.  We  then  have  what  is  commonly  spoken  of  as 
"  defibrinated  blood,"  which  does  not  give  a  blood  "clot.  Not  that 
the  coagulation  has  been  prevented,  but  the  material  essential 
for  the  formation  of  a  clot  has  been  removed  as  quickly  as 
formed,  and  instead  of  catching  the  corpuscles  in  the  meshes  of 
its  delicate  fibrils  to  form  the  clot  in  the  ordinary  way,  the 
stringy  shreds  of  fibrin  cling  around  the  beating  rod  as  a  jagged 
mass.  The  following  tables  show  the  relation  of  the  different 
constituents  of  coagulated  and  defibrinated  blood  respectively  :  — 

f  Plasma       ^  f  Serum  (appearing  as  clear  fluid). 

- 


,  pl  ^  ("Fibrin  (removed  on  the  rod). 

Living  blood  =  ^  £ia"  **,      V   =     \  Serum  +  \  Defibrinated 

\  Corpuscles/  1  Corpuscles}  blood. 

Many  circumstances  influence  the  rapidity  with  which  a  blood 
clot  is  formed.     Speaking  generally,  circumstances  which  tend 


248  MANUAL   OF   PHYSIOLOGY. 

to  injure  the  corpuscles  or  the  plasma,  and  give  rise  to  changes 
resulting  in  their  death,  promote  coagulation  ;  while,  on  the 
other  hand,  conditions  which  protect  the  corpuscles  and  impede 
fibrin  formation  must  retard  coagulation. 

These  may  be  arranged  categorically,  viz. 
(A)  Circumstances  promoting  coagulation  : — 

1.  Contact  with  foreign  bodies  is  of  the  first  importance  in 

hastening  coagulation.  The  greater  the  surface  of  con- 
tact with  the  vessel  or  the  air,  the  more  the  corpuscles 
are  exposed  to  injury,  and  the  more  rapid  are  the 
destructive  chemical  changes  inducing  fibrin  formation. 
Thus  a  drop  or  two  of  blood  falling  on  any  surface  so 
as  to  spread  out  in  a  thin  layer  clots  almost  instantly. 

2.  Motion,  by  renewing  the  points  of  contact  between  the 

blood  and  the  moving  agent,  hastens  coagulation. 
Thus,  by  whipping  fresh  blood,  all  the  fibrin  can  be 
removed  in  a  few  minutes,  and  the  defibrinated  blood 
left  without  a  clot. 

3.  Moderate  heat. — The  formation  of  the  fibrin  generators 

and  the  action  of  the  ferment  seem  to  go  on  more  rap- 
idly at  38°-40°  C.  than  at  any  other  temperature. 

4.  A  watery  condition  of  the  blood  causes  rapid  coagulation 

but  a  soft  clot.  This  is  seen  in  repeated  bleedings  or 
hemorrhages  ;  the  blood  which  flows  last  clots  first. 

5.  The  addition  of  a  small  quantity  of  water,  by  setting  up 

rapid  changes  in  the  corpuscles,  accelerates  coagula- 
tion. 

6.  A  supply  of  oxygen. — Oxygen  is  used  up  in  the  chemical 

changes  attendant  upon  the  death  of  the  blood,  and  its 
presence  aids  the  formation  of  firm  clots,  such  as  are 
produced  in  arterial  blood.  Exposure  to  the  air  in  a 
shallow  vessel  facilitates  coagulation,  partly  by  exten- 
sive contact  and  partly  by  a  free  supply  of  oxygen. 
But  exposure  to  air  is  not  necessary,  for  blood  collected 
in  mercury,  without  ever  coming  in  contact  with  the 
air,  coagulates  very  rapidly. 


COAGULATION   WITHIN   THE   VESSELS.  249 

* 

(B)  Circumstances  which  retard  coagulation  : — 

1.  Constantly  renewed  and  close  inter-relationship  with  the 

lining  of  healthy  blood  vessels  alone  affords  the  require- 
ments essential  for  the  preservation  of  the  living  cor- 
puscles and  plasma  in  their  normal  condition. 

2.  When  the  blood  is  surrounded  by  healthy  living  tissues 

interchanges  may  occur  between  them,  and  if  the  oxygen 
supply  is  deficient,  coagulation  is  much  delayed.  Thus 
considerable  quantities  of  blood  effused  into  the  tissues 
may  remain  liquid  and  black  for  many  days.  This 
dark  blood  clots  on  removal  and  exposure  to  the  air. 

3.  Low  temperature. — The   rate   of   coagulation    decreases 

below  38°  C.,  and  the  process  is  checked  at  0°  C. 

4.  The  addition  of  concentrated  solutions  of  neutral  salts 

(about  three  volumes  of  30  per  cent,  solution  of  mag- 
nesium sulphate)  quite  prevents  coagulation. 

5.  The  introduction  of  peptone  into  the  blood. 

6.  An  extract  of  the  mouth  of  the  leech  has  a  remarkable 

power  of  preventing  coagulation. 

7.  A  great  quantity  of  water  seems  to  render  the  action  of 

the  fibrin  factors  weak. 

8.  The  addition  of  egg  albumin,  syrup,  or  glycerine. 

9.  The  addition  of  small  quantities  of  alkalies. 

10.  The  addition  of  acetic  acid  until  very  slight  acid  reaction 

is  obtained. 

11.  Increase  in  the  amount  of  carbon  dioxide.     This,  together 

with  the  want  of  oxygen,  explains  why  venous  blood 
clots  more  slowly  and  loosely  than  arterial,  and  why 
the  blood  in  the  distended  right  side  of  the  heart  is  fre- 
quently liquid  after  death  from  suffocation. 

12.  The  blood  of  persons  suffering  from  inflammatory  disease 

coagulates  slowly,  but  forms  a  very  firm  clot,  which  is 
"  buffed  and  cupped." 

COAGULATION  WITHIN  THE  VESSELS. 

Since  the  blood  coagulates  spontaneously  when  removed  from 
the  body,  the  question  now  arises,  How  does  it  remain  fluid  in 
the  blood  vessels? 


250  MANUAL   OF    PHYSIOLOGY. 

Though  this  question  has  long  occupied  much  attention,  it  is 
still  difficult  to  formulate  a  definite  answer.  Nor  can  we  expect 
to  find  any  adequate  explanation  until  we  are  better  acquainted 
with  the  exact  details  of  the  origin  of  the  fibrin  generators.  It 
must  be  remembered  that  the  blood  may  be  regarded  as  a  tissue, 
made  up  of  living  constituents  requiring  constant  assimilation 
and  elimination  for  the  maintenance  of  its  perfectly  normal  con- 
ditions and  life.  We  can  confidently  say  that  coagulation  is  the 
outcome  of  certain  chemical  changes  concomitant  with  the  death 
of  the  blood,  and  that  while  it  lives  no  such  changes  take  place. 
But  such  an  answer  adds  little  to  our  knowledge  of  the  matter. 

Since  constant  chemical  intercourse  must  be  kept  up  between 
the  blood  and  its  surroundings  in  order  to  sustain  the  complex 
chemical  integrity  essential  for  its  life,  we  cannot  be  surprised 
that  its  waste  materials  accumulate,  and  that  it  soon  dies  when 
shed,  as  other  tissues  do  when  deprived  of  their  means  of  sup- 
port. The  formation  of  a  solid  and  the  separation  of  a  liquid 
form  of  proteid  is  in  no  way  unusual  as  a  first  step  in  the  decline 
from  exalted  chemical  construction,  for  similar  changes  occur  in 
other  tissues,  and  in  protoplasm  itself.  The  soft  contractile  sub- 
stance of  muscle  probably  tends  during  its  contraction,  and  cer- 
tainly at  its  death  does  undergo  almost  exactly  the  same  kind  of 
change  as  the  blood  in  coagulation. 

If  we  knew  accurately  the  nutritive  process  taking  place  in  the 
blood  itself,  and  with  which  of  its  surroundings  it  keeps  up  chem- 
ical interchange,  the  answer  would  be  much  simplified.  But  we 
have  in  the  blood  three  elements  that  probably  have  different 
modes  of  assimilation  and  elimination,  viz.,  plasma,  white  cells 
and  red  discs.  We  practically  know  nothing  of  the  changes 
they  undergo  during  their  nutrition;  or  whether  their  tissue 
changes  have  a  necessary  relation  to  those  of  the  neighboring 
tissues.  We  do  know,  however,  that  there  exists  some  very  inti- 
mate relation  between  the  membrane  lining  the  vessel  walls  and 
the  contained  blood.  They  seem  to  require  frequently  repeated 
contact  one  with  the  other  in  order  that  the  normal  condition  of 
both  may  be  maintained  in  perfect  vital  integrity.  That  fresh 
supplies  of  blood  are  required  by  the  vessel  wall  may  be  shown 


COAGULATION   WITHIN   THE   VESSELS.  251 

.by  the  fact  that  when  deprived  of  its  nutriment  by  a  stoppage  of 
the  blood  flow,  it  soon  loses  its  power  of  retaining  the  blood,  and 
admits  of  extravasation.  And  that  renewed  contact  with  the 
vessel  wall  is  equally  necessary  for  the  integrity  of  the  blood, 
is  seen  from  the  fact  that  the  cells  congregate,  the  discs  adhere 
together,  and  the  plasma  coagulates  when  stasis  interferes  with 
its  intercourse  with  fresh  parts  of  the  intima.  Probably  the 
chemical  changes  going  on  in  the  one  are  useful  for  the  nutrition 
of  the  other,  and  they  mutually  supply  one  another  with  some 
material  essential  for  their  life.  This  is  apparent  in  those  cases 
where  coagulation  takes  place  during  life  in  the  vessels.  It 
never  occurs  so  long  as  the  intima  of  the  vessel  is  perfect,  and  the 
blood  flow  constant,  but  it  follows  lesion  of  this  delicate  mem- 
brane, whether  caused  by  injury  or  mal-nutrition. 

The  gradual  occurrence  of  this  impairment  of  function  of  the 
intima  can  be  watched  under  the  microscope  in  the  small  vessels 
of  a  transparent  part  during  the  initial  stages  of  inflammation. 
Owing  to  the  arrest  of  the  flow  of  blood,  the  walls  of  the  small 
vessels  suffer  from  defective  nutrition,  and  may  be  seen  to  allow 
some  elements  to  escape,  while  the  discs  adhere  together  and  the 
plasma  coagulates. 

In  the  larger  vessels  the  same  thing  occurs  when  inflammation 
of  their  lining  membrane  destroys  its  capability  of  keeping  up 
the  necessary  nutritive  equilibrium.  Thus  clots  form  on  the 
inner  lining  to  the  walls  of  an  inflamed  vein,  often  growing  so 
as  to  fill  the  entire  vessel,  and  give  rise  to  a  condition  called 
thrombosis. 

On  the  valves  of  the  left  side  of  the  heart  and  in  the  arteries, 
where  the  delicate  intima  is  subjected  to  great  mechanical  strain, 
it  is  common  enough  to  find  slight  injuries  of  it  covered  over  with 
thin  clots.  To  the  surgeon  this  mutual  nutrition  of  intima  and 
blood  is  of  the  utmost  importance  in  attaining  the  occlusion  of 
vessels,  for  it  is  upon  this  fact  he  has  mainly  to  depend  for  the 
stoppage  of  hemorrhage  from  a  wounded  artery.  A  tightly-tied 
ligature  either  injures  the  inner  coats  mechanically,  or  starves 
the  intima  by  checking  the  flow  of  blood  through  the  vessel  up 
to  the  next  branch,  and  that  portion  of  the  vessel  is  filled  with 


252  MANUAL   OF   PHYSIOLOGY. 

stationary  blood,  which  soon  clots  and  forms  an  adherent  plug. 
But  if  the  ligature  be  applied  too  loosely,  a  slight  blood  current 
passes  through  the  point  where  the  vessel  is  tied,  and  this  suf- 
fices for  the  nutrition  of  the  intima  by  the  renewal  of  the  blood's 
contact,  so  that  no  clot  is  formed,  the  vessel  is  not  closed,  and 
most  probably,  when  the  ligature  has  cut  through  the  outer  coat, 
"secondary  hemorrhage"  occurs. 

It  has  also  been  shown  that  if  any  foreign  substance,  such  as 
a  thread,  be  introduced  into  the  blood  while  circulating,  a 
coagulum  will  form  around  it.  From  this  it  would  appear  that 
the  presence  of  a  substance  which  cannot  carry  on-  the  necessary 
chemical  intercourse  with  the  blood  will  excite  irritation  in  its 
elements,  and  so  effect  slight  local  death  of  the  plasma  and  the 
production  of  fibrin. 

The  time  required  for  the  production  of  intra-vascular  coagula- 
tion as  a  result  of  mere  stasis  is  happily  long,  for  it  has  been 
found  that  the  blood  current  may  be  stopped  in  a  limb,  by  pres- 
sure or  otherwise,  for  many  hours  without  coagulation  occurring. 
Indeed,  cases  have  occurred  where  a  tight  bandage  has  stopped 
the  circulation  for  an  entire  day  without  injurious  consequences. 
This  is  explained  by  the  fact  that  so  long  as  the  intima  lives,  the 
blood  remains  fluid ;  in  short,  the  tissues  die  before  the  blood 
clots  in  the  vessels. 

The  tissues  continue  to  live  for  some  time  after  the  animal  is 
dead,  and  so  we  see  the  blood  remains  fluid  in  the  vessels  a 
considerable  time,  in  fact,  as  long  as  the  vessel  wall  can  nourish 
itself  and  live.  Thus  it  has  been  shown  that  the  blood  in  a 
horse's  jugular  vein  separated  by  ligature  from  the  circulation, 
and  removed  from  the  animal,  will  remain  fluid  for  fully  twenty- 
four  hours. 

In  cold-blooded  animals  the  tissues  live  for  even  a  longer  time. 
The  heart  of  the  tortoise,  if  kept  under  suitable  conditions,  will 
beat  for  two  days  when  removed  from  the  body,  and  as  Briicke 
has  shown,  blood  contained  in  it  will  remain  fluid  until  after  the 
heart  is  dead. 

If  the  details  of  the  fibrin  formation  within  the  blood  vessels 
be  followed,  it  is  found  that  the  injured  spot  or  foreign  body  first 


COAGULATION   WITHIN   THE   VESSELS.  253 

becomes  covered  over  with  white  corpuscles,  around  which 
threads  of  fibrin  appear  attached  to  the  rough  surface.  As  more 
fibrin  is  formed  and  the  layer  thickens,  only  a  few  cells  can  be 
seen  in  its  meshes,  but  a  great  number  always  exist  on  the  sur- 
face of  the  new  fibrin,  forming  a  layer  between  it  and  the  blood. 
It  is  further  remarked  that  coagulation  has  some  relation  to  the 
abundance  of  white  cells  in  all  spontaneously  coagulated  fluids. 
The  more  cells,  the  firmer  the  clot.  In  pathological  exudations, 
also,  and  those  acute  serous  collections  which  coagulate  on 
removal  from  the  body,  fine  granular  threads  of  fibrin  seem 
to  start  from  the  white  cells,  and  radiate  from  them  in  a  stellate 
manner.  (Figs.  100  and  109.) 

When  white  cells  congregate  at  a  point  of  a  vessel  from  which 
the  intima  is  stripped,  their  more  active  exertion  possibly  pro- 
duces the  ferment,. etc.  And  at  the  same  time  they  remain  at 
the  injured  part  of  the  vessel  wall,  and  the  removal  of  the 
fibrin  factors  cannot  occur  at  the  place  of  injury,  since  the  intima 
is  destroyed.  Thus,  local  clots  are  formed  which  extend  over  the 
injured  surface,  and  by  a  process  of  organization  the  repair  of 
the  denuded  patch  is  accomplished. 

Some  believe  that  a  great  number  of  white  blood  cells  undergo 
chemical  disintegration  the  instant  the  blood  is  shed,  and  con- 
sider that  the  fibrin  ferment,  and  probably  other  fibrin  gener- 
ators, are  the  result  of  the  destruction  of  these  weak  cells,  and 
exclude  the  red  corpuscles  from  taking  any  share  in  the  process. 

There  is  some  evidence,  however,  that  the  plasma  and  the 
discs  can  give  rise  to  all  the  fibrin  factors,  and  we  know  that  in 
the  circulation  white  cells  must  be  destroyed  and  yet  cause  no 
coagulation. 

If  some  fresh  blood  be  allowed  to  flow  into  a  fine  capillary 
tube,  the  white  cells  can  be  seen  to  move  away  from  the 
red  discs,  and  the  formation  of  the  clot — a  delicate  fibrin  net- 
work enclosing  the  discs — may  be  watched.  Here  some  at  least 
of  the  white  cells  exhibit  manifestations  of  life  for  a  considerable 
time  after  the  clot  has  been  formed,  and  their  death  could  not 
have  been  the  source  of  the  fibrin  factors. 

In  conclusion,  then,  we  can  only  suppose  that,  as  in  other 


254  MANUAL   OP   PHYSIOLOGY. 

tissues,  some  chemical  changes  must  go  on  in  the  elements  of 
the  blood  in  order  to  preserve  its  integrity.  A  cessation  of  these 
changes  gives  rise  to  new  products  which  produce  fibrin,  and 
hence  cause  coagulation.  But  so  long  as  the  elements  of.  the 
blood  are  frequently  brought  into  close  relationship  with  a 
healthy  vessel  wall,  the  fibrin  factors  are  either  produced  in  such 
small  quantity  as  to  be  ineffectual,  or  they  are  altered,  destroyed, 
or  taken  up  by  the  intima  and  possibly  utilized  for  its  nutrition. 
When  the  blood  is  removed  from  the  vessels,  the  production  of 
the  fibrin  factors  proceeds  effectually,  either  on  account  of  the 
blood  elements  undergoing  destructive  changes,  the  products  of 
which  accumulate ;  or  owing  to  the  impossibility  of  re- integration, 
the  fibrin  factors  appear  as  a  product  of  lethal  chemical  change 
or  decomposition. 

In  accepting  the  first  view,  we  only  adopt  the  theory  of  John 
Hunter,  who  thought  coagulation  was  an  act  of  life.  If  we 
adopt  the  other  view,  we  must  needs  say  it  is  an  act  of  death. 
But,  after  all,  this  is  a  mere  difference  in  degree,  for  how  can  we 
distinguish  between  the  unsuccessful  attempt  of  a  living  tissue  to 
re-integrate,  or  regain  the  chemical  properties  upon  which  its 
life  depends,  and  the  inevitable  result  of  failure,  which,  if  pro- 
longed beyond  a  certain  point,  must  cause  its  death  ? 


THE   HEART. 


255 


CHAPTER  XVI. 
THE   HEART. 

The  course  taken  by  the  blood  in  its  way  to  the  various  parts 
of  the  body  is  called  the  circulation,  on  account  of  its  having  to 
make  repeatedly  the  circuit  of  vessels  leading  to  and  from  the 
heart.  The  heart  is  the  great  motor  power  which  drives  the 


FIG.  110. 


FIG.  111. 


R.H. 


L.H. 


Diagram  of  Circulation,  showing  right 
(R.  H.)  and  left  (L.  H.)  hearts,  and  the 
pulmonary  (p)  and  systemic  (s)  sets 
of  capillary  vessels. 


Capillary  Network  of  the  Choroid  of  Child  of  a 

few  months  old.    (Cadiat.) 

(a)  Artery.     (6)  Vein,  and  capillary  network 

intervening. 

blood  through  all  the  vessels,  of  which  there  is  one  set  belong- 
ing to  the  circulation  of  the  organs  of  the  system  generally,  and 
another  leading  to  and  from  the  lungs. 

Anatomists  speak  of  two  circulations — the  greater  or  systemic, 
and  the  lesser  or  pulmonary.  However,  if  we  follow  the  course 
of  the  blood,  we  see  that  both  these  sets  of  vessels  really  belong 
to  the  one  circulation,  and  in  fact  form  but  one  circuit.  In  all 


256  MANUAL   OF   PHYSIOLOGY. 

the  higher  animals  the  heart  forms  a  single  organ,  but  it  really 
is  composed  of  two  muscular  pumps  which  are  anatomically 
united  though  distinct  in  function.  These  functionally  distinct 
hearts  work  at  different  parts  of  the  circuit  traversed  by  the 
blood.  The  blood  on  its  way  through  the  lungs  and  systemic 
vessels  visits  the  heart  twice,  in  order  to  acquire  the  force  neces- 
sary to  overcome  the  resistance  of  these  two  sets  of  vessels.  The 
right  heart  is  visited  before  the  pulmonary  vessels,  and  is  the 
agent  for  pumping  the  blood  through  the  lungs.  The  left  heart 
is  placed  before  the  systemic  vessels  and  pumps  the  blood 
through  the  body  generally.  Thus  anatomically  there  appear  to 
be  two  circulations  and  but  one  heart ;  physiologically  there  is 
one  circulation  and  two  hearts ;  or  two  separate  points  of  resist- 
ance and  a  distinct  pumping  organ  to  drive  the  blood  through 
each. 

The  circulation  might  then  be  represented  by  a  simple  diagram 
(Fig.  110)  in  which  the  direction  of  the  current  is  indicated  by 
the  arrows.  L  H  shows  the  position  of  'the  left  or  systemic 
pump,  and  S  the  resistance  in  the  systemic  vessels.  R  H  rep- 
resents the  pulmonary  pump  and  P  the  second  obstacle  in  the 
circuit,  viz.,  the  vessels  of  the  lungs.  This  functional  distinction 
must  be  kept  in  view  in  studying  the  dynamics  of  the  circulation, 
although  the  two  pumping  organs  are  fused  into  one  viscus, 
with  two  distinct  and  separate  channels  for  the  passage  of  the  blood. 

In  each  system  of  blood  vessels  we  have  the  same  general 
arrangement  for  the  distribution  and  re-collection  of  the  blood. 

In  passing  from  either  the  right  or  left  side  of  the  heart  the 
blood  flows  into  tubes  called  arteries,  which  divide  and  subdivide 
until  the  branches  become  microscopical  in  size.  From  the  very 
minute  arteries  the  blood  passes  into  the  capillaries,  which  cannot 
be  said  to  branch,  but  form  a  network  of  delicate  tubes  with 
meshes  of  varying  closeness  according  to  the  tissue. 

Connected  with  the  meshes  of  the  capillaries  are  the  small 
veinlets  which  collect  the  blood  from  the  networks  (Fig.  111). 
These  unite,  gradually  forming  larger  vessels,  which  again  are 
but  the  tributaries  of  the  large  veins  which  convey  the  blood 
back  to  the  heart. 


THE   CIRCULATION   OF   THE   BLOOD. 


257 


FIG.  112. 


RA. 


RV. 


About  three  hundred  years  ago  the  true  course  of  the  blood 
current  through  the  systemic  and  pulmonary  heart,  arteries  and 
veins,  so  as  to  form  one  circle,  was  demonstrated  by  Harvey. 
Before  his  time  only  the  so-called  "  lesser  "  or  pulmonary  circuit 
was  known.  The  magnifying  glasses  at  his  disposal  did  not 
enable  him  to  see  the  capillaries,  which  were  first  described  by 
Malpighi  some  fifty  years  later. 

In  the  hope  of  making  their  different  functions  appear 
more  striking,  the  various  parts  of  the  circulatory  apparatus 
may  be  enumerated  as  follows,  and  roughly  illustrated  by  a 
diagram : — 

1.  The  left  (systemic)  heart  (L  H)  pumps  the  blood  into  the 
systemic   arteries,  and   thus  keeps 

these  vessels  over  filled. 

2.  The  larger  systemic   arteries 
(A),  by  their  elasticity,  exert  con- 
tinuous pressure  on  the  blood  with 
which  they  are  distended. 

3.  The  smaller  systemic  arterioles 
(A'),  by  their  vital   contractility, 
check  and  regulate  the  amount  of 
blood    flowing    out   of  the    larger 
arteries  into   the    capillaries,   and  / 
thus  keep  up  a  high  pressure  in  the 
larger  arteries. 

4.  In  the  systemic  capillaries  (S 
C),  the  essential  operations  of  the 
blood    are   carried    out,    viz.,   the 
chemical  interchanges   between  it 
and  the  tissues. 

5.  The  wide  systemic  veins  (V) 
are  the  passive  channels  conveying 
the  impure  blood  to  the  pulmonary 
heart. 

6.  The  right  (pulmonary)  heart 

(R  H)  pumps  the  blood  into  the  pulmonary  arteries  and  dis- 
tends them. 
22 


s.c. 


Diagram  of  the  Circulation  of  the  Blood 
and  the  absorbent  vessels.  For  details, 
see  text. 


258  MANUAL   OF   PHYSIOLOGY. 

7.  The   pulmonary   arteries  (P  A)  press   steadily  upon  the 
blood  and  force  it  through — 

8.  The  small  pulmonary  arterioles  (P  a),  regulate  the  flow 
into  the  capillaries  of  the  lungs. 

9.  In  the  pulmonary  capillaries  (P  C),  the  blood  is  exposed 
to  the  air,  and  undergoes  active  gas  interchange. 

10.  The  pulmonary  veins  (P  V)  carry  the  blood  to  the  left 
heart,  and  thus  complete  the  circuit.        ^ 

L/i  indicates  the  lymphatics,  which  drain  the  tissues,  and  Lc 
the  lacteals,  which  absorb  from  the  stomach  and  intestines  (I). 

Although  the  blood  enters  the  arteries  by  jerks,  its  motion 
through  the  capillaries  is  even,  because  the  arteries  constantly 
press  on  the  blood  they  contain,  their  elastic  walls  being  dis- 
tended by  the  pumping  of  the  heart,  which  fills  the  aorta 
and  arteries  more  quickly  than  they  can  empty  themselves, 
unless  the  adequate  pressure  has  been  attained.  The  contracting 
arterioles  are  the  chief  agents  in  resisting  the  outflow  and  keep- 
ing up  the  arterial  pressure. 

THE   HEAKT. 

The  heart  of  man  and  other  warm-blooded  animals  may  be 
said  to  be  made  up  of  two  muscular  sacs,  the  pulmonary  and 
systemic  pumps,  or,  as  they  are  commonly  termed,  the  right  and 
left  sides  of  the  heart ;  between  these  no  communication  exists 
after  birth.  Each  of  these  sacs  may  be  divided  into  two  cham- 
bers— one,  acting  as  an  ante-chamber,  receives  the  blood  from 
the  veins ;  it  has  very  thin  walls  and  is  called  the  auricle ;  the 
other,  the  ventricle,  is  the  powerful  muscular  chamber  which 
pumps  the  blood  into  and  distends  the  arteries.  (Figs.  113  and 
114.) 

In  the  empty  heart  the  great  mass  of  the  organ,  which  forms 
a  blunted  cone,  is  made  up  of  the  ventricles,  while  the  flaccid 
auricles  are  found  retracted  to  an  insignificant  size  at  its  base. 
The  four  cavities  have  the  same  capacity,  namely,  about  six 
ounces  or  eight  cubic  inches  when  distended. 

The  walls  of  both  the  auricles  are  about  the  same  thickness, 
while  the  amount  of  muscle  in  the  walls  of  the  ventricles  differs 


ARRANGEMENT    OF    MUSCLE    FIBRES. 


259 


materially.  The  wall  of  the  left  ventricle,  including  that  part 
which  forms  the  inter-ventricular  septum,  is  nearly  three  times  as 
thick  as  that  of  the  right  or  pulmonary  ventricle. 


FIG.  113. 


Interior  of  Right  Auricle  and  Ventricle  exposed  by  the  removal  of  a  part  of  their  walls. 

(Allen  Thomson.) 
1.  Superior  vena  cava.    2.  Inferior  vena  cava.    2'.  Hepatic  veins.    3,  3',  3".  Inner  wall 

of  right  auricle.    4,  4.  Cavity  of  right  ventricle.    4'.  Papillary  muscle.    5,  5',  5".  Flaps 

of  tricuspid  valve.    6.  Pulmonary  artery,  in  the  wall  of  which  a  window  has  been  cut. 

7.  On  aorta  near  the  ductus  arteriosus.    8,  9.  Aorta  and  its  branches.    10,  11.  Left 

auricle  and  ventricle. 


ARRANGEMENT  OF  MUSCLE  FIBRES. 

At  the  attachment  of  each  auricle  to  its  corresponding  ven- 
tricle there  is  situated  a  dense  ring  of  tough  connective  tissue, 


260 


MANUAL   OF   PHYSIOLOGY. 


which  surrounds  the  openings  leading  from  the  auricles  to  the 
ventricles.  Similar  tendinous  rings  (zona  tendinosa)  exist 
around  the  orifice  ol  the  aorta  and  pulmonary  arteries.  These 


FIG.  114. 


The  Left  Auricle  and  Ventricle  opened  and  part  of  their  walls  removed  to  show  their 

.,    -r,.  ,  ,       ,  cavities.    (Allen  Thomson.) 

^^l*  Short    V-uCavity  of  left  auricle.    3.  Thick  wall  of  left 
,n  °f  the  same  Wlth  papillary  muscle  attached.    5,  5'.  The  other 
seSment  of  the  mitral  valve.    7.  In  aorta  is  placed  over  the 


tendinous  rings  form  the  basis  of  attachment  for  the  muscle 
bundles  of  the  walls  of  both  the  ventricles  and  auricles. 

In  the  ventricles  many  layers  of  muscles  can  be  made  out. 


MINUTE   STRUCTURE. 


261 


FIG.  115. 


The  outer  fibres  pass  in  a  twisted  manner  from  the  base  toward 
the  apex,  where  they  are  tucked  in  so  as  to  reach  the  inner  sur- 
face of  the  ventricular  cavity.  They  then  pass  back  to  be 
attached  at  the  base ;  some  passing  into  the  papillary  muscles 
are  connected  with  the  cardiac  valves  through  the  medium  of 
the  chordae  tendinese ;  and  the  others,  forming  irregular  masses  of 
muscle  on  the  inner  surface  of  the  cavity,  pass  in  various  direc- 
tions toward  the  base,  to  be  fused  with  the  tendinous  rings 
around  the  arterial  orifices.  Another  set  of  layers  passes  trans- 
versely around  the  ventricle  lying  between  the  inner  and  outer 
sets,  and  passing  nearly  at  right  angles  to  them. 

The  muscular  fibres  forming 
the  thin  auricular  walls  have 
their  origin  from  the  zones  of 
the  auriculo-ventricular  orifices, 
and  pass  very  irregularly  around 
the  cavities.  The  outer  set  of 
fibres  have  a  transverse,  the 
inner  a  longitudinal  direction. 
Bands  of  fibres  encircle  the 
orifices  of  the  great  veins,  and 
extend  for  some  little  distance 
along  the  vessels,  particularly 
on  the  pulmonary  veins,  which 
have  thick,  circular,  muscular 
coats  after  they  leave  the  lungs. 

The  fibres  of  the  auricles  are 
not  directly  continuous  with 

those  of    the  Ventricles,  the   aU-    Striated  Muscle  Tissue  of  the  Heart,  show- 
,  ,  ,    .       ,  f,,  ing  the  trelliswork  formed  by  the  short 

riCUlar     and    ventricular     nbres       branching  cells,  with  central  nuclei. 

being  only  related  to  each  other 

by  their  points  of  origin,  viz.,  the  auriculo-ventricular  fibrous 

zones. 

MINUTE  STRUCTURE. 

The  muscle  tissue  of  the  heart  differs  both  in  structure  and 
mode  of  action  from  the  other  contractile  tissues  of  the  body. 
The  elements  are  firmly  united  with  one  another  to  form  irregular, 


262  MANUAL   OF    PHYSIOLOGY. 

close  networks,  which,  however,  can  be  broken  up  into  masses 
easily  recognizable  as  peculiar  cells.  These  cells  are  irregular 
prismoidal  blocks,  the  blunt  ends  of  which  are  often  split,  allow- 
ing connection  with  two  contiguous  cells.  They  contain  a  nucleus, 
situated  in  the  central  axis  of  the  cell.  The  cells  are  not  sur- 
rounded by  a  distinct  sheath  of  sarcolemma. 

Though  striated,  the  action  of  the  heart  muscle  is  peculiarly 
independent  of  the  higher  nervous  centres,  being  quite  invol- 
untary; it  is  characterized  by  a  definite  periodicity  and  is  inca- 
pable of  tetanus.  The  duration  of  its  contraction  is  very  long 
when  compared  with  that  of  the  skeletal  muscles,  but  is  much 
shorter  than  that  of  the  contracting  tissues  of  most  hollow 
viscera. 

VALVES. 

The  orifices  which  lead  into  and  out  of  the  ventricles  have 
peculiar  arrangements  of  their  lining  texture,  forming  valves 
which  allow  the  blood  to  pass  only  in  a  certain  direction.  These 
valves,  which  form  a  most  interesting  and  important  part  of  the 
economy  of  the  heart,  are  of  two  kinds,  each  differing  in  its 
mode  of  action.  One  prevents  the  passage  of  the  blood  from  the 
ventricles  to  the  auricles,  the  other  guards  the  openings  into  the 
great  arteries. 

The  auricula-ventricular  valves  have  a  sail-like  action.  They 
are  made  up  of  delicate  curtains  formed  of  thin  sheets  of  connec- 
tive tissue  arising  from  the  margins  of  the  auriculo-ventricular 
openings  which  form  the  fixed  attachment  of  each  of  the  curtains 
of  the  valves.  The  free  edges  and  ventricular  surfaces  of  the 
curtains  are  blended  with  the  tendinous  cords  coming  from  the 
papillary  muscles,  and  thus  give  points  of  tendinous  attachment 
to  some  of  the  bundles  of  muscle  fibres  in  the  wall  of  the  ven- 
tricle. At  the  right  auriculo-ventricular  opening  there  are  three 
chief  curtains ;  hence  it  is  called  the  "  tricuspid "  valve  (Fig. 
117,  RAV}.  The  opening  from  the  left  auricle  to  the  left  ven- 
tricle, which  is  about  one-third  smaller,  is  guarded  by  two  large 
valvular  flaps,  and  is  hence  called  the  "  bicuspid,"  or  more  com- 
monly "mitral,"  valve  (Fig.  116). 

The  aortic  and  pulmonary  valves  are  made  up  of  three  deep 


ACTION   OF   THE   VALVES. 


263 


semi-lunar  pockets  with  free  margins  looking  toward  the  vessel. 
The  convex  base  of  each  pocket  is  attached  to  the  arterial  orifice 
of  the  ventricle,  with  the  lining  membrane  of  which  it  is  contin- 
uous. 

FIG.  116. 


Portion  of  the  Wall  of  Ventricle  (d  d')  and  Aorta  (a  b  c),  showing  attachments  of  one 
flap  of  mitral  and  the  aortic  valves;  (A  and  g)  papillary  muscles;  (e,  e,  and/)  attach- 
ment of  the  tendinous  cords.  (Allen  Thomson.) 


ACTION  OF  THE  VALVES. 

Auriculo-ventricular  Valves,— The  mode  of  action  of  the  flaps 
of  the  tricuspid  and  mitral  valves  is  like  that  of  a  lateen  sail  of  a 
boat,  if  we  substitute  the  blood  stream  for  the  air  current;  the 
tendinous  cords  acting  as  the  "  sheet "  or  rope  which  restrains 
the  sail  when  filled  with  wind. 


264 


MANUAL   OF   PHYSIOLOGY. 


The  curtains  of  the  valves  may  at  first  be  considered  as  lying 
close  to  the  ventricular  wall.  As  the  ventricle  gradually 
becomes  filled,  the  flaccid  muscular  wall  is  moved  away  from  the 
valves,  which  are  held  in  the  midst  of  the  fluid  by  the  tendinous 
cords  coming  from  the  elastic  papillary  muscles.  When  the 
auricle  contracts,  a  column  of  blood  is  driven  into  the  ventricle, 
which,  though' not  distended,  is  already  filling  with  blood.  This 


LAV 


The  Orifices  of  the  Heart  seen  from  below,  the  whole  of  the  ventricles  being  cut  away, 
and  the  curtains  of  the  auriculo-ventricular  valves  drawn  down  by  threads  attached 
to  the  chordae  tendinese.  (Huxley.) 

RA  V.    Right  auriculo-ventricular  opening  surrounded  by  the  flaps  of  tricuspid. 

LA  V.    Left  auriculo-ventricular  opening  and  attached  mitral  valve. 

PA.    Pulmonary  valves  closed.  AO.  Aortic  valves  closed. 


sudden  central  inflow  gives  rise  to  lateral  back  eddies,  which  get 
behind  the  flaps  of  the  valves  and  carry  them  toward  the 
auricle.  By  the  time  the  auricle  has  emptied  itself  into  the 
ventricle,  the  flaps  of  the  valves  are  in  contact  with  each  other 
and  the  orifice  is  closed.  When  the  ventricle  begins  to  contract 
upon  its  contained  blood,  the  pressure  makes  the  valves  tense 


CARDIAC   CYCLE.  265 

and  the  fluid  bellies  out  the  sail-like  flaps  toward  the  auricles,  so 
that  their  convex  sides  come  into  still  closer  apposition  with  one 
another.  Their  free  margins  are  held  firmly  in  position  by  the 
papillary  muscles  contracting  and  tightening  the  cords.  The 
flaps  are  kept  at  much  the  same  tension  by  the  papillary  muscles 
shortening  in  proportion  as  the  ventricle  empties  itself  and  the 
cavity  diminishes  in  size.  By  this  mechanism  the  valves  are 
prevented  from  bulging  too  much  into  the  auricles,  or  allowing 
the  blood  to  pass  back  into  them. 

The  Arterial  Valves. — The  semilunar  valves  are  mere  mem- 
branous pockets,  and  have  no  tendinous  cords  attached  to  them  ; 
but  on  account  of  the  extent  of  their  convex  attachment,  when 
their  free  margin  is  made  tense  by  the  pocket  being  filled  from 
the  artery,  the  valves  can  only  pass  a  given  distance  from  the 
wall  of  the  vessel  and  are  thus  held  firmly  in  position.  The 
force  of  the  blood  leaving  the  ventricle  distends  the  vessel  and 
pushes  its  wall  away  from  the  less  elastic  valve.  When  the  force 
begins  to  diminish,  the  blood  passes  behind  the  semilunar  flaps 
and  raises  them  from  the  wall  of  the  distended  artery.  The 
moment  the  current  from  the  ventricle  has  ceased  to  flow,  the 
pockets  are  forced  back  by  the  aortic  blood  pressure  and  bulge 
into  the  lumen  of  the  vessel,  so  that  the  convex  surface  of  the 
lunated  portions  of  each  valve  is  pressed  against  corresponding 
parts  of  its  neighbors.  Their  union,  which  is  accomplished  by 
their  overlapping  to  some  extent,  forms  three  straight  radiating 
lines,  and  is  a  perfectly  impervious  barrier  to  any  backward  flow 
of  blood  (Fig.  118,  PA  and  Ao). 

CARDIAC  CYCLE. 

It  is  only  by  means  of  these  valvular  arrangements  that  the 
heart  is  enabled  to  perform  its  function  of  pumping  the  blood 
in  a  constant  direction  onward  to  empty  the  veins  and  fill  the 
arteries. 

This  pumping  is  carried  on  by  the  successive  contractions  and 
relaxations  of  the  muscular  walls  of  the  various  cavities. 

The  blood,  flowing  from  the  systemic  and  pulmonary  veins, 
passes  unopposed  into  the  right  and  left  auricles  respectively.  As 
23 


266 


MANUAL   OF   PHYSIOLOGY. 


soon  as  the  auricles  are  full  their  walls  suddenly  contract  and 
press  the  blood  into  the  right  and  left  ventricles,  upon  which  the 
ventricles  immediately  contract,  and  force  it  into  the  great 
arteries. 

The  contraction  of  each  pair  of  cavities  is  followed  by  their 
relaxation. 

The  blood  cannot  pass  back  into  the  veins  from  the  auricles 
when  they  contract,  because  the  auricular  contraction  com- 

FlG.  118. 


The  Orifices  of  the» Heart  seen  from  above,  both  the  auricles  and  the  great  vessels  being 

removed.    (Huxley.) 

PA.  Pulmonary  artery  and  its  semilunar  valves.         Ao.  Aorta  and  its  valves. 
RA  V.  Tricuspid,  and  LA  V.  Bicuspid  valves. 

mences  in  the  bundles  of  muscular  fibre  which  surround  the 
orifices  of  the  great  venous  trunks ;  and  it  cannot  flow  back  to 
the  auricles,  because,  as  has  been  seen,  the  force  of  the  blood 
current  on  its  entry  into  the  ventricles  closes  the  valves;  while  a 
backward  flow  from  the  large  arteries  is  at  once  prevented  by  the 
current  distending  the  semilunar  pockets,  and  thus  firmly  closing 
the  valves. 

When  viewed  for  the  first  time,  the  beat  of  the  heart  appears 


SYSTOLE   OF   THE   HEART. 


267 


FIG.  119. 


to  be  a  single  act,  so  rapidly  does  the  ventricular  follow  the 
auricular  beat.  More  careful  examination  shows  that  this  single 
action  is  composed  of  different  phases  of  activity  and  repose, 
which  together  make  up  the  cycle  of  the  heart  beat.  The  contrac- 
tion of  the  cavities  of  the  heart  is  called  their  systole,  the  period 
of  rest  is  called  their  diastole. 

Systole   of  the   Heart. — The   systole    of   the   corresponding 
cavities  of  both  sides  of  the  heart  is  exactly  synchronous ;  that  is 
to  say,  the  two  auricles  contract  simultaneously,  and  the  contrac- 
tion of  the  two  ventricles 
follows  immediately  that  of 
the  auricles. 

The  ventricular  systole 
follows  that  of  the  auricle 
so  closely  that  no  interval 
can  be  appreciated.  The 
rapidly  succeeding  acts  of 
auricular  and  ventricular 
systole  are  followed  by  a 
period  during  which  both 
auricles  and  ventricles  are 
in  diastole,  which  is  com- 
monly spoken  of  as  the 
passive  interval  or  pause. 

While  the  auricles  are 
contracting  the  ventricles 
are  relaxed,  and  the  relax- 
ation of  the  auricles  com- 
mences immediately  the 
ventricular  contraction  be- 
gins. 

The  entire  cycle  of  the 
heart  beat,  occupying  near- 
ly a  second  in  the  healthy 
adult,  may  be  divided  into  three  stages : — 


Curves  drawn  on  a  moving  surface  by  three 
levers,  which  are  connected  with  the  interior 

f  the  heart,  viz.  -.- 


ring  in  the  right  auricle; 

Centre  line  shows  the  pressure  changes  within 
the  right  ventricle ; 

Lower  line  shows  the  changes  of  pressure  occur- 
ring in  the  left  ventricle. 

(The  smoked  surface  is  moved  from  right  to 
left.)  (After  Chauvian.) 


Auricular  systole. 
Ventricular  systole. 
General  diastole. 


268  MANUAL   OF    PHYSIOLOGY. 

The  exact  time  occupied  by  each  phase  of  the  cycle  can  only 
be  calculated  approximately.  This  may  be  done  either  by  regis- 
tering graphically  the  motions  of  the  auricles  and  ventricles 
directly  communicated  to  levers  brought  into  contact  with  their 
surface,  or  by  recording  graphically  the  pressure  changes  which 
occur  within  the  cavities,  by  introducing  into  them  little  elastic 
sacks  filled  with  air,  whence  the  pressure  changes  are  communi- 
cated to  an  ordinary  "  tambour,"  and  registered  on  a  smoked 
surface. 

Of  the  whole  period  of  the  cycle  the  passive  interval  or  pause 
is  the  longest  and  the  most  variable,  for  in  the  ordinary  changes 
in  the  heart's  rhythm  the  pause  alone  varies.  Next  in  duration 
is  the  ventricular  systole,  while  the  shortest  is  the  auricular 
systole. 

The  following  figures  give  approximately  the  proportion  of 
time  occupied  by  each  part  of  the  cycle  in  the  case  of  a  horse, 
whose  intra-cardiac  tension  was  registered  in  the  manner  just 
referred  to  while  his  heart  beat  about  fifty  times  in  the  minute : — 

Proportion  Duration 

*  of  cycle.  in  seconds. 

Auricular  systole £  0.2" 

Ventricular  systole f  =  0.4" 

Passive  interval £  0.6" 

Or  if  we  assume  the  human  heart  to  beat  some  seventy  times 
a  minute,  each  cycle  would  occupy  about  •£$  of  a  second,  made 
up  as  follows  : — 

Auricular  systole =  TV  of  a  second. 

Ventricular  systole =  -fa        " 

Pause =  T% 

The  duration  of  the  auricular  and  ventricular  systole  varies 
little  except  under  abnormal  circumstances,  but  the  pause  is  con- 
stantly undergoing  slight  changes.  In  fact,  the  duration  of  the 
general  diastole  depends  upon  the  rate  of  the  heart  beat,  being 
less  in  proportion  as  the  heart  beats  more  quickly. 

CARDIAC  MOVEMENTS. 

If  the  thorax  of  a  recently  killed  frog  be  opened,  the  heart  can 
be  observed  beating  in  situ,  and  the  different  acts  in  the  cycle 
studied  without  difficulty. 


CARDIAC   MOVEMENTS.  269 

In  mammalians,  in  order  to  see  the  heart  in  operation,  it 
is  necessary  to  keep  up  artificial  respiration,  during  which  the 
heart  continues  to  beat  regularly,  though  the  thorax  be  opened. 
A  careful  inspection  of  the  beating  heart  shows  that  during  its 
cycle  of  action  certain  changes  take  place  in  the  shape  and  rela- 
tive position  of  its  cavities.  This  is  owing  partly  to  the  change 
in  the  amount  of  their  blood  contents  and  partly  to  the  form 
assumed  by  the  muscular  wall  when  contracting. 

During  the  passive  interval  the  auricles  are  seen  to  swell  grad- 
ually on  account  of  the  blood  flowing  into  them  from  the  veins: 
when  the  auricular  cavities  are  nearly  full,  a  contraction,  com- 
mencing in  the  great  venous  trunks  near  the  heart,  passes  with 
increasing  force  over  the  auricles  and  gives  rise  to  their  rapid 
systolic  spasm.  The  auricles  suddenly  diminish  in  size,  and  ap- 
pear to  become  pale.  When  the  blood  is  being  propelled  through 
the  auriculo-ventricular  openings,  the  flaccid  walls  of  the  ven- 
tricles appear  to  be  drawn  over  the  liquid  mass  by  the  contrac- 
tion of  the  muscular  walls  of  the  auricles  (just  as  a  stocking  is 
drawn  over  the  foot  by  the  hands),  and  the  base  of  the  ventricles 
is  thus  drawn  upward.  The  moment  the  ventricles  have  received 
their  full  charge  of  blood  from  the  auricles  they  contract,  becom- 
ing shorter  by  the  movement  of  the  base  toward  the  apex,  and 
thicker  by  their  elongated  cone  becoming  rounder.  The  great 
arteries  are  at  the  same  time  distended  with  the  blood  from  the 
ventricle  and  elongated,  their  elastic  walls  being  drawn  down 
over  the  liquid  wedge.  The  soft  elastic  tissues  are  thus  in  turn 
made  to  slide,  as  it  were,  over  the  incompressible  fluid  that  forms 
the  fulcrum,  which  the  muscular  walls  use  as  a  purchase. 

During  the  systole,  when  the  thorax  is  open,  the  ventricles 
rotate  slightly  on  their  long  axis,  so  that  the  left  comes  a  little 
forward,  and  the  apex  also  forward  and  toward  the  right.  When 
the  systole  of  the  ventricles  ceases,  they  become  flaccid  and 
flattened,  and  the  gradual  refilling  of  the  cavities  begins,  as  there 
is  nothing  to  prevent  the  blood  flowing  from  the  veins  through 
the  auricles  into  the  ventricles,  where  the  pressure,  as  in  all 
parts  of  the  thorax,  is  negative.  The  semilunar  valves  being 
closed,  the  large  arteries  grasp  firmly  the  blood,  and  by  their 


270  MANUAL   OF   PHYSIOLOGY. 

steady  resilient  pressure  force  it  on  toward  the  distal  vessels. 
During  this  pause  the  arteries  seem  to  become  shorter  and  to 
draw  the  base  of  the  heart  up  again  by  lengthening  the  flaccid 
ventricles. 

The  part  of  the  heart  which  changes  its  position  most  is  the 
line  between  the  auricles  and  ventricles,  while  the  apex  remains 
fixed  in  one  position,  only  making  a  very  slight  lateral  and  for- 
ward motion,  which  probably  does  not  take  place  within  the 
thorax.  If  a  thin  needle  with  a  straw  attached  be  made  to  enter 
the  apex  through  the  wall  of  the  chest,  the  straw  does  not  move 
in  any  definite  direction  during  the  systole,  but  simply  shakes. 

FIG.  120. 


Cardiac  Tambour,  which  can  be  strapped  on  to  chest  wall,  so  that  the  central  button 
lies  over  the  heart  beat,  and  the  pressure  may  be  regulated  by  the  screws  at  the  side. 
To  the  tube  bent  at  right  angles  is  attached  the  rubber  tube  which  connects  the  air 
cavity  with  that  of  the  writing  tambour  shown  in  Fig.  119. 

If,  on  the  other  hand,  the  needle  be  placed  in  the  base  of  the 
ventricles,  the  straw  moves  up  and  down  with  each  systole  and 
diastole. 

HEART'S  IMPULSE. 

The  heart  communicates  its  motion  to  the  chest  wall,  and  the 
movement  can  be  felt  and  seen  over  a  limited  area,  which  varies 
with  the  thinness  of  the  individual.  This  cardiac  impulse,  as  the 
stroke  is  called,  can  best  be  felt  in  the  fifth  intercostal  space,  a 
little  to  the  median  side  of  the  left  nipple.  It  is  found  to  be 
synchronous  with  the  ventricular  systole.  During  this  period — 


HEART'S  IMPULSE.  271 

ventricular  systole — the  base  of  the  ventricles  moves  downward 
and  becomes  thicker.  The  flaccid  cone  which  is  formed  by  ven- 
tricles during  diastole  is  somewhat  flattened  against  the  chest 
wall,  but  during  systole  it  becomes  rounded  and  bulges  forward, 
pushing  the  chest  wall  before  it.  This  change  in  shape  is  the 
chief  cause  of  the  cardiac  impulse. 

If  the  ventricles  be  gently  held  between  the  fingers  during 
their  systole,  a  most  striking  sensation  is  given  by  the  change  of 
shape  and  the  sudden  hardening  of  the  muscle.  The  mass  in  the 
ventricles,  from  being  quite  soft  and  compressible  during  diastole, 
suddenly  acquires  a  wooden  hardness,  owing  to  the  tightness  with 
which  the  muscle  grasps  the  fluid,  and  the  greater  firmness  of  the 
contracting  tissue. 

This  hardening  gives  the  sensation  of  a  sudden  enlargement. 
No  matter  on  what  surface  the  finger  be  placed,  the  heart  seems 
to  give  a  slight  knock  in  that  direction.  Thus,  when  grasped 
between  the  forefinger  placed  below  the  diaphragm  and  the 
thumb  on  the  antero-superior  aspect,  the  impulse  is  equally  felt 
by  each  digit. 

The  important  items  in  causing  the  impulse  are,  then,  the 
change  in  shape  of  the  ventricles  from  a  flattened  to  a  rounded 
cone,  and  their  simultaneous  hardening,  which  no  doubt  helps  to 
make  the  movement  more  distinctly  felt  through  the  wall  of  the 
chest. 

The  point  at  which  the  impulse  is  best  felt  corresponds  to  the 
anterior  surface  of  the  ventricles  at  a  considerable  distance  above 
the  apex ;  it  is  therefore  erroneous  to  call  the  impulse  the  "  apex 
beat." 

The  cardiac  impulse  is  a  valuable  measure  of  the  strength  of  the 
systole,  and  hence  is  of  great  importance  to  the  clinical  physician. 
It  may  be  registered  by  means  of  an  instrument  called  the  Car- 
diograph. Many  such  instruments  have  been  devised,  most  of 
which  work  on  the  same  principle,  and  make  a  record  on  a 
moving  surface  with  a  lever  attached  to  a  tambour,  to  which  the 
movements  of  the  chest  wall  are  transmitted  from  a  somewhat 
similar  drum  by  means  of  air  tubes.  In  using  this  plan,  so  gener- 
ally employed  by  Marey,  one  air  tambour  (Fig.  120)  is  applied 


272 


MANUAL   OF   PHYSIOLOGY. 

over  the  heart,  the  motions  of  which 
cause  a  variation  in  the  tension  of  the  air 
|  it  contains;  these  variations  are  trans- 
|  mitted  by  a  tube,  /  (Fig.  121),  to  the 
j  other  tambour  (6),  where  they  give  rise 
12  to  a  motion  in  its  flexible  surface,  to  which 
|  a  delicate  lever  is  attached  at  (a). 
'£ 

|>  HEART  SOUNDS. 

§  The  heart's  action  is  accompanied  by 
g  two  distinct  sounds,  which  can  be  heard 
g>  by  bringing  the  ear  into  firm,  direct  con- 
l  tact  with  the  prsecordial  region,  or  indi- 
j>  rectly  by  the  use  of  the  stethoscope.* 
|  One  sound  follows  the  other  quickly, 
c;  and  then  comes  a  short  pause ;  conse- 
^  quently,  they  are  spoken  of  as  the  first 
|  and  second  sounds. 

The  first  sound  is  heard  at  the  begin - 
3  ning  of  the  ventricular  systole.  It  is  a 
®  low,  soft,  prolonged  tone,  and  is  most  dis- 
o  tinctly  heard  over  the  fifth  intercostal 
~  space. 

The  second  sound  is  heard  at  the  mo- 

o 

a  ment  when  the  two  sets  of  sernilunar 
£  valves  are  closed  and  made  tense,  that 
"  is,  when  the  blood  ceases  to  escape  from 
g  the  ventricles.  It  is  a  sharp,  short  sound, 
3  and  is  best  heard  at  the  second  costal 
^  cartilage  on  the  right  side. 

The  cause  of  the  first  sound  is  not  so 
|   evident.     Possibly  several  factors  aid  in 
its   production.      The    principal    events 


*A  flexible  stethoscope  to  listen  to  one's  own  heart 
sounds  can  easily  be  made  by  fitting  the  mouthpiece  to 
one  end  of  a  piece  of  rubber  tubing  about  18  inches 
long,  and  to  the  other  end  the  bowl  of  a  wooden  pipe. 
The  bowl  is  applied  over  the  different  regions  of  the 
heart,  and  the  mouthpiece  firmly  fitted  in  the  ear. 


HEART   SOUNDS. 


273 


occurring  at  the  same  time  as  the  first  sound  may  be  enumerated 
thus : — 

1.  The  heart's  impulse. 

2.  The  rush  of  blood  into  the  arteries. 

3.  The  contraction  of  the  heart  muscle. 

4.  The  sudden  tension  of  the  ventricular  chambers  and  the 

auriculo-ventricular  valves. 

It  has  already  been  seen  that  the  heart's  impulse  is  caused  by 
a  sudden  change  in  shape  and  density  of  the  muscle,  and  not  by 
a  knock  against  the  chest.  The  first  sound  is  heard  more  clearly 
when  the  chest  wall  is  removed,  so  that  the  apex  beating  against 
the  thorax  cannot  help  to  cause  the  sound. 

The  character  of  the  sound  is  quite  unlike  that  which  could  be 
produced  by  the  passage  of  the  blood  through  the  arterial  orifices. 

The  sound  is  not  unlike  the  muscular  tone  which  accompanies 
the  continuous  (tetanic)  contraction  of  the  skeletal  muscles.  It 
corresponds  in  time  with  the  contraction  of  the  cardiac  muscle. 
In  disease  where  the  heart  muscle  is  weak,  the  sound  becomes 
faint  or  inaudible,  although  the  valves  are  made  tense  by  an  intra- 
ventricular  force  sufficient  to  overcome  the  pressure  in  the 
arteries.  Otherwise  the  circulation  would  cease.  An  abnormal 
presystolic  sound,  like  in  character  to  the  systolic  sound,  is  now 
supposed  by  some  physicians  to  be  produced  by  the  auricular 
systole  ;  but  this  cannot  depend  on  the  vibrations  of  valves. 

All  this  evidence  tends  to  show  that  the  sound  is  produced  by 
the  contraction  of  the  muscle  tissue  of  the  heart,  or,  in  short,  that 
it  depends  upon  some  sudden  physical  change  occurring  during 
the  cardiac  muscle  contraction. 

Against  the  view  that  the  muscular  tone  is  the  cause  of  the 
first  sound  is  urged  the  supposition  that  only  tetanus  causes  a 
muscle  sound,  and  a  single  contraction  is  not  accompanied  by 
any  tone.  Though  in  many  ways  it  differs  from  the  single 
contraction  of  other  muscles,  yet  the  heart  beat  is  no  doubt  a 
single  contraction.  But  the  tone  which  may  be  heard  during 
the  normal  contraction  of  skeletal  muscle  has  not  been  proved  to 
depend  on  regularly  recurrent  contractions  such  as  occur  in  the 


274  MANUAL   OF    PHYSIOLOGY. 

tetanus  produced  by  an  interrupted  current ;  and  a  kind  of  thud, 
very  like  the  first  sound  of  the  heart,  may  be  elicited  by  the 
single  stimulation  of  a  skeletal  muscle. 

On  the  other  hand,  the  auricula-ventricular  valves  are  made  tense 
at  the  beginning  of  the  sound,  and  injury  or  disease  of  these 
valves  is  said  to  be  associated  with  a  weak  or  altered  first  sound : 
this  is  often  observed  in  disease  of  the  mitral  valve.  The  blood 
is  said  by  some  to  be  necessary  for  the  production  of  the  sound, 
because  the  gentle  closure  and  immediate  subsequent  tension  of 
these  valves  have  a  share  in  causing  it. 

As  before  remarked,  the  valvular  tension  would  not  account 
for  the  presystolic  sound  occasionally  heard,  and  there  is  no 
doubt  that  the  first  sound  can  be  heard  in  an  empty  heart, 
removed  from  the  animal,  in  which  the  valves  cannot  become 
tense,  or  even  in  the  ventricles  after  they  are  separated  from 
the  valves. 

The  sound  has  been  analyzed  with  suitable  resonators,  and  two 
distinct  tones  made  out — one  high  and  short,  corresponding  to 
the  tension  of  the  valves;  the  other  long  and  low,  corresponding 
in  duration  with  the  muscle  contraction. 

The  reasons  given  for  thinking  that  the  heart  muscle  cannot 
produce  a  tone  suggest  that  the  sudden  state  of  tension  of  the 
ventricular  wall  when  tightened  over  the  blood  may  give  rise  to 
vibrations,  and  be  an  important  item  in  causing  the  first  sound. 
This  would  explain  the  faintness  of  the  sound,  both  when  the 
valves  were  injured  and  the  muscle  weak,  and  when  the  blood 
was  prevented  from  entering.  It  would  also  explain  the  presys- 
tolic sound,  which  requires  a  certain  auricular  tension  for  its 
production. 

From  the  foregoing  statements  it  would  appear  probable  that 
both  the  tension  of  the  valves  and  the  muscle  are  concerned  in 
the  production  of  the  first  sound. 

The  production  of  the  second  sound  is  more  easily  explained. 
Occurring  just  after  the  ventricle  is  emptied,  it  is  synchronous 
with  the  closure  and  sudden  tension  of  the  semilunar  valves  at 
the  aorta  and  pulmonary  orifices.  The  blood  in  the  aorta  forci- 
bly closes  the  valves  as  soon  as  the  ventricular  pressure  begins 


INNERVATION    OF    THE    HEART.  275 

to  wane.  This  sudden  motion  causes  a  vibration  of  the  valves, 
which  is  rapidly  checked  by  the  continuous  pressure  of  the  col- 
umn of  blood. 


INNERVATION  OF  THE  HEART. 

A  most  interesting  phenomenon  in  the  heart's  action,  and  one 
difficult  to  explain,  is  the  wonderful  regularity  of  its  rhythmical 
contractions  under  normal  circumstances,  and  the  extreme  deli- 
cacy of  the  nervous  mechanism  by  which  it  is  regulated. 

The  vast  majority  of  the  active  contractile  tissues  of  the  higher 
animals  is  under  the  immediate  direction  of  the  central  nervous 
system.  Thus  the  skeletal  muscles  are  connected  with  the 
cerebro-spinal  axis  by  means  of  nerves,  along  which  impulses 
pass  stimulating  the  contractile  tissue  to  action. 

Some  muscular  organs,  as  has  been  seen  in  the  pharynx, 
oesophagus,  etc.,  though  not  under  the  control  of  the  will,  are 
governed  altogether  by  the  cerebro-spinal  axis  ;  while  others,  of 
which  the  most  striking  example  is  the  heart,  have,  in  immediate 
relation  to  the  tissue,  nerve  elements  capable  of  exciting  them  to 
contraction. 

It  will  materially  help  us  in  comprehending  the  nervous 
mechanisms  of  the  heart  if  we  bear  in  mind  the  fact  that  the 
muscle  tissue  of  the  heart  of  some  animals  has — quite  independ- 
ently of  any  nervous  influences — an  inherent  tendency  to  rhyth- 
mical contraction.  This  is  shown  by  the  following  facts.  The 
heart  muscle  cannot,  under  any  circumstances,  remain  contracted 
like  a  skeletal  muscle  in  tetanus,  or  like  an  unstriated  muscle  in 
tonus,  except  when  its  tissue  is  spoiled  by  deficient  nutrition,  etc. 
The  heart  of  many  of  the  invertebrate  animals  contracts  rhyth- 
mically without  any  nerve  elements  being  found  in  it  by  the 
most  careful  microscopic  examination.  A  strip  cut  from  the 
ventricle  of  the  tortoise  can,  by  rapid  gentle  excitations,  be  made 
to  beat  with  an  automatic  rhythm  without  the  help  of  any  known 
nerve  mechanism.  The  lower  part  of  the  frog's  ventricle — which 
is  commonly  admitted  not  to  contain  any  nerves — beats  quite 
rhythmically  if  stimulated  with  a  gentle  stream  of  serum  and 
weak  salt  solution.  There  is  no  reason  to  assume  that  we  cannot 


276  MANUAL   OF    PHYSIOLOGY. 

concede  to  muscle  tissue,  as  we  do  to  nerve  cells,  the  property  of 
acting  with  an  automatic  rhythm. 

Although  the  heart  muscle  may  itself  have  this  tendency  to 
rhythmical  contraction,  there  is  no  doubt  that  in  all  vertebrate 
animals  the  rhythm  is  controlled  and  regulated  by  nerves. 
These  may  be  divided  into  an  intrinsic  and  extrinsic  set. 

INTRINSIC  NERVE  MECHANISMS. 

In  cold-blooded  animals,  such  as  a  frog  or  tortoise,  the  heart 
will  beat  for  days  after  its  removal  from  the  animal,  if  it  be  pro- 
tected from  injury  and  prevented  from  drying.  In  warm- 
blooded animals  the  tissues  lose  their  vitality  very  soon  after 
they  are  deprived  of  their  blood  supply ;  however,  spontaneous 
rhythmical  movements  can  be  seen  in  the  mammalian  heart  if 
removed  at  once  after  death.  The  hearts  of  oxen,  rapidly 
slaughtered,  give  a  few  beats  after  their  removal  from  the  thorax. 
If  a  blood  current  be  caused  to  flow  through  the  vessels  of  the 
heart  tissue  this  spontaneous  contraction  will  go  on  for  some  time, 
or  will  even  recommence  after  having  ceased. 

The  hearts  of  two  criminals  who  were  hanged  were  found  to 
continue  to  beat  for  four  and  seven  minutes  respectively  after  the 
spinal  cord  and  the  medulla  had  been  separated. 

These  facts  prove  conclusively  that  the  stimulus  which  causes 
the  heart  to  beat  rhythmically  arises  in  the  muscle  tissue  of  the 
organ  or  in  close  relation  to  it.  Upon  physiological  grounds 
alone  we  might  conclude  that  in  the  heart  tissue  of  the  vertebrata 
there  exist  nerve  elements  capable  of  sustaining  the  rhythmical 
action,  even  if  we  had  not  anatomical  proof  of  the  existence  of 
the  ganglionic  cells  with  which  we  are  familiar. 

Such  collections  of  nerve  elements  are  called  automatic  centres, 
and  are  made  up,  like  all  other  origins  of  nerve  force,  of  gan- 
glionic cells. 

Since  the  heart  of  mammalian  animals  soon  ceases  to  beat,  it 
forms  an  unsatisfactory  subject  for  experimental  inquiry.  The 
heart's  innervation  is,  therefore,  best  studied  in  a  cold-blooded 
animal,  where  also  the  mechanisms  are  probably  more  simple. 

The  frog,  being  readily  obtainable,  is  commonly  chosen. 


INTRINSIC   NERVE   MECHANISMS.  277 

After  the  cycle  of  the  heart's  beat  has  been  carefully  watched 
in  situ,  and  when  removed  from  the  animal,  if  the  apex  of  the 
ventricle  be  separated  from  the  auricles  and  sinus  venosus  and 


FIG.  122. 


Diagrammatic  Plan  of  the  Cardiac  Nerve  mechanism.  The  direction  of  the  impulses  is 
indicated  bv  the  arrows.  The  right  and  left  sides  of  the  figure  are  used  to  show  one- 
half  of  the  fibres. 

not  stimulated  in  any  way,  it  remains  motionless,  while  the 
auricles  continue  to  beat.  But  it  responds  by  an  ordinary  single 
contraction  to  short  direct  stimulus,  and  if  the  stimulus  be  kept 


278  MANUAL   OF   PHYSIOLOGY. 

up  it  beats  rhythmically.  If  the  auricles  be  removed  from  the 
ventricle  so  as  to  leave  the  line  of  union  attached  to  it,  both  con- 
tinue to  beat.  But  each  part  beats  with  a  different  rhythm,  and 
under  like  conditions  the  auricles  continue  to  beat  longer  than 
the  ventricles.  If  the  heart  be  made  into  three  zigzag  strips  by 
a  couple  of  partial  transverse  incisions,  the  rhythm  of  the  sinus 
is  carried  by  the  muscle  tissue  to  the  very  apex  (Engelmann). 

The  auricles  beat  even  when  subdivided ;  and  the  dilated 
termination  of  the  great  vein,  called  the  sinus  veuosus,  opening 
into  the  right  auricle,  when  quite  separated  from  the  rest  of  the 
heart,  continues  to  beat  longer  and  more  regularly  than  any 
other  part.  When  the  entire  heart  is  intact  this  sinus  seems  to 
be  the  starting  point  of  the  heart  beat. 

This  experimental  evidence  of  the  presence  of  nerve  centres  in 
certain  parts  of  the  heart  muscle  of  the  frog  is  supported  by  the 
results  of  anatomical  investigations,  for  the  microscope  shows  that 
there  are  many  ganglionic  cells  distributed  throughout  the  heart 
tissue,  and  that  they  are  located  just  where  we  should  expect 
from  the  above  facts.  That  is  to  say,  there  are  none  in  the 
substance  of  the  ventricles,  while  there  are  several  groups  of 
cells  scattered  around  its  base  in  the  auriculo-ventricular  groove 
(Bidder).  There  are  others  in  the  walls  of  the  auricles, 
particularly  in  the  septum,  and  the  greatest  number  are  found 
in  the  walls  of  the  sinus  venosus  (Remak). 

The  ganglia  in  the  sinus  venosus  are  most  easily  stimulated, 
and  are  probably  the  only  ones  which  habitually  act  as  auto- 
matic centres.  They  certainly  take  the  initiative  in  the  ordinary 
heart  beat,  and  regulate  the  rhythm  of  the  contraction  of  the 
auricles  and  ventricles. 

This  seems  more  than  probable  from  the  following  facts:  1. 
The  ordinary  contraction  wave  starts  from  the  sinus  venosus. 
2.  This  part  beats  longer  and  more  steadily  than  the  others  when 
separated  from  the  animal.  3.  When  cut  off  from  the  sinus  the 
beat  of  the  heart  becomes  weak,  uncertain,  and  changes  its 
rhythm.  4.  When  the  sinus  venosus  is  physiologically  separated 
by  a  ligature  from  the  auricles  and  ventricle,  both  the  latter 
cease  to  beat,  while  the  motions  of  the  sinus  continue.  If  a 


EXTRINSIC    CARDIAC    NERVES.  279 

slight  stimulus,  such  as  the  touch  of  a  needle,  be  then  applied  to 
the  auriculo-ventricular  margin,  it  gives  rise  to  a  series  of  rhyth- 
mical contractions.  Or  if  the  ventricle  be  separated  from  the 
auricles  by  incision  through  the  auriculo-ventricular  groove,  the 
former  commences  to  beat  rhythmically,  while  the  auricles  com- 
monly remain  motionless. 

These  latter  observations  (experiments  of  Stanni us)  have  been 
explained  in  various  ways,-  supposing  the  ligature  either  (1)  to 
excite  some  inhibitory  nerve  mechanism  or  (2)  cut  off  the  excit- 
ing influence  of  the  sinus.  The  most  probable  explanation  seems 
to  be  the  following.  When  cut  off  by  ligature  from  the  sinus 
venosus,  the  heart  fails  to  contract  spontaneously  because  the 
initiatory  stimulus,  which  habitually  arrived  from  the  sinus  by 
means  of  the  conducting  power  of  the  muscle  tissue,  can  no 
longer  pass  the  block  in  that  tissue.  When  the  ventricle  is  cut 
away  from  the  auricles,  the  incision  is  sufficient  stimulus  to  the 
cells  in  the  groove  to  make  them  excite  its  rhythmical  contrac- 
tions. 

Although  we  cannot  adequately  explain  the  relationships 
borne  by  the  different  sets  of  ganglia  in  the  frog's  heart  to  one 
another,  there  seems  no  doubt  that  the  following  conclusions 
may  be  accepted  as  proven,  and  are,  in  all  probability,  applicable 
to  the  hearts  of  mammals.  That  nerve  centres  exist  in  the 
muscle  tissue  of  the  heart,  some  of  which  are  capable  of  originat- 
ing stimuli  for  the  rhythmically  contracting  muscle.  That  there 
exist  other  ganglionic  groups  which  help  to  regulate  and  dis- 
tribute the  stimuli  in  sequence  throughout  the  several  cavities. 

EXTRINSIC  CARDIAC  NERVES. 

The  intrinsic  nerve  mechanism  of  the  heart  just  described  is 
under  the  immediate  control  of  the  great  nervous  centres  through 
the  medium  of  fibres  passing  from  the  medulla  oblongata  by  the 
vagus  and  sympathetic  nerves. 

Some  of  these  fibres  check  the  action  of  the  intrinsic  ganglia, 
and  cause  the  heart  to  beat  more  slowly  ;  hence  they  are  called 
inhibitory.  Others  quicken  the  beat,  and  are  called  acceleratory. 


280  MANUAL   OF   PHYSIOLOGY. 

INHIBITORY  NERVES  OP  THE  HEART. 

It  was  observed  by  Weber  (1)  that  electric  stimulation  of  the 
vagus  nerve  caused  a  slowing  of  the  heart's  rhythm,  and  if 
increased  gave  rise  to  a  standstill  of  the  heart  in  diastole  ;  (2) 
that  the  heart  beat  gradually  recommenced  soon  after  the  stim- 
ulus had  been  removed. 

On  the  other  hand  (3)  the  section  of  both  vagi  produced  an 
increase  in  the  rapidity  of  the  heart  beat,  varying  according  to 
the  kind  of  animal  experimented  upon.  Section  of  only  one 
vagus,  however,  has  not  this  effect. 

From  these  experiments  it  would  appear — 1.  That  some  fibres 

FIG.   123. 


Tracing,  showing  the  effect  of  weak  Stimulation  of  Vagus  Nerve.    Stimulus  appliid 
between  vertical  lines.    (Recording  surface  moved  from  left  to  right.) 

of  the  vagus  bear  impulses  of  a  checking  or  inhibitory  nature  to 
the  intrinsic  nerves  of  the  heart.  2.  That  these  influences  are 
constantly  in  operation,  or,  in  other  words,  the  vagi  exert  a  tonic 
inhibitory  influence  on  the  rapidity  of  the  heart  beat.  3.  The 
tonic  action  of  one  vagus  bears  inhibitory  influence  sufficient  to 
regulate  the  heart's  action.  This  tonicity  of  the  vagus  inhibition 
is  more  marked  in  dogs  and  man  than  in  rabbits,  and  is  reduced 
to  a  minimum  in  frogs,  where  section  of  the  vagi  produces  very 
little  effect  on  the  rate  of  the  beat. 

Vagus  inhibition  is  increased  by  the  following  circumstances — 
(a)  certain  psychical  phenomena,  such  as  terror,  which  may  pro- 


THE   ACCELERATOR   NERVES.  281 

duce  a  temporary  standstill ;  (b)  deficiency  of  arterial  blood  in 
the  medulla  oblongata ;  (c)  increase  of  the  blood  pressure  within 
the  cranium ;  and  (d)  reflexly  by  the  stimulation  of  many 
afferent  nerves,  particularly  those  bearing  impulses  from  the 
abdominal  viscera  to  the  medulla,  and  the  afferent  fibres  of  the 
opposite  vagus. 

The  following  drugs  affect  the  cardiac  nerve  mechanisms : 
Muscarin  produces  diastolic  standstill  of  the  heart  by  exciting  the 
local  inhibitory  ganglia  or  vagus  terminals.  Atropin  causes 
quickening  of  the  heart's  action  by  paralyzing^  the  endings  of  the 
vagus,  and  also  those  intrinsic  mechanisms  which  are  supposed 
to  have  an  inhibitory  effect.  Nieotin  produces  at  first  a  slowing 
of  the  heart  by  stimulating  the  inhibitory  tone  of  the  vagus. 
This  is  soon  followed  by  exhaustion  of  the  terminals  and  a  con- 
sequent quickening  of  the  heart  beat.  Large  doses  of  curare 
paralyze  the  inhibitory  fibres.  Digitalis  excites  the  vagus  centre 
in  the  medulla,  and  thereby  reduces  the  rapidity  of  the  heart's 
beat. 

THE  ACCELERATOR  NERVES. 

It  has  been  found  that  stimulation  of  the  cervical  portion  of 
the  spinal  cord  causes  quickening  of  the  heart  beat.  This  occurs 
even  after  the  possibility  of  increase  of  blood  pressure  has  been 
removed  by  section  of  the  splanchnic  nerves,  and  the  tonic 
inhibition  of  the  vagi  has  been  cut  off  by  their  section.  In  the 
cervical  portion  of  the  spinal  cord  nerve  channels  must  exist 
which  are  capable  of  stimulating  the  muscle  fibres  of  the  heart, 
so  as  to  cause  it  to  beat  more  quickly.  These  accelerator 
fibres  pass  from  the  cord  through  the  communicating  branches 
to  the  last  cervical  or  first  dorsal  sympathetic  ganglion,  and 
thence  to  the  heart.  Stimulation  of  the  ganglia,  or  of  the 
branches  passing  thence  to  the  heart,  quickens  its  beat.  The 
effect  of  stimulus  applied  to  these  nerves  does  not  begin  to  show 
itself  until  a  comparatively  long  time  after  it  has  been  applied, 
and  the  acceleratory  effort  continues  for  a  considerable  time  after 
the  stimulus  is  removed.  Stimulation  of  the  accelerator  fibres 
has  less  effect  than  the  inhibition  of  the  vagus,  which  follows 
stimulation  whether  the  accelerators  are  stimulated  or  not,  while 
24 


282  MANUAL   OF   PHYSIOLOGY. 

the  action  of  the  accelerators  is  suspended  so  long  as  the  vagus 
is  being  stimulated. 

An  analogy  exists  between  the  nervous  mechanism  of  the 
heart  and  that  of  the  blood  vessels  (to  be  described  in  a  future 
chapter)  which  may  help  in  their  better  comprehension.  Both 
the  heart  and  vascular  muscles  can  work  automatically ;  though 
no  ganglionic  cells  can  be  found  in  the  latter.  Both  are 
regulated  by  central  influences.  The  heart  receives  constant 
inhibitory  dilator  impulses  by  the  vagus,  and  occasional  motor 
(accelerator)  impulses  by  the  sympathetic.  The  vessels  receive 
constant  motor  (constrictor)  impulses  by  the  sympathetic  and 
occasional  inhibitory  (dilator)  impulses  from  other  nerves. 

The  motor  influences  are  supposed  to  act  by  increasing  the 
chemical  activity  of  the  tissue  (anabolic  action),  while  the 
inhibitory  impulses  lessen  the  tissue  change  (katabolic  action). 

AFFERENT   CARDIAC   NERVES. 

Besides  the  nerve  channels  bearing  impulses  to  the  heart, 
others  pass  from  the  heart  to  the  medulla,  probably  having  their 
origin  in  the  inner  lining  of  the  heart,  which  is  the  part  most 
sensitive  to  stimulus. 

These  fibres  appear  to  be  of  two  kinds,  one  of  which  (in  vagi) 
affects  the  cardio-inhibitory  centre  and  diminishes  the  pulse  rate  ; 
the  other  (depressor)  affects  the  vaso-inhibitory  centre  and 
lowers  the  blood  pressure.  Increase  of  the  intra- ventricular 
pressure  stimulates  both  these  sets  of  fibres,  and  thus  we  see  that 
over-filling  of  the  heart  from  increase  of  blood  pressure,  etc., 
causes  retardation  of  its  beat,  and  an  equilibrium  is  established 
between  the  general  blood  pressure  and  the  force  of  the  heart 
beat. 


ARTERIES.  283 


CHAPTER  XVII. 
THE   BLOOD  VESSELS. 

The  channels  which  carry  the  blood  through  the  body  form  a 
closed  system  of  elastic  tubes,  which  may  be  divided  into  three 
varieties : — 

1.  Arteries. 

2.  Capillaries. 

3.  Veins. 

The  arteries  and  veins  serve  merely  to  conduct  the  blood  to  and 
from  the  capillaries,  where  the  essential  function  of  the  blood, 
viz.,  its  chemical  interchange  with  the  tissues,  is  carried  on. 

ARTERIES. 

The  arteries  are  those  vessels  which  carry  the  blood  from  the 
heart  to  the  capillaries.  The  great  trunk  of  the  aorta,  which 
springs  from  the  left  ventricle,  gives  off  a  series  of  branches, 
which  in  turn  subdivide  more  and  more  freely  in  proportion  to 
their  distance  from  the  heart.  Arterial  twigs  of  considerable 
size  here  and  there  form  connections  with  those  of  a  neighboring 
trunk  (anastomoses) ;  but  these  unions  are  simple  junctions  of 
single  branches,  never  so  complex  as  to  be  worthy  of  the  name  of 
a  network  or  plexus,  such  as  those  seen  in  the  capillaries  or  in 
the  veins. 

The  walls  of  the  arteries  are  made  up  of  three  coats  : — 

1.  An  external  tough  layer  of  white  fibrous  tissue,  which  gives 
strength  to  the  vessels,  restricts  their  elasticity  like  the  webbing 
in  the  wall  of  rubber  water  hose,  and  also  acts  as  a  bond  of  union 
between  them  and  the  neighboring  tissues.     This  coat  (tunica 
adventitia)  carries  the  minute  vessels,  necessary  for  the  nutrition 
of  the  vessel  wall,  and  nerves. 

2.  The  middle  coat  (tunica  media)  forms  the  more  characteris- 
tic part  of  the  arterial  structure,  being  a  mixture  of  elastic  tissue 


284 


MANUAL   OF   PHYSIOLOGY. 


and  unstriated  muscle.     It  is  much  thicker  in  the  arteries  than 
in  the  veins,  where  its  special  functions  are  not  required.     It 


FIG.  124. 


FIG.  125. 


Transverse  Section  of  part  of  the  Wall  of  the  Posterior  Tibial  Artery  (man).    (SchUfer.) 

(a)  Endothelium  lining  the  vessel,  appearing  thicker  than  natural  from  the  con- 
traction of  the  outer  coats. 

(b)  The  elastic  layer  of  the  intima. 

(c)  Middle  coat  composed  of  muscle  fibres  and  elastic  tissue. 

(d)  Outer  coat  consisting  chiefly  of  white  fibrous  tissue. 

differs    somewhat  in  character  in  arteries  of  different  calibre, 
being  much  thicker  in  the  large  vessels.     This  change  occurs 

gradually  on  passing  along  the 
diminishing  branches.  In  the 
large  arteries  and  the  arterioles 
the  middle  coat  differs  essen- 
tially both  in  structure  and  in 
function,  and  in  each  class  of 
vessel  it  forms  the  most  impor- 
tant part  for  the  due  perform- 
ance of  their  respective  func- 
tions. In  the  large  vessels  it  is 
made  up  of  fibres  and  sheets  of 
elastic  tissue  woven  into  a  dense 
feltwork,  interspersed  with  a 

Portion  of  Small  Artery  from  Submucous  few  muscle  Cells.     In  the  Smallest 
Tissue  of  Mouse's  Stomach,  stained  with 

gold  chloride,  showing  the  nuclei  of  the  arteries     Or     arterioles,     On     the 

muscle   cells  (M)   passing   transversely  ,,         ,         ,    ,, 

around  the  vessel  to  form  the  middle  other  hand,  the  great  mass  oi  the 

coat,  outside  which  is  the  fibrous  tissue         'jj-i  . 

of  the  outer  coat  (F).    Around  the  vessel    middle      COat     IS     made     Up     01 

several   fine  nerve  fibrils    form  a  net-  i  n        ,1  , 

work  (N).  muscle   cells,  the   elastic   tissue 

being  but  sparsely  represented. 
Between  the  large  arteries  and  the   capillaries  every  grade  of 


CAPILLARIES.  285 

transition  may  be  found ;  the  elastic  tissue  gradually  becoming 
less  abundant  and  the  muscle  elements  relatively  more  numerous 
in  proportion  as  the  capillaries  are  approached. 

3.  The  internal  lining  (tunica  intima)  of  the  arteries  is  com- 
posed of  a  delicate,  elastic,  homogeneous  membrane  lined  with 
a  single  layer  of  endothelial  cells.  The  intima  may  be  said  to 
be  continuous  throughout  all  the  vessels  and  the  heart  cavities. 

It  is  thus  seen  that  the  large  arteries  have  extremely  elastic 

FIG.  126. 


Capillary  Network  of  a  Lobule  of  the  Liver. 

and  firm  walls,  capable  of  sustaining  considerable  pressure. 
The  smaller  the  calibre  of  the  arteries  becomes  the  more  the 
general  property  of  elasticity  and  resiliency  is  reinforced  by 
that  of  vital  contractility  due  to  the  greater  relative  number  of 
muscle  cells  contained  in  the  middle  coat. 

CAPILLARIES. 

The  frequently  branching  arterioles  finally  terminate  in  the 
capillaries,  in  which  distinct  branches  can  no  longer  be  recognized, 


286 


MANUAL   OF   PHYSIOLOGY. 


but  the  thin  canals  are  interwoven  into  a  network  of  blood 
channels,  the  meshes  of  which  are  made  up  of  vessels,  all  of 
which  have  about  the  same  calibre.  They  communicate  indefi- 
nitely with  the  capillary  meshworks  of  the  neighboring 
arterioles,  so  that  any  given  capillary  area  appears  to  be  one 
continuous  network  of  tubules,  connected  here  and  there  with 
the  similar  networks  from  distinct  arterioles,  and  thus  any  given 
capillary  area  may  be  fed  with  blood  from  several  different 
sources.  The  walls  of  the  capillaries  are  composed  of  a  single 
layer  of  elongated  endothelial  cells  (possibly  lining  an  invisible 
membrane)  cemented  edge  to  edge  to  form  a  tube.  They  are 

FIG.  127. 


Capillary  Network  of  Fat  Tissue.    (Ktein.) 

soft  and  elastic,  and  permeable  not  only  to  the  fluid  portion 
of  the  blood,  but  also,  under  certain  circumstances,  to  the 
corpuscles. 

It  is,  in  fact,  in  these  networks  that  the  essential  function  of 
the  circulation  is  carried  on,  viz.,  the  establishment  of  a  free 
interchange  between  the  tissues  and  the  blood. 

The  characters  of  the  capillary  network  vary  in  the  different 
tissues  and  organs ;  the  closeness  and  wideness  of  the  meshes 
may  be  said  to  be  in  proportion  to  the  functional  activity  or 
inactivity  of  the  organ  or  tissue  in  question,  a  greater  amount  of 
blood  being  required  in  the  parts  where  energetic  duties  are  per- 
formed. 


SECTIONAL   AREA   OF   VESSELS.  287 

VEINS. 

The  veins  arise  from  the  capillary  network,  commencing  as 
radicles  which  unite  in  a  way  corresponding  to  the  division  of 
the  arterioles,  but  they  form  wider  and  more  numerous  channels. 
They  rapidly  congregate  together  to  make  comparatively  large 
vessels,  which  frequently  intercommunicate  and  form  coarse  and 
irregular  plexuses.  The  general  arrangement  of  the  structures 
in  the  walls  of  the  veins  is  like  that  of  the  arteries ;  they  also 
have  three  coats,  the  external,  middle  and  internal;  the  tissues  of 
each  differing  but  little  from  those  of  the  arteries.  The  external 
coat  is  like  that  of  the  arteries,  but  is  not  quite  so  strong.  The 
middle  coat,  however,  in  the  large  veins,  is  easily  distinguished 
from  that  of  the  large  arteries  by  being  much  thinner,  owing  to 
the  paucity  of  yellow  elastic  tissue.  It  is  also  characterized  by 
its  relative  richness  in  muscle  fibre.  The  structure  of  the  middle 
coat  of  the  small  veins  can  be  distinguished  from  that  of  the 
arterioles  by  the  comparative  sparseness  of  the  muscle  cells  run- 
ning around  the  tubes.  The  inner  coat  of  the  veins  is  practically 
the  same  as  that  of  the  arteries. 

The  veins  are  capable  of  considerable  distention,  but,  though 
possessed  of  a  certain  degree  of  elasticity,  they  are  much  inferior 
to  the  arteries  in  resiliency. 

In  a  large  proportion  of  veins,  valve- like  folds  of  their  lining  coat 
exist,  which  prevent  the  backward  flow  of  blood  to  the  capillaries 
and  insure  its  passage  toward  the  heart.  These  valves  resemble 
in  their  general  plan  the  pocket  valves  that  protect  the  great 
arterial  orifices  of  the  heart.  They  vary  much  in  arrangement, 
there  being  commonly  two  or  sometimes  only  one  flap  or  pocket 
entering  into  the  formation  of  the  valve.  They  are  closely  set 
in  the  long  veins  of  the  extremities,  in  which  the  blood  current 
has  to  move  against  the  force  of  gravity. 

AGGREGATE  SECTIONAL  AREA  OF  THE  VESSELS. 

The  general  aggregate  diameter  of  the  different  parts  of  the 

vascular  system  varies  greatly.     The  combined  calibre  of  the 

branches  of  an  artery  exceeds  that  of  the  parent  trunk,  so  that 

the  aggregate  sectional  area  of  the  arterial  tree  increases  as  one 


288 


MANUAL   OF   PHYSIOLOGY. 


proceeds  from  the  aorta  toward  the  capillaries.  After  the 
muscular  arterioles  are  passed  the  general  diameter  of  the 
vascular  system  suddenly  increases  immensely,  and  in  the  capil- 
laries it  reaches  its  maximum,  the  aggregate  sectional  area  of 
which  is  said  to  be  several  (5  to  8)  hundred  times  as  great  as  that 
of  the  aorta. 

The  aggregate  sectional  area  of  the  veins  diminishes  as  the  tribu- 
taries unite  to  form  main  trunks,  and  reaches  its  minimum  at  the 
entrance  of  the  vena  cava  into  the  right  auricle. 

FIG.  128. 


Diagram  intended  to  give  an  idea  of  the  aggregate  sectional  area  of  the  different  parts  of 

the  vascular  system. 

(A)  Aorta.  (C)  Capillaries.  (V)  Veins. 

The  transverse  measurement  of  the  shaded  part  may  be  taken  as  the  width  of  the 
various  kinds  of  vessels,  supposing  them  fused  together. 


The  capacity  of  the  veins  is,  however,  everywhere  much  greater 
than  that  of  the  corresponding  arteries,  the  least  difference  being 
near  the  heart,  where  the  calibre  of  the  venae  cavse  is  more  than 
twice  that  of  the  aorta. 

After  this  brief  anatomical  sketch,  the  most  important  proper- 


PHYSICAL   FORCES   OF   THE   CIRCULATION.  289 

ties  of  each  part  of  the  vascular  system  may  be  summarized 
thus  : — 

1.  The  structure  of  the  walls  of  the  large  arteries  shows  them 

to  be  capable  of  sustaining  considerable  pressure,  and  of 
exerting  powerful  and  continuous  elastic  recoil  on  the 
blood. 

2.  In  the  small  arteries,  as  well  as  this  elasticity,  frequent 

variation  in  their  calibre  occurs,  dependent  on  the  con- 
traction of  their  muscular  coat  which  regulates  the 
blood  flow. 

3.  In  the  capillaries  we  find  extreme  thinness,  elasticity,  and 

permeability  of  their  wall,  which  presents  an  immense 
surface,  so  as  to  allow  free  interchange  between  the 
blood  and  the  surrounding  textures. 

4.  The  veins  have  yielding  and  distensible  walls,  capacity  to 

accommodate  a  large  quantity  of  blood,  and  valves  to 
prevent  its  backward  flow  upon  the  capillaries. 

5.  The  aggregate  sectional  area  of  the  systemic  capillaries  is 

about  three  hundred  times  that  of  the  great  veins,  and 
seven  hundred  times  that  of  the  aorta,  so  that  the 
current  of  the  blood  must  be  proportionately  slower  in 
the  capillary  network. 

PHYSICAL  FORCES  OF  THE  CIRCULATION. 

A  liquid  flows  through  a  tube  as  the  result  of  a  difference  of 
pressure  in  the  different  parts  of  the  tube.  The  liquid  moves 
from  the  part  where  the  pressure  is  higher  toward  that  where 
it  is  lower,  except  where  sudden  and  great  variations  of  calibre 
occur.* 

The  energy  of  the  flow  corresponds  with  the  amount  of  differ- 
ence in  the  pressure,  and  varies  in  proportion  to  it,  being  con- 


*  Although  in  the  whole  course  of  any  system  of  tubes  the  flow  of  liquid  must  take 
place  from  the  part  of  higher  to  that  of  lower  pressure,  yet  if  a  narrow  tube  open 
abruptly  into  one  the  diameter  of  which  for  a  short  length  is  much  greater,  the  diminu- 
tion of  velocity  in  the  wide  tube  may  cause  the  local  pressure  in  it  to  exceed  that  in  the 
narrower  tube  immediately  preceding;  so  that  the  liquid  would  be  actually  flowing,  for 
a  short  distance,  from  a  point  of  lower  to  a  point  of  higher  pressure. 

25 


R.H. 


L  H 


290  MANUAL   OF   PHYSIOLOGY. 

tinuous  so  long  as  the  pressure  is  unequal  in  different  parts,  and 
ceasing  when  it  is  equalized  throughout  the  tube. 

If  liquid  be  forcibly  pumped  into  one  extremity  of  a  long  tube, 
such  as  a  garden  hose,  a  pressure  difference  is  established,  the 

pressure  becoming  greater  at  the 
end  into  which  the  liquid  is 
pumped,  a  current  consequently 
takes  place  toward  the  open  end. 

So  }™s  aus  the  free  or  distal  end 

of  the  tube  is  quite  open  and  on 
the  same  level  as  the  rest,  no 
very  great  pressure  can  be 
brought  to  bear  on  the  walls  of 

Diagram   of  Circulation,   showing    right    the  tube,  HO  matter  how  forcibly 
(R  H)  and  left  (L  H)  hearts,  and  the  pul-    ,  ,  .  ,  , 

monary     (p)  and    systemic  (s)  sets  of    the    pumping  may   gO  On,  as   the 

liquid  easily  escapes,  and  there- 

fore flows  the  more  quickly  as  the  pumping  becomes  more  ener- 
getic. If,  however,  the  outflow  be  impeded  by  raising  the  distal 
end  of  the  tube  to  any  considerable  height,  or  by  partially  clos- 
ing the  orifice  with  a  nozzle  or  rose,  then  the  pressure  within  the 
tube  can  be  greatly  increased  by  energetic  pumping,  and  the 
tube  being  elastic  will  be  distended. 

It  can  be  further  observed  in  this  common  operation  that  the 
smaller  the  orifice  of  the  nozzle  the  greater  the  pressure  in  the  tube 
with  a  given  rate  of  working  the  pump  ;  and,  the  orifice  remain- 
ing the  same,  the  pressure  will  increase  in  proportion  as  the  pump 
is  more  energetically  worked.  Or  in  other  words,  the  pressure 
within  the  tube  will  depend  on  (a)  the  energy  used  at  the  pump, 
and  (6)  the  degree  of  impediment  offered  to  the  outflow. 

If  the  tube  be  resilient,  and  the  nozzle  have  a  small  orifice  so 
that  a  high  pressure  can  be  established  within  the  tube,  it  will 
be  found  that  the  liquid  will  flow  from  the  nozzle  in  a  continuous 
stream,  and  will  not  follow  the  jerks  communicated  by  the  pump. 
That  is  to  say,  the  interrupted  energy  of  the  pump  is  stored  up 
by  the  elastic  tube  and  converted  into  a  continuous  pressure 
exerted  on  the  fluid.  But  if  the  tube  be  quite  rigid,  or  the  ori- 
fice too  wide  to  allow  the  pressure  within  the  tube  to  be  raised 


BLOOD    PRESSURE.  291 

sufficiently  high,  then  the  fluid  will  flow  out  of  the  end  of  the 
tube  in  jets  which  correspond  with  the  strokes  of  the  pump;  i.e., 
the  outflow  will  follow  closely  the  pressure  difference  caused  by 
the  pump  at  the  point  of  inflow. 

These  simple  facts,  which  can  be  verified  experimentally  with 
an  ordinary  enema  bag,  a  yard  of  elastic  tubing,  and  a  short 
glass  tube  drawn  to  a  point,  form  the  key  to  the  most  important 
dynamic  principles  of  the  circulation. 

BLOOD  PRESSURE. 

The  cause  of  the  blood's  motion  is  simply  a  difference  in  the 
pressure  within  the  various  parts  of  the  vascular  system,  for  the 
heart  acts  as  the  pump  filling  the  tube  represented  by  the  large 
elastic  arteries,  which  can  be  more  or  less  distended,  according 
as  (1)  the  outflow  is  impeded  or  facilitated  by  the  contraction  or 
relaxation  of  the  muscular  arterioles  which  form  the  outlet,  or 
as  (2)  the  inflow  is  increased  or  diminished  by  the  greater  or  less 
activity  of  the  heart's  action. 

From  the  foregoing  facts,  and  what  has  been  said  of  the  direc- 
tion of  the  blood  current,  namely,  that  it  flows  from  the  arteries 
through  the  capillaries  into  the  veins,  it  would  appear  that  the 
pressure  in  the  arteries  exceeds  that  in  the  capillaries,  and  that 
in  the  capillaries  must  in  turn  be  greater  than  that  in  the  veins, 
the  blood  flowing  in  the  direction  in  which  the  pressure  becomes 
less. 

The  different  manner  in  which  blood  flows  from  a  cut  artery 
and  a  cut  vein  shows  that  a  great  difference  exists  in  the  pressure 
within  the  two  sets  of  vessels. 

When  a  small  artery  is  cut  and  the  orifice  directed  upward 
the  blood  spurts  out  two  or  three  feet,  in  jerks.  When  a  vein  is 
cut,  the  blood  only  trickles  gently  from  the  orifice,  the  force  of 
the  flow  depending  much  upon  the  position  of  the  part.  It  is 
well  to  remember  that  bleeding  from  a  vein  in  the  leg  or  arm 
can  be  stopped  by  placing  the  limb  in  a  position  more  elevated 
than  the  rest  of  the  body,  so  as  to  prevent  the  force  of  gravity 
from  acting  on  the  blood. 

By  means  of  a  special  form  of  gauge  (the  mercurial  manom- 


292  MANUAL   OF    PHYSIOLOGY. 

eter), — which  will  presently  be  described — the  exact  difference 
in  the  pressure  exerted  by  the  blood  against  the  vessel  walls  in 
the  different  parts  of  the  circulation  can  be  accurately  estimated", 
and  it  has  been  found  by  direct  experiment  that  the  pressure 
varies,  just  as  one  would  be  led  to  expect  from  a  consideration  of 
its  physical  relationships,  namely,  with  the  direction  and  rate  of 
the  current  and  the  varying  width  of  the  bed  in  which  it  flows. 

The  fall  in  pressure  observed  in  the  vessels  when  passing  from 
the  left  ventricle  to  the  right  auricle  is  not  even.  In  the 
arterioles  it  falls  suddenly,  and  a  great  difference  always  exists 
between  the  arterial  and  venous  pressure  (p.  299).  It  is  on 
account  of  the  permanent  high  pressure  in  the  arteries  and  com- 
paratively low  pressure  in  the  capillaries  and  veins,  that  there 
is  a  continuous  and  permanent  flow  through  the  capillaries  from 
arteries  to  veins. 

Sustentation  of  the  Arterial  Blood  Pressure. — The  fundamental 
problem  that  must  be  clearly  understood  in  studying  the 
dynamics  of  the  circulation  is  how  the  high  pressure  in  the 
arteries  is  kept  up,  or,  in  other  words,  how  the  arteries  can  exert 
so  much  pressure  on  the  blood  when  the  capillary  outflow  is  so 
wide  and  free. 

From  the  description  already  given  of  the  action  of  the  heart, 
it  appears  that  each  beat  of  the  ventricle  pumps  some  six  ounces 
of  blood  into  the  aorta.  Though  coming  to  the  left  ventricle 
from  "the  pulmonary  circulation,  the  blood  may,  on  account  of 
the  exact  cooperation  of  the  two  sides  of  the  heart,  be  regarded 
as  being  pumped  out  of  the  systemic  veins.  Thus,  as  far  as  the 
general  consideration  of  the  physical  forces  is  concerned,  the  pul- 
monary circulation  may  be  left  out  of  the  question.  This  pumping 
occurs  some  seventy  times  a  minute,  so  that  a  great  quantity  of 
blood  is  removed  from  the  veins  and  forced  into  the  arteries. 
The  ventricles  in  filling  the  arteries  have  to  work  against  consid- 
erable pressure,  and  may  be  said  to  pump  up  the  blood  from  the 
low-pressure  veins  into  the  high-pressure  arteries,  and  the  result 
of  this  work  is  the  different  pressure  in  the  two  sets  of  vessels. 
During  the  contraction  of  the  heart  the  ventricular  pressure 
exceeds  that  of  the  aorta,  while  during  the  diastole  it  falls  to  that 


BLOOD    PRESSURE.  293 

of  the  auricle  or  even  of  the  great  veins.  The  heart  then  is  the 
most  essential  agent  in  keeping  the  arteries  stretched  and  over- 
filled, and  in  emptying  the  veins. 

The  second  important  factor  in  enabling  the  high  arterial 
blood  pressure  to  be  kept  up,  is  the  resiliency  of  the  middle  coat  of 
the  arteries.  It  is  only  on  account  of  the  great  elasticity  of  their 
arterial  walls,  that  these  vessels  are  capable  of  being  so  overfilled, 
and  because  of  the  perfect  resiliency  of  the  elastic  coat,  that  they 
are  able  to  exert  such  powerful  pressure  on  the  blood  for  such  an 
unlimited  time.  If  the  arteries  were  rigid  tubes,  to  distend  them 
with  a  fluid,  itself  inelastic,  would  of  course  be  out  of  the  ques- 
tion ;  the  outflow  from  the  distal  extremity  would  only  take  place 
when  the  additional  charge  of  blood  was  injected  by  the  heart. 

With  each  contraction  the  ventricle  overcomes  arterial  pressure, 
and  further  stretches  the  elastic  artery.  The  act  of  injecting  the 
blood  into  the  aorta  only  occupies  about  one-quarter  of  each 
heart  beat.  The  semilunar  valves  bear  the  pressure  of  the  blood 
in  the  aorta  for  the  rest  of  the  time.  The  whole  force  of  the 
ventricle  is  therefore  used  up  in  causing  arterial  distention. 
During  the  greater  part  of  the  heart's  cycle,  the  arteries  are 
closed  at  their  cardiac  end  by  the  aortic  valves,  and  open  at  their 
distal  end  to  the  capillaries. 

As  the  result  of  this,  the  blood  flows  constantly  out  of  the  dis- 
tended arteries,  through  the  capillaries,  into  the  veins,  and  tends 
to  equalize  the  pressure  in  the  veins  and  arteries. 

But  why  does  not  this  constant  outflow  allow  the  pressure  in 
the  arteries  to  fall  to  the  level  of  that  in  the  veins  ?  Or,  in  other 
words,  what  is  the  impediment  offered  to  the  escape  of  the  blood 
that  thus  keeps  the  arteries  distended  ?  If  the  arteries  and 
veins  were  a  set  of  continuous  wide  tubes  of  similar  construction 
and  capacity  throughout,  it  would  be  impossible  for  the  heart  to 
empty  the  veins,  overfill  the  arteries,  and  establish  the  great 
pressure  difference  that  normally  exists.  Therefore  some  resist- 
ance equal  to  the  pressure  must  be  offered  to  the  flow  of  the 
blood  from  the  arteries  into  the  veins. 

This  resistance  is  made  up  of  several  items,  of  which  one  alone, 
namely,  the  vital  contraction  of  the  arterioles,  is  sufficient  to  keep 


294  MANUAL   OF   PHYSIOLOGY. 

up  the  arterial  pressure.  No  doubt  the  great  increase  of  surface 
over  which  the  blood  has  to  move  in  the  capillaries,  and  the 
pressure  exercised  upon  them  by  the  surrounding  elastic  tissues, 
have  influence  in  impeding  the  emptying  of  the  arteries.  But 
the  contractility  of  the  arterioles  is  the  most  important  item,  as 
may  be  seen  from  the  following  consideration.  The  resistance 
offered  by  the  capillaries  is  insignificant  when  compared  with  the 
arterial  blood  pressure,  for  the  increase  of  friction  accompanying 
their  greater  extent  of  surface  is  counterbalanced  by  the  decrease 
of  friction  dependent  upon  the  great  total  capacity  of  the  capil- 
laries in  comparison  with  that  of  the  small  arteries.  The 

PIG.  130. 


Tracing,  showing  the  effect  of  weak  Stimulation  ot  Vagus  Nerve.     Stimulus  applied 
between  vertical  lines.     (Recording  surface  moved  from  left  to  right.) 


capillary  resistance  alone  is  therefore  not  sufficient  to  restrain  the 
blood  from  rushing  into  the  veins.  This  is  seen  when  the  arteri- 
oles are  paralyzed  by  the  destruction  of  the  nervous  mechanism 
controlling  them  ;  the  blood  then  flows  readily  through  the 
capillary  network,  the  veins  become  engorged,  the  arterial  blood 
pressure  falls,  and  the  circulation  comes  to  a  standstill,  in  spite 
of  the  heart's  more  rapid  beats.  We  know  that  beyond  the  arteri- 
oles the  pressure  falls  suddenly,  and  in  the  capillary  network  it 
is  always  very  low. 

The  four  great  factors  in  keeping  up  the  arterial  blood  pres- 
sure may   be  thus  enumerated:    1,  the  heart  beat;  2,  perfect 


BLOOD   PRESSURE. 


235 


aortic  valves;  3,  the  elastic  resiliency  of  the  large  arteries;  4, 
the  resistance  offered  by  the  contraction  of  the  muscular  arteri- 
oles. 

Heart  Beat. — If  any  factor  fail,  the  mechanism  of  the  circula- 
tion is  at  once  impaired.  For  example,  the  heart's  beat  may  be 
stopped  by  the  stimulation  of  the  inhibitory  nerve  fibres  of  the 


FIG.  131. 


Mercurial  Manometer  for  measuring  and  recording  the  blood  pressure, 
(a)  Proximate  limb  of  manometer.    (6)  Union  of  two  limbs  of  manometer,    (c)  The  rod 
floating  on  mercury  and  carrying  the  writing  point,     (d)  Stop-cock  through  which 
the  sodium  bicarbonate  can  be  introduced  between  the  blood  and  mercury  of  manom- 
eter. 

vagus,  in  which  case  the  blood  pressure  rapidly  falls,  as  shown 
by  the  curve  taken  by  the  graphic  method.  Or  weakness  of  the 
heart  beat  may  arise  from  disease  (fatty  degeneration)  of  the 
muscle,  when  signs  of  low  arterial  tension  can  be  recognized  in 
the  human  subject. 

Valves. — Any  insufficiency  of  the  aortic  valves  that  permits 


296 


MANUAL   OF   PHYSIOLOGY. 


the  blood  to  flow  backward  into  the  ventricle,  allows  the  arterial 
pressure  to  fall  between  each  ventricular  systole,  and  gives  rise 
to  the  characteristic  "  pulse  of  unfilled  arteries,"  as  it  is  called 
by  the  physician. 

Elasticity  of  Arteries. — The  resiliency  of  the  arterial  coats  may 
also  be  destroyed  to  a  certain  extent  by  degeneration  of  the  tissue, 
in  which  case  the  large  arteries  become  greatly  distended,  and 
unable  to  exert  their  normal  steady  pressure  on  the  blood. 


FIG.  132. 


The  ordinary  modern  form  of  rotating  blackened  cylinder  (R),  which  is  moved  by  clock- 
work in  the  box  (A)  by  means  of  the  disc  (D)  pressing  upon  the  wheel  (»),  which  can 
be  raised  or  lowered  by  the  screw  (L),  so  as  to  come  in  contact  with  any  part  of  the 
disc  more  or  less  near  the  centre,  and  thus  rotate  at  different  rates.  The  cylinder  can 
be  raised  by  the  screw  (v),  which  is  turned  by  the  handle  (u).  (Hermann.) 

Contractile  Arterioles. — Injuries  of  the  nervous  centres  are  often 
associated  with  paralysis  of  the  muscular  arterioles  and  fall  of 
blood  pressure ;  but  the  effect  upon  the  blood  pressure  of  dilata- 
tion of  the  small  arteries  can  be  best  seen  by  experimenting  on 
the  nerves  that  control  their  contraction.  If  paralysis  or  inhibi- 
tion^of  the  vasomotor  mechanisms  be  experimentally  produced, 


MEASUREMENT   OF   BLOOD   PRESSURE.  297 

the  result  on  the  arterial  pressure  is  the  same,  a  sudden  fall, 
which  may  reach  that  of  the  atmosphere.  The  chief  opposition 
to  the  outflow  of  blood  from  the  arteries  being  removed,  they 
cease  to  be  tense,  even  though  the  ventricle  continue  to  beat  and 
pump  the  blood  into  them. 

MEASUREMENT  OF  THE  BLOOD  PRESSURE. 

The  first  attempt  at  direct  measurement  of  blood  pressure 
was  made  by  the  Rev.  Stephen  Hales  about  the  middle  of  the  last 
century,  who,  wishing  to  compare  the  motion  of  fluids  in  animals 
with  that  in  plants,  connected  a  tube  with  an  artery  of  a  living 
animal,  and  found  that  the  blood  was  ejected  with  considerable 
force,  and  that  when  the  artery  of  a  horse  was  brought  into  union 
with  a  long  upright  tube,  the  blood  reached  a  height  of  about 
three  yards. 

The  column  of  blood  is  not  now  used  as  a  measure,  because  so 
much  blood  leaving  the  vessels  tends  to  empty  them  and  to 
reduce  the  pressure  in  the  arteries ;  besides,  the  coagulation  of 
the  blood  soon  stops  the  experiment.  We  now  employ  the  mer- 
curial manometer,  \vhich  consists  of  a  column  of  mercury  in  a 
U-shaped  tube.  To  prevent  coagulation,  the  tube  between  the 
mercury  and  blood  is  filled  with  a  solution  of  sodium  carbonate, 
the  pressure  being  regulated  to  equalize  as  nearly  as  possible  that 
of  the  blood.  A  rod  is  made  to  float  upon  the  mercury,  in  the 
open  side  of  the  tube,  and  to  the  upper  extremity  of  this  a  writing 
apparatus  can  be  attached,  so  that  by  the  movements  of  the  mer- 
cury, a  graphic  record  of  the  blood  pressure  and  its  variation 
can  be  traced  on  a  regularly  moving  surface.  This  instrument, 
known  as  Ludwig's  Kymograph,  is  that  used  in  all  ordinary 
measurements  and  experiments  on  blood  pressure. 

In  order  to  overcome  the  inertia  of  the  mercurial  column, 
another  manometer  has  been  devised,  which  will  be  mentioned 
in  speaking  of  the  character  of  the  curve  (p.  302).  When  an 
experiment  of  long  duration  has  to  be  made,  a  recorder  with  a 
rolled  strip  of  paper  can  be  employed  (Fig.  133). 

The  modern  accurate  methods  of  research  have  taught  us  the 
differences  in  pressure  that  exist  in  the  various  parts  of  the 


298 


MANUAL   OF   PHYSIOLOGY. 


vascular  system.  However,  direct  measurement  can  only  be 
accomplished  in  vessels  of  such  a  size  as  to  admit  a  cannula, 
hence  the  pressure  in  the  capillaries  in  the  very  minute  arteries 
and  veins  can  only  indirectly  be  estimated.  The  pressure  in  all 
parts  of  the  vascular  system  is  subject  to  frequent  variations  to 
be  presently  mentioned,  but  this  table  may  be  useful  in  giving  a 


FIG.  133. 


Ludwig's  Kymograph  with  continuous  paper. 

The  instrument  consists  of  an  iron  table,  above  which  the  recording  surface  is  slowly 

drawn  past  the  writing  points  from  an  endless  roll  of  paper  on  the  left  by  the  motion 

I  the  cylinder  (c),  and  rolled  up  on  a  spindle  next  the  driving-wheel  on  the  right. 

The  mercurial  manometers  are  so  fixed  on  (D)  that  the  open  ends  come  in  front  of  the 

firm  roller  upon  which  the  paper  rests.  The  writing  style  can  be  seen  rising  from  these 

tubes  while  the  other  limbs  of  the  manometers  lead  through  the  stop-cocks  to  the 

tubes  which  are  in  communication  with  the  blood  vessels.    The  time  is  recorded  by 

means  of  a  pen  attached  to  the  electro-magnet  (M)    which  by  a  "breaking"  clock  is 

demagnetized  every  second.    The  moment  at  which  a  stimulus  is  applied  is  marked 

he  zero  line  by  a  key  to  which  another  pen  is  attached  near  the  time  marker. 

general  idea  of  the  average  permanent  differences  that  exist  in 
the  different  vessels  of  large  animals  and  man. 

Large  arteries  (Carotid,  Horse)  +  160  ram.  mercury. 
Medium             (Brachial,  Man)  -f  320  mm.          " 
Capillaries  of  Finger                    -j-    38  mm.  " 

Small  Veins  of  Arm  -j-      9mm.  " 

Large  Vein  of  Neck  —  1  to  — 3  mm.  " 


RECORD    OF    BLOOD    PRESSURE. 


299 


If  the  different  parts  of  the  circulation  be  represented  on  the 
base  line  H.  A.  c.  v.,  these  letters  corresponding  to  heart,  arteries, 
capillaries,  and  veins  respectively,  and  if  the  height  of  the  blood 
pressure  be  represented  on  the  vertical  line  in  mm.  Hg.,  the 
curve  h,  a,  c,  v,  would  give  about  the  relative  pressure  in  the 
various  parts  of  the  circulation.  This  shows  that  in  the  receiv- 
ing chamber  of  the  heart  the  pressure  is  negative,  while  the 


i.  134. 


Diagram  showing  the  relative  height  of  the  blood  pressure  in  the  different 

regions  of  the  vessels. 

H.  Heart,  a.  Arterioles.  v.  Small  Veins.  A.  Arteries,  c.  Capillaries,  v.  Large  Veins. 
H.  v.  being  the  zero  line  (==  atmospheric  pressure),  the  pressure  is  indicated  by  the 
height  of  the  curve.  The  numbers  on  the  left  give  the  pressure  (approximately)  in 
mm.  of  mercury. 

ventricular  pump  drives  it  to  the  height  of  the  arterial  pressure 
160  mm.  Hg.  In  the  arteries  the  pressure  is  everywhere  high, 
while  just  before  the  blood  reaches  the  capillaries  a  sudden  fall 
occurs.  The  variation  after  this  is  merely  a  gentle  descent  until 
the  large  venous  trunks  are  reached,  where  the  blood  pressure  is 
below  zero,  i.  e.,  below  the  pressure  of  the  atmosphere. 

From  a  purely  physical  point  of  view  the  ventricle  may  be 


300  MANUAL   OF   PHYSIOLOGY. 

regarded  as  pumping  the  blood  up  to  an  elevated  high-pressure 
reservoir  of  small  capacity  (the  arteries),  from  which  it  rapidly 
falls  by  numerous  outlets  into  an  expansive,  low-lying  irrigation 
basin  (the  wide  capillaries),  while  it  slowly  trickles  back  to  the 
well  (the  auricle),  which  lies  below  the  surface  pressure. 
From  this  diagram  the  following  points  can  be  gathered  : — 

1.  The  great  difference  between  the  pressure  on  the  arterial 

and  venous  sides  of  the  circulation. 

2.  The  comparatively  slight  difference  in  pressure  in  the 

different  parts  of  the  arterial  or  of  the  venous  systems 
respectively. 

3.  The  suddenness  of  the  fall  in  the  pressure  between  the 

small   arteries   and  the   capillaries,   where   the   great 
resistance  to  the  outflow  is  met  with. 

4.  In  the  large  veins  the  pressure  of  the  blood  is  habitually 

below  that  of  the  atmosphere,  only  becoming  positive 
during  forced  expirations. 

VARIATIONS  IN  THE  BLOOD  PRESSURE. 

If  the  blood  pressure  be  recorded  with  Ludwig's  Kymograph, 
a  tracing  will  be  obtained  which  shows  that  the  pressure  under- 
goes periodic  elevations  and  depressions  of  two  different  kinds. 
The  smaller  oscillations  are  found  to  correspond  with  the  heart 
beat,  the  larger  waves  have  the  same  rhythm  as  the  respiratory 
movements,  and  the  average  elevation  of  the  mercurial  column 
is  spoken  of  as  the  mean  pressure.  In  the  large  arteries  of  the 
warm-blooded  animals  this  mean  pressure  varies  with  the  size  of 
the  animal  from  90  mm.,  mercury,  to  more  than  200  mm.  In 
cold-blooded  animals  it  is  comparatively  low,  from  22  mm.  in 
the  frog  (Volkmann)  to  84  mm.  in  a  large  fish. 

The  general  mean  pressure  in  the  arteries  is  increased  by  (1), 
increased  action  of  the  heart ;  (2),  increased  contraction  of.  the 
muscular  coat  of  the  arteries ;  (3),  sudden  increase  in  the  quantity 
of  blood.  When  the  change  is  gradual,  the  vessels  adapt  them- 
selves to  the  increase. 

The  opposite  of  these  conditions  may  be  said  to  have  a  contrary 
effect. 


INFLUENCE    OF    RESPIRATION    ON    BLOOD    PRESSURE.       301 

The  character  of  the  change  in  presssure  which  accompanies 
the  heart's  systole  is  not  shown  exactly  in  the  tracing  obtained  by 
the  mercurial  manometer,  owing  to  the  sluggishness  of  the  move- 
ment of  the  mercurial  column,  which,  as  it  were,  rubs  off  the 
apices  of  the  curves.  But  with  the  spring  manometer  of  Fick,  the 
details  of  these  oscillations  are  marked.  They  are  of  course 
synchronous  with  the  arterial  pulse,  and  follow  the  variations  of 

FIG.  135. 


Blood-pressure  Curve,  drawn  by  mercurial  manometer.  0 — a;=zero  line,  y — y'  =  curve 
with  large  respiratory  waves  and  small  waves  of  heart  impulse.  A  scale  is  introduced 
to  show  height  of  pressure. 

tension,  as  will  be  described  when  treating  of  that  subject.     (See 
Figs.  136  and  137.) 

INFLUENCE  OF  RESPIRATION  ON  BLOOD  PRESSURE. 
The  explanation  of  the  respiratory  undulations  in  this  tracing 
of  the  blood  pressure  is  difficult.     Though  many  causes  have 
been  assigned,  no  single  one  appears  to  explain  adequately  all  the 


302 


MANUAL   OF   PHYSIOLOGY. 


changes  that  may  occur  in  this  phenomenon.     At  first  sight  the 
respiratory  movements  and  consequent  pressure  changes  within 


FIG.  136. 


FIG.  137. 


Fick's  Spring  Manometer. 

A  hollow  C-shaped  spring  (A),  made  of  extremely  thin  metal,  is  fixed  at  (bb),  where  its 
cavity  communicates  with  the  tube  (K).  The  top  of  the  C  is  connected  at  (a)  with  the 
writing  lever.  Any  increase  of  pressure  in  the  tube  (K)  causes  the  spring  to  expand 
and  move  the  writing  point  (G)  up  and  down. 

the  thorax  would  seem  to  give  a  simple  mechanical  explanation  of 
the  variation  in  pressure.  But  if  the 
change  occurring  in  the  intra-thoracic 
pressure  be  examined  carefully,  it  will 
be  found  not  to  correspond  exactly 
with  the  so-called  respiratory  wave  of 
the  pressure  curve  in  the  arterial 
system. 

The  amount  of  pressure  exercised 
on   the   pericardial   contents    by  the 
respiratory   movements.     It   is  slightly 


Tracing  of  blood  pressure  taken 
with  Fick's  manometer. 


lungs   varies  with  the 


INFLUENCE   OF   RESPIRATION    ON   BLOOD    PRESSURE.      303 

decreased  during  inspiration  and  increased  during  expiration. 
The  differences  thus  produced,  however,  are  during  ordinary 
respiration  very  slight  (probably  1  mm.,  mercury).  So  slight  a 
variation  as  1  mm.,  mercury,  cannot,  by  direct  action  on  the 
aortic  arch,  cause  the  change  of  several  millimetres  which  we  see 
in  the  respiratory  undulation  in  the  arterial  pressure.  We  must, 
therefore,  seek  the  explanation  in  the  changes  it  causes  in  the 
great  veins. 

Owing  to  the  lungs  being  very  elastic  and  constantly  tending 
to  shrink  away  from  the  costal  pleura,  the  pressure  in  the  pleural 
cavity  is  less  than  that  of  the  atmosphere  which  distends  the  lungs, 

FIG.  138. 


Blood  pressure  ami  Respiratory  Tracings  recorded  synchronously — recording  surface 
moving  from  right  to  left— showing  that  the  variations  in  pressure  in  the  arteries 
(continuous  line)  and  in  the  thoracic  cavity  (dotted  line)  do  not  exactly  correspond, 
the  latter  continuing  to  fall  after  the  blood  pressure  has  commenced  to  rise. 

i.  e.,  the  pleural  pressure  is  negative.  All  the  viscera  in  the 
thoracic  cavity  are  habitually  under  the  influence  of  the  negative 
pressure.  Thus  the  elastic  lungs  exert  a  kind  of  traction  on  the 
pericardium,  and  tend  to  cause  a  negative  pressure  within  the 
heart  arid  great  systemic  vessels,  both  arteries  and  veins.  The 
influence  is  more  felt  by  the  thin-walled  venae  cava3  in  which  the 
blood  pressure  is  low  than  in  the  thick-walled  arteries  where  it 
is  high. 

The  flow  of  blood  into  the  left  auricle  from  the  pulmonary 
vessels  is  not  influenced  by  the  negative  pressure,  as  pressure  of 
the  atmosphere  cannot  reach  them. 


304  MANUAL   OF   PHYSIOLOGY. 

It  has  been  suggested  that  by  facilitating  the  flow  into  the  thorax 
from  the  great  veins,  the  amount  of  blood  entering  the  right 
auricle  during  inspiration  may  be  increased,  and  thus  the  left 
ventricles  may  be  better  filled  and  made  to  beat  more  actively, 
so  as  to  cause  an  elevation  in  the  arterial  pressure. 

The  sequence  of  events  may  be  read  as  follows.  During  inspi- 
ration the  negative  pressure  on  the  right  heart  is  increased ; 
the  atmospheric  pressure  acting  on  the  tributaries  of  the  superior 
vena  cava  is  unchanged,  while  the  pressure  in  the  abdominal 
cavity  is  increased,  and  the  inferior  vena  cava  compressed  by  the 
muscular  action.  The  blood  thus  flows  more  readily  into  the 
right  heart,  and  consequently  the  lungs  receive  a  larger  supply 
of  blood  during  this  period.  In  expiration,  on  the  other  hand, 
the  intra-thoracic  pressure  becomes  less  negative,  the  compression 
of  the  abdominal  viscera  is  relieved,  and  the  flow  into  the  auricle 
loses  somewhat  in  force. 

But  this  view  appears  to  leave  the  pulmonary  circulation  out 
of  the  question  in  a  way  hardly  justifiable,  since  the  lungs  must 
be  traversed  by  the  blood  before  the  increased  inspiratory  inflow 
to  the  right  auricle  can  affect  the  left  ventricle  or  the  systemic 
arteries. 

It  must  be  carefully  borne  in  mind  that  the  left  side  of  the 
heart  works  under  different  conditions,  for  the  variations  of  pres- 
sure affect  both  the  pulmonary  veins  and  the  left  auricle  simi- 
larly, since  they  are  both  included  in  the  thoracic  cavity,  and  are 
both  subjected  to  a  slightly  varying  negative  pressure.  The  aid 
given  to  the  flow  into  the  right  heart  by  the  low  intra-thoracic 
pressure  is  quite  absent  on  the  left  side,  as  the  inflow  is  not 
assisted  by  atmospheric  pressure;  so  that  the  thoracic  movements 
do  not  exert  any  influence  on  the  flow  of  blood  from  the  pul- 
monary veins  to  the  systemic  arteries.  While  inspiration  is 
taking  place,  the  lungs  receive  a  larger  supply  of  blood.  From 
the  relatively  large  amount  of  blood  in  these  organs  it  is  pro- 
bable that  this  slight  excess  has  little  or  no  influence  on  the 
amount  entering  the  left  side  of  the  heart.  The  left  ventricle 
may  receive  an  amount  of  blood  during  expiration  slightly  in 
excess  of  that  which  it  receives  during  inspiration.  This  can 


INFLUENCE   OF    RESPIRATION    ON    BLOOD    PRESSURE.      305 

have  but  little  direct  effect  on  the  pressure  in  the  great  arterial 
trunks. 

It  is  more  than  probable  that  excess  of  blood  in  the  heart 
cavities  does  not  mechanically  influence  the  beat  or  the  blood 
pressure,  but  rather  acts  as  a  nervous  stimulus,  and  excites  the 
inhibitory  centre  of  the  heart  and  the  depressor  centres  which 
control  the  arterioles. 

The  rejection  of  this  indirect  mechanical  explanation  appears 
necessary  from  the  following  facts  : — 

1.  The  rise  in  pressure  is  not  exactly   synchronous  with 

expiration  or  inspiration. 

2.  The  heart   beats   more  slowly   during  expiration   than 

inspiration. 

3.  This  difference  at  once  disappears  if  the  vagi  be  cut  and 

the  respiratory  wave  becomes  greatly  modified. 

4.  Variations  in  the  pressure  like   the   respiratory   wave 

occur  after  the  respiratory  movements  have  quite 
ceased. 

5.  The  respiratory  wave  is  observed  when  artificial  respira- 

tion is  employed,  in  which  the  forcing  of  air  into  the 
lungs  is  the  cause,  and  not  the  result  of  the  thoracic 
movements,  so  that  the  pressure  effects  are  reversed. 

We  may  conclude  that  a  sympathy  in  action  can  be  recognized 
in  the  working  of  the  respiratory,  vascular  and  cardiac  nerve 
mechanisms. 

The  undulations  known  as  Traube's  Curves  occurring  in  cura- 
rized  animals  when  no  respiratory  movements  are  performed, 
have  been  explained  by  referring  them  to  a  stimulation  by 
impure  blood  of  the  vasomotor  centre,  which  by  rhythmical 
impulses  increases  the  contraction  of  the  arterioles  and  causes  a 
rhythmical  variation  in  the  blood  pressure.  This  explanation 
when  applied  to  the  respiratory  waves  seems  to  be  rendered 
unsatisfactory  by  the  fact  that  these  undulations  go  on  even 
when  the  arterioles  are  cut  off  from  their  chief  nerve  centres  by 
sections  of  the  spinal  cord.  So  that  if  these  undulations  are  to 
be  referred  to  nerve  mechanism  we  are  ignorant  of  the  course 
26 


306  MANUAL    OF    PHYSIOLOGY. 

taken  by  the  nerve  impulses,  for  any  rhythmical  sympathy 
existing  between  the  respiratory  and  vasomotor  nerve  centres  in 
the  medulla  cannot  well  influence  the  vessels  when  the  cord  is 
cut. 

Thus  we  seem  forced  to  fall  back  upon  the  muscular  coats  of 
the  arteries  for  an  explanation  of  the  respiratory  variation  in 
the  blood  pressure,  and  to  accord  to  this  tissue  automatic  rhyth- 
mical contractility. 

The  blood  pressure  in  the  capillaries  cannot  be  directly  measured 
by  the  means  above  described ;  it  is  difficult  to  estimate,  and 
very  variable.  The  slightest  change  of  pressure  in  the  corre- 
sponding veins  or  arteries  causes  the  pressure  in  the  capillaries 
to  rise  or  fall.  Thus,  variations  in  pressure  are  constantly 
occurring  in  the  capillaries,  which  cause  an  alteration  in  the 
rate  of  flow,  or  even  a  retrograde  stream  in  some  parts  of  the 
network. 

The  regulation  of  the  blood  supply,  and,  therefore,  of  the  pres- 
sure in  the  capillaries,  is  under  the  control  of  the  small  arterioles 
which  supply  them  ;  a  slight  relaxation  of  the  muscle  of  the 
arterioles  causes  great  increase  in  the  amount  of  blood  flowing 
through  the  capillaries,  as  can  readily  be  seen  with  the  micro- 
scope. 

The  blood  pressure  in  the  veins  must  be  less  than  that  in  the 
capillaries,  and,  as  has  been  said,  must  diminish  as  the  heart  is 
approached,  where  in  the  great  veins  (superior  cava)  the  pres- 
sure is  said  to  be  rather  below  that  of  the  atmosphere  (  —  3  to 
—  5  mm.,  mercury).  During  inspiration  the  minus  pressure  may 
become  further  lowered,  while,  on  the  other  hand,  it  is  only  by 
very  forced  expiration  that  it  ever  becomes  equal  to  or  at  all 
above  that  of  the  atmosphere. 

This  is  a  most  important  fact,  as  the  suction  considerably  helps 
the  flow  of  blood  from  the  veins,  and  also  the  current  of  fluid 
from  the  thoracic  duct  that  bears  the  chyle  from  the  intestines 
and  the  fluid  collected  from  the  tissue  drainage  back  to  the 
blood. 

The  pressure  of  the  blood  in  the  veins  may  be  said  to  be  gen- 
erally nil,  since  the  veins  are  nowhere  overfilled  with  blood. 


THE    ARTERIAL   PULSE.  307 

The  pressure,  on  the  other  hand,  that  can  be  registered  and 
measured  depends  upon  forces  communicated  from  without, 
namely:  (1)  gravity;  (2)  the  elastic  pressure  of  the  surrounding 
tissue;  and  (3)  the  pressure  exerted  by  the  muscle  during  con- 
traction. This  pressure  is  increased  by  any  circumstance  which 
impedes  the  flow  of  blood  through  the  right  side  of  the  heart, 
through  any  large  vein,  or  through  the  pulmonary  circulation  ; 
but  when  no  abnormal  obstacle  exists  in  the  venous  blood  cur- 
rent the  pressure  in  those  vessels  can  never  attain  any  great 
height,  for,  as  we  have  seen,  the  large  trunks  are  constantly  being 
emptied  by  the  heart's  action. 

Most  circumstances  which  tend  to  lower  arterial  pressure  also 
tend  to  raise  the  pressure  in  the  veins,  so  that  when  the  heart's 
action  is  weak  or  its  mechanism  faulty  the  venous  pressure 
rises. 

In  the  veins  of  the  extremities  the  pressure  greatly  depends  on 
the  position  of  the  limb,  as  it  varies  almost  directly  with  the 
effect  of  gravity. 

In  the  pulmonary  circulation  the  direct  measurement  of  the 
intra-vascular  pressure  is  rendered  extremely  difficult,  and  pos- 
sibly erroneous,  by  the  fact  that  to  ascertain  it  the  thorax  has 
to  be  opened.  It  has  been  found  in  the  pulmonary  artery  to  be 
in  a  dog  29.6  mm.,  in  a  cat  17.6  mm.,  and  in  a  rabbit  12  mm.  of 
mercury. 

THE  ARTERIAL  PULSE. 

Each  systole  of  the  ventricle  sends  a  quantity  of  blood  into 
the  aorta,  and  thus  communicates  a  stroke  to  the  blood  in  that 
vessel.  The  incompressible  fluid  causes  the  tense  arterial  wall 
to  distend  still  further,  and  the  shock  to  the  column  of  .blood  is 
not  transmitted  onward  directly  by  the  fluid,  but  causes  the  elas- 
tic walls  of  the  arteries  to  yield  locally,  and  thus  it  is  converted 
into  a  wave  which  passes  rapidly  along  those  vessels.  This 
motion  in  the  walls  of  the  vessel  can  be  felt  wherever  the  artery 
can  be  reached  by  the  finger,  but  best,  as  is  the  case  in  the 
radial  and  temporal  arteries,  where  the  vessel  is  superficial  and 
lies  on  some  unyielding  structure,  such  as  bone. 


308  MANUAL   OF   PHYSIOLOGY. 

This  motion  of  the  vessel  wall  is  called  the  arterial  pulse.  It 
consists  of  a  simultaneous  widening  and  lengthening  of  the  artery. 
The  arteries  near  the  heart  are  more  affected  by  the  pulse  wave 
than  those  more  remote,  the  wave  becoming  fainter  and  fainter 
as  it  travels  along  the  branching  arteries.  In  the  smallest  arteries 
it  is  hardly  recognizable,  and  under  ordinary  circumstances  is 
quite  absent  in  the  capillaries  and  veins. 

The  diminution  in  the  pulse  wave  in  the  smaller  arteries 
chiefly  depends  upon  the  fact  that  the  force  of  the  wave  is  used 
up  in  distending  the  successive  parts  of  the  arteries.  In  the 
small  arteries  the  extent  of  surface  to  which  the  pulse  wave  is 
communicated  is  great,  and  thereby  the  wave  is  much  decreased. 
It  is  probable  that  reflected  waves  pass  from  the  peripheral  end 
of  the  arterial  tree — the  contracted  arterioles — and  meeting  the 
pulse  wave  in  the  small  arteries  help  to  obliterate  it.  So  long  as 
the  arterioles  are  contracted  to  the  normal  degree  no  pulsation  is 
communicated  to  the  capillaries,  because  the  wave,  reaching  the 
arterioles,  is  reflected  by  them. 

The  pulse  wave  can  easily  be  shown  to  take  some  time  to  pass 
along  the  vessels.  Near  the  orifice  of  the  aorta  the  arterial 
distention  occurs  practically  at  the  same  time  as  the  ventricular 
systole,  but  even  with  comparatively  rough  methods  the  radial 
pulse  can  be  observed  to  be  a  little  later  than  the  heart  beat. 
The  difference  of  time  between  the  pulse  in  the  facial  and  the 
dorsal  artery  of  the  foot  has  been  estimated  to  be  one-sixth  of  a 
second,  and  the  difference  in  the  distance  of  these  vessels  from  the 
heart  is  about  1500  mm.,  so  that  the  rate  at  which  the  pulse  wave 
travels  is  nearly  10  metres  per  second.  The  velocity  of  the  wave 
is  said  to  depend  upon  the  degree  of  elasticity  of  the  walls  of  the 
vessels,  and  it  would  appear  to  be  quicker  in  the  lower  than  in 
the  upper  extremities. 

The  time  that  the  wave  takes  to  pass  any  given  point  must  be 
equal  to  the  time  taken  to  produce  it,  that  is  to  say,  the  time  the 
ventricle  occupies  in  sending  a  new  charge  of  blood  into  the 
aorta,  which  is  about  one-third  of  a  second.  Knowing  the  rate 
at  which  the  wave  travels  (10  m.  per  sec.)  and  the  time  it  takes 
to  pass  any  given  point  (i  sec.),  its  length  may  be  calculated  to 


PULSE   TRACINGS. 


309 


be  about  three  metres,  or  about  twice  as  long  as  the  longest 
artery.  Thus  the  pulse  wave  reaches  the  most  distant  artery 
in  one-sixth  of  a  second,  or  about  the  middle  of  the  ventricular 


FIG.  139. 


Marey's  Sphygmograph. 

The  frame  (B,  B,  B)  is  fastened  to  the  wrist  by  the  straps  at  B,  B,  and  the  rest  of  the 
instrument  lies  on  the  forearm.  The  end  of  the  screw  (v)  rests  on  the  spring  (R),  the 
button  of  which  lies  on  the  radial  artery.  Any  motion  of  the  button  at  R  is  com- 
municated to  v,  which  moves  the  lever  (L)  up  and  down.  When  in  position,  the 
blackened  slip  of  glass  (P)  is  made  to  move  evenly  by  the  clockwork  (H)  so  that  the 
writing  point  draws  a  record  of  the  movements  of  the  lever. 

systole,  and  when  the  wave  has  passed  from  the  arch  of  the  aorta, 
its  summit  has  just  reached  the  arterioles. 

Numerous  instruments  have  been  invented  for  the  demonstration 
and  graphic  representation  of  the  pulse  in  the  human  being.  Of 
these  the  one  in  general  use  is  Marey's  Sphygmograph  (Fig.  139), 

FIG.  140. 


Tracing  drawn  by  Marey's  Sphygmograph.  The  surface  moved  from  right  to  left.  The 
vertical  upstrokes  show  the  period  when  the  shock  is  given  by  the  systole  of  the 
ventricle.  The  upper  wave  on  the  downstroke  shows  when  the  blood  has  ceased  to 
enter  the  aorta.  Then  comes  the  dicrotic  depression,  which  is  a  negative  wave 
produced  by  the  momentary  backflow  in  aorta,  and  the  dicrotic  elevation  caused  by 
the  closure  of  the  valves. 


by  means  of  which  a  graphic  record  of  the  pulse  is  made,  in  the 
form  of  a  tracing  of  a  series  of  elevations  and  depressions  (Fig. 
140).  The  elevations  correspond  to  the  onset  of  a  wave,  and  the 


310  MANUAL   OF   PHYSIOLOGY. 

depressions  to  its  departure,  or  to  the  temporary  rise  and  fall  of 
the  arterial  pressure.  In  the  falling  part  of  the  curve  an  irreg- 
ularity caused  by  a  slight  second  wave  is  nearly  always  seen. 
This  is  called  the  dicrotic  wave.  Sometimes  there  are  more  than 
one  of  these  secondary  waves,  the  most  constant  of  which  is  a 
small  wave  preceding  the  dicrotic,  called  predicrotic;  but  the 
dicrotic  is  always  more  marked  than  any  other.  Several  waves 
of  oscillation  can  be  seen  as  a  gradually  decreasing  series  in 
tracings  taken  from  elastic  tubes,  but  we  cannot  say  positively 
that  they  occur  in  the  arteries.  When  several  secondary  waves 
exist  in  the  pulse  curve,  the  smaller  ones  probably  depend  on 
oscillations  caused  by  the  lever  of  the  instrument. 

The  dicrotic  wave  does  not  depend  on  the  instrument,  because 
in  most  cases  the  skilled  finger  laid  on  the  radial  artery  at  the 
wrist  can  easily  detect  it,  and  it  can  be  directly  seen  in  the  vessel 
when  the  pulsation  in  the  arteries  is  visible,  or  when  a  jet  of 
blood  escapes  from  an  artery. 

When  a  new  charge  of  blood  is  shot  into  the  aorta  the  elastic 
wall  of  the  vessel  is  suddenly  stretched.  At  the  same  time  a 
shock  is  given  to  the  column  of  blood,  and  the  fluid  next  the 
valves  is  moved  forward  with  great  velocity.  Owing  to  its 
inertia  the  fluid  tends  to  pass  onward  from  the  valves,  and  thus 
allows  a  momentary  fall  in  pressure  which  is  at  once  followed 
by  a  slight  reflux  of  the  blood  and  the  forcible  closure  of  the 
valves. 

The  first  crest  or  apex  of  the  pulse  curve  corresponds  to  the 
shock  given  by  the  systole,  and  is  greatly  exaggerated  by  the 
inertia  of  the  lever.  The  crest  of  the  predicrotic  wave  marks 
the  moment  when  the  blood  ceases  to  flow  from  the  ventricle, 
and,  therefore,  it  is  the  real  head  of  the  pulse  wave. 

The  dicrotic  wave  has  been  explained  as  (1)  a  wave  of  oscil- 
lation, (2)  a  wave  reflected  from  the  periphery,  or  (3)  a  wave 
from  the  aortic  valves. 

1.  If  the  first,  it  should  be  less  marked  than  the  predicrotic, 
which  by  this  theory  is  said  to  be  the  first  wave  of  oscillation, 
for  each  succeeding  oscillation  is  less  than  its  forerunner.  But, 
as  already  mentioned,  the  dicrotic  is  invariably  the  larger. 


PULSE   TRACINGS.  311 

2.  There  are  many  reasons  why  it  cannot  be  a  wave  of  reflec- 
tion from  the  periphery  of  the  arterial  tree ;  viz.,  (1)  Its  curve 
is  not  found  to  be  nearer  the  primary  wave  when  the  peripheral 
vessels  are  approached.      (2)    The  arterioles  which  form   the 
peripheral  resistance  are  at  too  irregular  distances  to  give  one 
definite  wave  of  reflection.     (3)  It  is  seen  in  the  spurting  of  an 
artery  cut  off  from  the  periphery.     (4)  It  increases  with  greater 
elasticity  and  low  tension,  which  cause  the  reflected  waves  to 
diminish. 

3.  The  dicrotic  notch  then  most  probably  depends  upon  a 
negative  centrifugal  wave,  caused  by  the  sudden  stoppage  of  the 
inflow  and  the  momentary  reflux  of  blood  during  the  closure  of 
the  valves ;    and  the  dicrotic  crest  is,  no  doubt,  produced  by 
the  completion  of  their  closure,  at  which  moment  the  sudden 
check  given  to  the  reflux  of  the  blood  column  causes  a  positive 
centrifugal  wave  to  follow  the  primary  wave  of  the  pulse. 

The  view  that  the  reflux  of  blood  and  the  closure  of  the  valves 
produce  the  dicrotic  wave  is  supported  by  the  fact  that  the  con- 
ditions which  increase  the  dicrotism — viz.  (1)  sharp,  strong  sys- 
tole, (2)  low  tension,  and  (3)  perfect  resiliency — promote  the 
recoil  and  closure ;  and,  on  the  other  hand,  the  conditions  which 
interfere  with  the  closure  of  the  valves  also  diminish  the  dicrotic 
wave  in  the  most  marked  degree,  viz.  (1)  inefficiency  of  the 
aortic  valves,  and  (2)  a  rigid  calcareous  condition  of  the  arteries. 

It  can  be  shown  in  an  elastic  tube,  fitted  with  a  suitable  pump 
and  sphygmographs,  that  when  its  outlet  is  closed  a  positive  wave 
is  reflected  from  the  distal  end  back  to  the  pump,  and  when  the 
outlet  is  opened  a  negative  centripetal  wave  is  reflected.  This 
fact  assists  in  explaining  the  variations  in  the  character  of  the 
pulse  curve  of  the  radial  artery  where  the  equidistance  of  the 
derived  arterioles  enables  the  reflected  waves  to  have  consider- 
able effect.  When  the  arterioles  are  constricted  (a  condition 
corresponding  to  the  closure  of  tube)  a  positive  centripetal  wave 
is  reflected,  and  is  added  to  the  pulse  wave  so  as  to  diminish  the 
dicrotic  notch,  and  give  the  curve  known  as  characteristic  of  the 
"high-tension"  pulse  seen  in  Bright's  disease.  (Fig.  141,  n.) 
On  the  other  hand,  when  the  arterioles  are  widely  dilated  (cor- 


312  MANUAL   OF    PHYSIOLOGY. 

responding  to  the  open  condition  of  the  tube)  a  negative  wave  is 
reflected,  and  is  subtracted  from  the  force  of  the  pulse  wave  so  as 
to  exaggerate  the  dicrotic  notch,  and  give  the  tracing  charac- 
teristic of  the  "  low-tension  "  pulse  seen  in  fever,  etc.  (Fig.  141, 
in.) 

The  mean  rate  of  the  pulse  varies  in  different  individuals, 
seventy-two  per  minute  being  a  fair  average  for  a  middle-aged 
adult.  It  varies  also  with  many  circumstances,  which,  though 
purely  physiological,  must  be  borne  in  mind  in  taking  the  pulse  as 
a  clinical  guide. 

1.  Age.     At  birth  it  is  about  140  per  minute,  and  is,  generally 

FIG.  141. 


I.  Scheme  of  Normal  Pulse  Curve :  a,  Entrance  of  ventricular  stream  into  the  aorta,  the 
lever  is  jerked  too  high,  reaching*;  ab  shows  real  summit  of  waves  ;  b,  point  at  which 
stream  from  ventricle  ceases ;  c,  negative  wave  caused  by  (1)  sudden  cessation  of  inflow 
and  slight  reflux  of  blood;  d,  point  of  closure  of  aortic  valves;  e,  positive  wave  from 
valves  (dicrotic  wave).    The  time  may  be  measured  on  abscissa  at  a'  b'  d'. 

II.  Scheme  of  High  Tension  Pulse  Curve  (constricted  arterioles).    A.  Curve  of  radial 
pulse,  which  is  the  resultant  of  positive  reflected  wave  c  added  to  the  primary  curve  B. 

HI.  Scheme  of  Low  Tension  Pulse  Curve  (dilated  arterioles).  A.  Radial  pulse  curve, 
which  is  the  resultant  of  the  negative  reflected  wave  c  subtracted  from  the  primary 
wave  B.  (After  Grashey.) 

speaking,  quicker  in  young  than  in  old  people,  commonly  falling 
to  60  in  aged  persons. 

2.  Sex.    It  is  more  rapid  in  females  than  in  males. 

3.  Posture.     It  is  quicker  standing  than  lying,  particularly  if 
a  patient  who  has  been  lying  down,  stand  or  sit  up,  the  pulse 
becomes  more  rapid. 

4.  The  time  of  day.     At  its  minimum  at  midnight,  it  gains  in 
rapidity  till  9  o'clock  in  the  morning ;  falls  in  the  daytime,  and 
rises  in  the  evening  till  6  o'clock. 

5.  Muscular  exercise  quickens  it. 


VELOCITY    OF   BLOOD   CURRENT.  313 

6.  It  is  quicker  during  inspiration  than  expiration. 

7.  It  increases  with  increase  of  temperature. 

8.  It  is  variously  affected  by  emotions. 

VELOCITY  OF  THE  BLOOD  CURRENT. 

The  velocity  of  the  blood  must  not  be  confounded  with  the 
velocity  of  the  pulse  wave,  which  bears  to  it  the  same  relation 
as  the  surface  waves  on  a  river  do  to  the  rate  of  the  stream  of 
water. 

It  has  already  been  mentioned  that  the  general  bed  of  the 
blood  increases  from  the  aorta  to  the  capillaries,  and  decreases 
from  the  capillaries  to  the  vena  cava.  The  branches  or 
tributaries  of  an  artery  or  vein  have  collectively  a  larger 
sectional  area  than  the  vessel  from  which  they  spring  or  to  which 
they  lead  respectively ;  or,  in  other  words,  if  we  imagined  the 
whole  vascular  system  fused  together  into  one  tube  it  would  form 
two  somewhat  irregular  cones,  one  corresponding  to  the  arteries 
and  the  other  to  the  veins,  with  their  bases  placed  at  the  capil- 
laries and  their  apices  at  the  heart.  Between  the  two  cones  a  still 
wider  portion  would  represent  the  aggregate  sectional  area  of 
the  capillaries.  (Fig.  128,  p.  288.) 

Since  the  same  quantity  of  blood  must  pass  through  each 
section  of  these  cones  in  a  given  time,  the  rate  at  which  it  flows 
must  vary  greatly  in  the  different  parts,  being  faster  in  proportion 
as  the  diameter  of  the  part  is  narrower,  in  accordance  with  the 
well-known  physical  law  that  with  the  same  quantity  of  liquid 
flowing,  its  velocity  changes  inversely  with  the  square  of  the 

diameter  of  the  tube  (V00^)-     Thus,  the  mean  velocity  of  the 

flow  in  the  arteries  becomes  slower  as  the  capillaries  are 
approached,  and  in  the  wide  bed  of  the  latter  the  rate  of  the 
current  is  reduced  to  a  minimum.  In  the  small  veins  the  rate 
is  slower  than  in  the  larger  trunks,  but  on  the  venous  side  its 
rapidity  never  reaches  that  of  the  aorta,  where  it  may  be  said  to 
move  at  least  twice  as  quickly  as  in  the  vena  cava. 

The  following  table  may  be  useful  in  giving  a  general  idea  of 
the  average  velocity  in  different  parts  of  the  circulation  : — 
27 


314  MANUAL   OF    PHYSIOLOGY. 

Near   valves  of  aorta — while  the   ventricles  are  contracting  it 
reaches  1200  mm.  per  sec. 

Descending  aorta 300-600 

Carotid 205-357 

Radial 100  mm.  per  sec. 

Metatarsal 57 

Arterioles 50 

Capillaries 0.5 

Venous  radicles 25 

Small  veins  on  dorsum  of  hand 50 

Vena3  cavse 200 

In  the  aorta  near  the  valves  the  blood  current  varies  in 
rapidity,  because  the  flow  through  the  aortic  orifice  is  inter- 
mittent, and  this  variation  must  be  more  or  less  communicated  to 
the  neighboring  arteries  in  the  form  of  an  increase  of  rapidity 
coincident  with  the  beat  of  the  arterial  pulse.  The  variation  in 
the  rate  of  the  blood  flow  which  is  caused  by  the  heart  beat 
diminishes  with  the  force  of  the  pulse  as  the  smaller  arteries  are 
approached,  and  finally  ceases  completely  in  the  capillaries, 
where  under  ordinary  circumstances  the  flow  is  perfectly 
continuous.  In  the  first  part  of  the  aorta  the  velocity  of  the 
blood  flow  is  reduced  to  nil  after  each  ventricular  beat,  while  in 
the  capillaries  no  change  is  perceived.  Between  these  two 
extremes  all  gradations  may  be  found,  which  follow  the  same 
rules  as  the  pulse. 

The  general  mean  velocity  varies  directly  with  the  blood 
pressure,  which  bears  a  generally  inverse  relation  to  the  calibre 
of  the  arteries.  The  velocity  in  any  one  artery  and  its  branches 
will  vary  with  the  calibre  of  those  vessels,  which  are  constantly 
undergoing  local  changes  in  size. 

Generally  speaking,  quick  heart  beats  cause  increase  in  velocity 
of  the  stream,  but  no  definite  or  invariable  relation  exists  between 
the  rate  of  the  heart  beat  and  the  current  of  the  blood.  The 
vasomotor  influences  have,  no  doubt,  much  more  effect  than  the 
heart  beat  on  the  rate  of  the  stream  in  the  smaller  vessels  the 
calibre  of  which  they  control. 

In  looking  at  the  blood  passing  through  the  small  vessels  of  a 
transparent  tissue,  such  as  the  frog's  tongue  or  web,  it  appears 
that  different  parts  of  the  column  of  fluid  move  with  different 
velocities.  Down  the  centre  of  the  stream  the  red  corpuscles  are 


VELOCITY    OF   BLOOD   CURRENT. 


315 


seen  coursing  rapidly,  while  between  the  central  part  and  the 
vessel  wall  on  each  side  a  pale  line  of  plasma  can  be  recognized, 


FIG.  142. 


Small  poition  of  Frog's  Web,  very  highly  magnified.    (Huxley.) 

A..  Wall  of  capillary  vessels.  B.  Tissue  lying  between  the  capillaries,  c.  Epithelial  cell 
of  skin,  only  shown  in  part  of  specimen  where  the  surface  is  in  focus.  D.  Nuclei  of 
epithelial  cells.  E.  Pigment  cells  contracted.  F.  Bed  corpuscles  (oval  in  the  frog). 
G.  H.  Red  corpuscles  squeezing  their  way  through  a  narrow  capillary,  showing  their 
elasticity,  i.  White  blood  cells. 


316  MANUAL   OF   PHYSIOLOGY. 

which  seems  to  flow  more  slowly  and  to  carry  with  it  only  a  few 
white  corpuscles. 

In  the  veins  the  velocity  varies  greatly  with  a  variety  of  cir- 
cumstances which  have  little  or  no  effect  on  the  arterial  flow. 
Thus,  the  position  of  the  body  or  limb,  the  activity  of  the  neigh- 
boring muscles  and  the  respiratory  movements  alter  it,  but  as  a 
general  rule  the  flow  in  the  veins  is  pretty  steady,  there  being 
no  pulsation  or  corresponding  variation  of  velocity.  In  the 
large  vessels  the  onward  flow  is  affected  by  the  contraction  of 
the  auricles.  During  the  auricular  systole  the  veins  cannot 
empty  themselves,  and  therefore  there  is  a  slight  check  to  the 
onward  flow,  and  the  velocity  of  the  current  is  correspondingly 
reduced.  In  cases  where  the  auricles  are  dilated  and  distended 
with  blood  this  may  cause  a  definite  pulsation,  which  becomes 
visible  in  the  great  veins  of  the  neck. 

WORK  DONE  BY  THE  HEART. 

The  amount  of  work  done  by  any  form  of  engine  may  be 
expressed  as  so  many  kilogrammetres  per  hour.  That  is  to  say, 
the  numbers  of  kilogrammes  it  could  raise  to  the  height  of  one 
metre  in  that  time. 

The  left  ventricle  moves  with  each  systole  about  0.188  (Volk- 
mann)  kilogrammes  of  fluid  against  an  arterial  pressure  corre- 
sponding to  3.21  (Donders)  metres  height  of  blood,  i.  e.,  0.188  X 
3.21  =  0.604  kilogrammetres  for  each  systole.  This  at  75  per 
minute  for  24  hours  would  be  0.604  X  75  X  60  X  24^65,230 
kilogrammetres. 

The  right  ventricle  does  about  one-third  as  much  work  as  the 
left,  making  a  total  of  86,970  kilogrammetres  for  the  ventricles. 
Or,  in  other  words,  the  heart  of  a  man  weighing  twelve  stone 
does  as  much  work  in  twenty-four  hours  as  would  be  required  to 
lift  his  body  1248  yards  into  the  air,  i.  e.,  nearly  ten  times  as 
high  as  the  steeple  of  St.  Paul's  Cathedral. 


VASOMOTOR   NERVES.  317 

CONTROLLING  MECHANISMS  OF  THE  BLOOD  VESSELS. 
VASOMOTOR  NERVES. 

That  the  arteries  possessed  elastic  resiliency  and  vital  contrac- 
tility which  regulated  the  amount  of  blood  flowing  to  any  given 
part  was  observed  by  John  Hunter  in  studying  inflammation. 

The  muscle  cells  have  long  since  been  clearly  demonstrated  in 
the  middle  coats  of  the  arteries,  but  nothing  was  known  of  the 
nervous  channels  which  bore  the  stimulus  to  the  vessels,  or  the 
nerve  centres  which  regulated  their  contraction,  until  compara- 
tively recent  times. 

The  first  definite  knowledge  concerning  special  nerves  for  the 
control  of  the  muscular  wall  of  the  vessels  is  due  to  Claude  Ber- 
nard. He  showed  that  cutting  the  sympathetic  nerve  in  the 
neck  was  followed  by  an  increase  in  temperature  of  that  side  of 
the  head,  and  a  great  dilatation  of  the  arteries. 

It  was  further  observed  that  stimulation  of  the  superior  gan- 
glion of  the  sympathetic  brought  about  an  opposite  result, 
namely,  a  fall  in  temperature  and  contraction  of  the  vessels  on 
the  side  at  which  the  stimulus  was  applied.  If  the  stimulus  was 
increased,  the  vessels  contracted  more  than  the  normal,  but  on 
cessation  of  the  stimulus  they  became  dilated  above  the  normal 
and  the  temperature  again  rose ;  the  effects  of  the  stimulus 
gradually  passed  off.  From  this  it  was  concluded  that  the  sym- 
pathetic in  the  neck  contained  constrictor  fibres  which  conveyed 
impulses  causing  habitual  tonic  contraction  of  the  vessel  wall, 
corresponding  to  what  was  already  recognized  as  arterial  tone. 
On  section  of  the  nerve  the  tonic  contraction  disappeared,  but  on 
gentle  stimulation  it  reappeared,  and  if  more  strongly  stimulated 
an  excessive  contraction  set  in  causing  occlusion  of  many  of  the 
vessels. 

Subsequent  experiments  have  shown  that  all  the  vessels  are 
supplied  with  similar  vasomotor  (constrictor)  nerves,  section  of 
which  causes  dilatation,  while  stimulation  causes  contraction 
of  the  vessels  in  the  territory  presided  over  by  the  stimulated 
nerves. 

It  has  also  been  shown  that  the  vaso- constrictor  nerves  for  all 
parts  of  the  body  come  from  the  cerebro-spinal  axis,  passing  out 


318  MANUAL   OF    PHYSIOLOGY. 

from  the  spinal  cord  as  extremely  fine  medullated  fibres  (white 
rami  communicantes)  by  the  anterior  roots  of  all  the  spinal 
nerves  between  the  2d  thoracic  and  2d  lumbar.  They  join  the 
sympathetic,  which  may  be  regarded  as  a  chain  of  vasomotor 
ganglia,  and  are  distributed  to  the  vessels  either  as  special  nerves, 
branches  of  the  sympathetic,  as  the  splanchnics,  or  with  the  gen- 
eral peripheral  nerve  trunks. 

Although  stimulation  of  almost  any  nerve  causes  vascular 
contraction,  it  has  been  shown  that  in  some  parts  stimulation 
gives  rise  to  an  opposite  result,  viz.,  vascular  dilatation.  Thus, 
stimulation  of  the  chorda  tympani  or  nervi  erigentes  is  followed 
by  dilatation  of  the  arterioles  of  the  submaxillary  gland  and 
penis  respectively.  This  dilatation  is  caused  by  the  nerve  impulses 
checking  the  normal  contraction  by  inhibiting  the  activity  of 
the  vascular  muscles.  It  is  believed  that  all  arterioles  may  be 
influenced  by  such  fibres,  but  the  greater  power  of  the  constric- 
tor fibres  in  most  nerves  prevents  their  demonstration. 

These  vaso-dilator  fibres  also  come  from  the  central  nervous 
system,  but  leave  it  by  routes  quite  different  from  those  traversed 
by  the  vaso-constrictor  fibres,  and  are  not  connected  with  the 
sympathetic  ganglia.  They  pass  out  above  by  the  vagus  and 
glosso-pharyngeal  nerves  and  below  with  the' lower  sacral  nerves. 
No  vaso-inhibitory  fibres  have  been  found  to  pass  by  the  other 
spinal  roots. 

VASOMOTOR  CENTRES. 

The  nerve  cells  which  govern  the  majority  of  the  vasomotor 
channels,  lie  in  the  upper  part  of  the  medulla  oblongata  in  the 
floor  of  the  fourth  ventricle.  This  is  proved  by  two  facts:  1st, 
most  of  the  brain  may  be  removed  without  diminishing  the 
arterial  tone;  and  2d,  if  the  spinal  cord  be  cut  below  the 
medulla  (artificial  respiration  of  course  being  kept  up),  the  mean 
blood  pressure  is  found  to  fall  immediately,  almost  to  the  level 
of  the  atmospheric  pressure,  owing  to  the  relaxation  of  the 
smaller  arteries  consequent  on  the  paralysis  of  their  muscular 
coat. 

The  changes  in  the  capillary  circulation  caused  by  vascular 
paralysis  can  be  seen  in  the  web  of  a  frog  in  which  the  medulla 


VASOMOTOR   CENTRES. 


319 


has  been  destroyed  (pithed)  while  the  circulation  is  being 
studied.  The  small  arteries  dilate  and  the  pulse  becomes  appar- 
ent in  the  capillaries,  and  even  in  the  veins. 

It  seems  probable  that  in  the  medulla  oblongata  a  vasomotor 
centre  exists,  which  can  regulate  the  contraction  of  all  the  vessels, 
and  keep  them  constantly  more  or  less  contracted.  This  slight 
general  vascular  constriction  is  spoken  of  as  the  arterial  tone. 
The  existence  of  such  a  centre  in  the  medulla,  and  of  nerve 
channels  in  the  cord  leading  from  it,  is  made  certain  by  the  fact 

FIG.  143. 


UUUUUUULJl^JUL^ 

Kymographic  tracing  showing  the  effect  on  the  blood-pressure  curve  of  stimulating  the 
central  end  of  the  depressor  nerve  in  the  rabbit.  The  recording  surface  moving  from 
left  to  right,  (c)  Commencement  and  (o)  cessation  of  stimulation.  There  is  consider- 
able delay  (latency)  in  both  the  production  and  cessation  of  the  effect.  (T)  Marks  the 
rate  at  which  the  recording  surface  moves,  and  the  line  below  is  the  base  line.  (Foster.) 

that  if  a  gentle  stimulus  be  applied  to  a  certain  part  of  the  me- 
dulla, or  just  below  it,  simultaneous  general  vascular  constriction 
sets  in,  as  indicated  by  a  great  and  sudden  rise  in  the  blood  pres- 
sure. 

Pressor  Influences. —  The  action  of  the  vasomotor  centre  can 
be  increased,  the  tone  of  the  vessels  elevated,  and  the  pressure 
raised,  either  by  (1)  direct  or  (2)  reflex  excitation.  Directly,  if 
the  blood  flowing  through  the  medulla  contains  too  little  oxygen 
or  too  much  waste  products  it  stimulates  the  centre  and  the 


320  MANUAL   OF   PHYSIOLOGY. 

blood  pressure  rises.  This  may  be  seen  by  temporarily  suspend- 
ing artificial  respiration  during  an  experiment  on  blood  pressure, 
when  the  pressure  rises  considerably.  Eeflexly,  the  activity  of 
the  vasomotor  centre  can  be  increased  by  (1)  the  stimulation  of 
any  large  sensory  nerve  or  (2)  by  sudden  emotion  (fear). 

Depressor  Influences. — The  tone  of  the  arteries  may  be  dimin- 
ished by  inhibiting  the  activity  of  the  vasomotor  centre  by  the 
stimulation  of  a  certain  afferent  nerve,  the  anatomy  of  which  has 
been  made  out  in  the  rabbit  and  some  other  animals,  and 
probably  has  its  analogue  in  man.  It  passes  from  the  inner 
surface  of  the  heart  to  the  vasomotor  centre  in  the  medulla. 
The  effect  of  stimulation  of  this  nerve  in  lowering  the  blood 
pressure  is  so  great  that  it  is  called  the  depressor  nerve.  Some 
emotions  (shame)  may  also  reduce  the  activity  of  the  centre/as 
seen  in  blushing,  which  is  simply  dilatation  of  the  facial  vessels. 

Subsidiary  Centres. — Besides  this  chief  vasomotor  centre  it  is 
probable  that  in  the  higher  animals,  as  certainly  is  the  case  in 
the  frog,  other  centres  are  distributed  throughout  the  spinal  cord 
which  are  able  to  take  the  place  of  the  great  primary  centre. 
After  the  spinal  cord  has  been  cut  high  up,  the  hinder  extremities 
more  or  less  recover  their  vasomotor  power  in  a  few  days,  and 
destruction  of  the  lower  part  of  the  spinal  cord  causes  renewed 
vasomotor  paralysis.  In  frogs  this  recovery  takes  place  rapidly, 
the  centres  being  less  confined  to  the  medulla  than  is  the  case  in 
the  more  highly  organized  animals,  but  in  the  rabbit  and  dog  it 
has  been  observed  to  occur  more  slowly. 

Besides  keeping  up  the  normal  tone,  the  arterial  nervous 
mechanisms  have  the  function  of  regulating  the  amount  of  blood 
supplied  to  various  organs  or  parts  at  different  times.  Both  vaso- 
motor and  dilator  or  inhibitory  impulses  are  probably  employed 
for  this  purpose. 

REGULATION  OF  THE  DISTRIBUTION  OF   THE  BLOOD. 
The  various  experimental  results  recently  obtained  on   this 
subject   (too   numerous  to  be  mentioned   here),  show  that  the 
vascular  nerve  mechanisms  are  very  complex.  The  supposition  of 
some  such  arrangements  as  the  following  may  help  the  student. 


REGULATION    OF   THE    DISTRIBUTION    OF   THE    BLOOD.      321 

1.  The  blood  vessels  have  muscular  elements  which,  though 
commonly  controlled  by  nerves,  are  capable  of  automatic  activity. 
A  supply  of  arterial  blood  is  sufficient  stimulus  for  their  moderate 
action,  and  mechanical    or   other  local  stimulus   is   capable  of 
exciting  increased  constriction.     We  know  that  such  automatic 
contractile  elements  exist  in  some  of  the  lower  animals  (snail's 
heart,  hydra,  etc.),  and  we  have  no  reason  to  doubt  their  existence 
in  mammals.     Moreover,  such  a  view  obviates  the  necessity  of 
supposing   that   local   nerve   elements   exist   which    cannot   be 
recognized  morphologically. 

2.  In  the  medulla  oblongata  there  exist  nerve  cells  which  exert 
a  constant  influence  over  the  activity  of  the  vascular  muscles. 
These  groups  of  nerve  cells  which  compose  the  vascular  nerve 
centres  may  be  divided  into  motor  and  inhibitory.     From  these 
centres  impulses  of  two  distinct  kinds  emanate,  the  one  increasing 
the  action  of  the  contractile  elements,  and  the  other  diminish- 
ing it.     They  are  intimately  connected  with  the  centres  which 
preside  over  the  functional  activity  of  the  various  viscera,  and 
are  also  closely  related  to  the  nerves  coming  from  all  parts  of 
the  circulatory  apparatus. 

3.  Direct  communication  between  these  vasomotor  and  vaso- 
inJiibitory  centres  and  the  blood  vessels  is  kept  up  by  means  of 
efferent  nerve  channels,  some  bearing  stimulating  (vaso-constric- 
tor)  others  inhibitory  (vaso-dilator)  impulses. 

4.  The  activity  of  the  contractile  elements  of  any  given  vas- 
cular area  may  be  altered  by  influences  from  different  sources. 
(a)  Local  influences  are  brought  but  little  into  play,  but,  if  the 
part  be  cut  off  from  the  nervous  centres,  they  are  capable  of 
controlling  the  local  blood  supply  by  changing  the  degree  of  local 
arterial  constriction.      (/?)  Central  influences  from  the  medulla 
are  habitually  in  action,  affecting  all  the  vessels  and  keeping  up 
the  vascular  tone.     These  impulses  are  variously  modified  by 
changes  occurring  in  distant  parts  of  the  circulatory  apparatus, 
and  can  be  regarded  as  a  general  regulating  mechanism.     They 
pass  through  the  sympathetic  chain.      (/)   Special  influences, 
which  are  associated  with  the  functions  of  the  different  parts  and 
organs,  are  only  called  into  operation  during  the  performance  of 


322  MANUAL   OF    PHYSIOLOGY. 

the  function,  whatever  it  may  be.  These  impulses  are  probably 
conveyed  by  the  same  nerves  as  excite  the  various  forms  of  func- 
tional activity. 

These  three  modes  of  regulation  have  different  powers  in  dif- 
ferent parts,  and  thus  we  find  that  section  or  stimulation  of  cer- 
tain nerves  gives  vasomotor  effects  which  appear  contradictory. 

Section  of  a  sensory  nerve  causes  temporary  vasomotor  pa- 
ralysis, owing  to  the  tonic  constrictor  influence  being  cut  off. 
Stimulation  of  the  peripheral  stump  causes  vaso-constriction 
from  excitation  of  the  fibres  bearing  these  impulses. 

The  stimulation  of  a  motor  nerve  causes  an  increase  in  the 
flow  of  blood  through  the  muscle,  i.  e.,  is  associated  with  a  vaso- 
dilator effect,  probably  dependent  on  the  inhibitory  influence  of 
certain  efferent  fibres  which  check  the  local  vascular  agencies. 

Thus  we  must  suppose  that  there  exist  local  agents  under  the 
control  of  the  medullary  centres,  and  that  there  are  distinct  sets 
of  efferent,  exciting  and  inhibitory  fibres  passing  between  the 
centre  and  periphery.  One  set  of  fibres  lies  in  the  ordinary 
functional  nerve  of  the  part,  the  other  in  the  sympathetic,  which 
to  a  great  extent  runs  along  the  vessels  themselves,  and  forms 
intricate  networks. 

As  far  as  we  know  anatomically  there  are  no  local  agents 
other  than  the  muscles  in  the  wall  of  the  vessels.  Since  the 
impulses  from  the  centres  which  can  stimulate  or  inhibit  the 
activity  of  the  local  agents  travel  by  different  fibres,  all  the 
observed  phenomena  may  be  explained  without  supposing  local 
nerve  centres  to  exist. 


MECHANISM   OF    RESPIRATION.  323 


CHAPTEK  XVIII. 

THE  MECHANISM  OF  RESPIRATION. 

The  blood  undergoes  a  series  of  modifications,  and  is  constantly 
being  altered  as  it  passes  from  one  part  or  organ  to  another. 

It  has  already  been  seen  that  a  quantity  of  nutrient  material 
is  taken  up  by  the  blood  on  its  way  through  the  capillaries  of  the 
alimentary  tract,  and  a  stream  of  lymph  and  chyle  is  poured 
into  it  when  it  reaches  the  great  venous  trunks ;  so  that  from  two 
sources  the  blood  is  obviously  increased  in  quantity.  The  most 
essential  change  that  takes  place  in  the  circulatory  fluid  is  the 
respiratory,  and  the  addition  it  most  urgently  demands  is  that 
which  it  receives  in  the  capillaries  of  the  lungs.  All  the  blood 
passes  through  these  organs  in  order  to  ensure  the  elimination 
of  the  carbonic  acid  acquired  in  the  general  systemic  capillaries, 
and  the  recharging  of  the  red  corpuscles  with  oxygen. 

These  gas  interchanges  will  form  the  subject  matter  of  the 
present  chapter ;  and  the  more  especial  modifications  which  the 
blood  undergoes  in  the  ductless  glands,  the  spleen,  the  liver,  etc., 
as  well  as  in  the  kidneys  and  other  excretory  glands,  will  be 
considered  subsequently. 

As  has  already  been  pointed  out  (Chapter  v),  an  animal 
during  its  life  may  be  said  to  use  the  substances  supplied  to  it  in 
food  as  fuel,  and  thus  to  acquire  the  energy  which  is  bound  up 
in  them ;  for  the  activities  of  the  various  tissues  are  really  com- 
bustions, being  invariably  associated  with  an  oxidization  of  some 
of  the  carbon  compounds,  so  as  to  produce  carbon  dioxide  and 
water.  In  order  that  the  structures  may  be  able  to  undergo  this 
change  they  must  have  a  ready  supply  of  oxygen  constantly  at 
hand,  and,  moreover,  the  carbon  dioxide  which  is  formed  in  the 
process  must  be  removed.  The  regular  income  of  oxygen  and 
the  regular  discharge  of  carbon  dioxide  are  the  first  essentials 
to  life;  hence  we  find  in  almost  all  animals  special  arrangements 
known  as  the  respiratory  apparatus,  by  means  of  which  these 


324  MANUAL   OF   PHYSIOLOGY. 

gases  can  find  their  way  to  and  from  the  tissues  and  external  air 
respectively. 

Here,  as  in  the  case  of  the  nutritive  materials,  the  blood  acts 
as  the  carrier.  The  pulmonary  half  of  the  circulation  is  devoted 
to  the  gas  interchange  between  the  blood  and  the  atmosphere, 
and  is  sometimes  spoken  of  as  external  respiration.  The  gas  inter- 
change between  the  blood  and  the  tissues  takes  place  in  the  gen- 
eral systemic  capillaries,  and  has,  therefore,  been  spoken  of  as 
the  internal  or  tissue  respiration. 

In  mammalia  the  pulmonary  apparatus  is  so  far  perfected  that 
all  the  necessary  gas  interchange  can  be  carried  on  by  the  lungs, 
and  the  respiratory  influence  of  the  external  skin  or  the  mucous 
passages  may  be  regarded  as  insignificant.  But  it  should  be 
remembered  that,  whenever  the  blood  is  in  close  relation  to  oxy- 
gen, as  in  the  case  of  swallowed  air,  the  oxygen  is  soon  absorbed 
by  the  blood. 

In  some  of  the  lower  animals  the  cutaneous  surface  aids  very 
materially  in  respiration  ;  for  example,  frogs  can  live  by  this 
cutaneous  respiration  alone  for  an  almost  indefinite  time. 

The  change  in  the  lungs  consists  in  (1)  oxygen  being  taken 
from  the  atmospheric  air*  by  the  blood  and  (2)  carbon  dioxide 
being  given  off  from  the  blood  to  the  air.  In  the  capillaries,  on 
the  other  hand,  the  blood  takes  the  carbon  dioxide  from  the  tis- 
sues, and  yields  to  them  a  great  portion  of  its  oxygen. 

KESPIRATORY  MECHANISM  IN  LOWER  ANIMALS. 

In  the  lowest  class  of  animals  (e.  g.,  amoeba),  we  find  no  special 
organs  for  the  purpose  of  respiration,  the  gas  interchange  being 
sufficiently  provided  for  by  the  exposure  of  the  general  surface 
of  their  bodies  to  the  medium  in  which  they  live,  namely,  water. 

All  higher  animals  have  some  special  apparatus  for  the  pur- 


*  The  composition  of  the  atmosphere  is  everywhere  remarkably  constant,  in  spite  of 
its  oxygen  being  used  up  by  living  beings.    It  consists  of — 

Oxygen 21     vols.  percent. 

Nitrogen 79        "        "      " 

Carbonic  acid  gas  (variable) 04    "        "      " 

Moisture  (variable) 8        "        "      " 


RESPIRATORY   MECHANISM   IN    LOWER   ANIMALS. 


325 


FIG.  144. 


pose  of  respiration.  This  apparatus  has  always  the  same  essen- 
tial object,  that  of  exposing  their  tissues  to  a  medium  containing 
oxygen,  and  of  removing  the  carbonic  acid  gas. 

In  some  of  the  invertebrate  animals  it  suffices  to  distribute  the 
medium  containing  oxygen  throughout  the  tissues  of  the  animal 
by  means  of  tubes.  Thus  in  the  Echinodermata  a  water  vascular 
system  exists  which  seems  to  carry  on  the  function  of  respiration. 
A  somewhat  similar  method  of  distribution  of  oxygen  takes  place 
in  arthropoda,  in  which  delicately  branching  open  tubes  (tra- 
cheae) distribute  air  to  the  tissues  of  the  animal's  body. 

When  more  active  changes  occur  in  the  tissues  there  is  always 
a  perfect  blood  vascular  system. 
The  blood  is  invariably  used  as  the 
distributing  and  collecting  agent  of 
the  gases  in  the  tissues,  and  by  flow- 
ing through  some  special  organ  ex- 
posed to  the  surrounding  medium  it 
ensures  the  gas  interchange  between 
the  body  and  the  outer  world. 
These  organs  are  formed  on .  two 
general  types:  (1)  external  vascular 
fringes  and  (2)  internal  vascular 
sacs. 

Animals  living  in  water  have 
commonly  the  external  fringe  ar- 
rangement (gills),  while  those  living 
in  air  have  sacs  (lungs).  Some  ani- 
mals (frogs,  toads,  etc.)  have  gills 
in  the  early  stages  of  their  life, 
and  lungs  when  they  are  more  fully 
developed.  In  frogs  and  serpents 
the  lungs  are  simple  sacs,  with  the 

inner  surface  increased  by  folds  of  the  lining  membrane,  which 
gives  it  a  honeycomb  appearance ;  into  each  sac  opens  one  of  the 
divisions  of  the  air  tube.  In  crocodiles  the  air  tubes  divide  into 
several  branches,  which  open  into  a  series  of  anfractuous,  vascular 
recesses  communicating  one  with  another. 


Diagram  of  the  Respiratory  Organs. 
The  windpipe  leading  down  from 
the  larynx  is  seen  to  branch  into 
two  large  bronchi,  which  subdi- 
vide after  they  enter  their  respec- 
tive lungs. 


326 


MANUAL   OF   PHYSIOLOGY. 


In  birds  wide  bronchial  tubes  pass  through  the  lung  tissue  to 
reach  large  air  cavities.  The  walls  of  the  tubes  are  studded  with 
the  openings  of  innumerable  air  cells  lined  with  capillary  blood 
vessels.  The  terminal  air  cavities  are  not  vascular  as  in  the 
mammalian  lung. 

STRUCTURE  OF  THE  LUNG  AND  AIR  PASSAGES. 
The  respiratory  apparatus  of  mammals  consists  of  (1)  vascu- 
lar sacs  filled  with  air,  known  as  the  lung  alveoli ;  (2)  channels 


FIG.  145. 


FIG   146. 


Muscles  of  Larynx,  viewed  from  above. 
Section  of  small  portion  of  Lung  in  which  are  Th.  Thyroid  cartilage.    Or.  Cricoid  carti- 


seen  a  bronchial  tube  with  its  plicated  lining 
mucous  membrane  in  the  centre,  and  the 
large  blood  vessels  at  the  sides  cut  across. 
Loose  areolar  tissue  and  numerous  lymphatics 
surround  the  large  vessels  and  separate  them 
from  the  lung  tissue. 


lage.  V.  Edges  of  the  vocal  cords. 
Ary.  Arytenoid  cartilages.  Th.  A. 
Thyro-arytenoid  muscle,  c.  a.  I.  Lateral 
crico-arytenoid  muscle,  c.  a.  p.  Pos- 
terior crico-arytenoid  muscle.  Ar.p. 
Posterior  arytenoid  muscle. 


by  which  these  sacs  are  ventilated — the  air  passages  ;  (3)  motor 
arrangements,  which  carry  on  the  ventilation  of  the  lungs — the 
thorax. 

1.  The  lungs  are  made  up  of  innumerable  minute  cavities 
(alveoli),  with  thin  septa  springing  from  the  inner  surface  so  as 
to  divide  the  space  into  several  compartments  or  air  cells.  Each 
of  these  cavities  forms  a  dilatation  on  the  terminal  twig  of  a 
branching  bronchus,  and  may  be  regarded  as  an  elementary 
lung.  The  aggregate  of  these  cavities,  and  the  branches  of  the 


STRUCTURE    OF   THE    LUNG   AND   AIR    PASSAGES.  327 

air  passages  and  vessels  distributed  to  them  make  up  the  struc- 
ture of  the  lung. 

The  walls  of  the  cavities  are  formed  chiefly  of  fine  elastic  fibres, 
and  the  surface  is  lined  with  exceptionally  delicate  and  thin-celled 
epithelium.  Supported  in  the  delicate  framework  of  elastic  and 
connective  tissue  is  the  remarkably  close- set  network  of  capilla- 
ries, in  which  the  blood  is  exposed  to  the  air.  The  delicate  wall 
of  the  vessel  and  the  thin  body  of  the  epithelial  lining  ceil  are 
the  only  structures  interposed  between  the  blood  and  the  air. 

Pleura. — The  external  surface  of  the  lungs  is  invested  by  a 


FIG.  147. 


Transverse  section  of  part  of  the  wall  of  a  medium-sized  bronchial  tube.    X  30. 

(F.  E.  SchuUze.) 

a.  Fibrous  layer  containing  plates  of  cartilage,  glands,  etc.  b.  Coat  composed  of  unstri- 
ated  muscle,    c.  Elastic  sub-epithelial  layer,    d.  Columnar  ciliated  epithelium. 

serous  membrane,  the  pleura,  which  is  reflected  to  the  wall  of 
the  thorax  from  the  roots  of  the  lungs,  and  completely  lines  the 
cavity  in  which  they  lie.  Thus  the  lungs  are  only  attached  to 
the  thorax  where  the  air  passages  and  great  vessels  enter,  the 
rest  of  their  surface  being  able  to  move  over  the  inner  surface 
of  the  thorax,  and  to  retract  from  the  chest  wall  if  air  be  admitted 
into  the  pleural  sac. 

2.  The  air  passages  are  kept  permanently  open  during  ordinary 
breathing  by  the  elasticity  of  their  tissues.  The  trachea  and 
bronchi  have  special  cartilaginous  springs  for  the  purpose.  These 


328 


MANUAL   OF    PHYSIOLOGY. 


are  closely  attached  to  the  fibro-elastic  tissues  which  complete  the 
general  foundation  of  the  walls  of  the  tubes.  The  air  passages 
are  throughout  lined  with  ciliated  columnar  epithelium,  which, 
at  the  entrance  to  the  infundibula,  loses  its  cilia,  and  is  converted 
into  a  single  layer  of  flattened  cells. 

The  air  passages  are  supplied  with  muscle  tissue  of  different 
kinds.  Besides  the  ordinary  striated  muscles  that  control  the 
opening  of  the  anterior  and  posterior  nares  and  pharynx,  a 
special  set  surrounds  the  upper  part  of  the  larynx,  and  is  capable 

FIG.  148. 


Section  of  a  portion  of  Lung  Tissue,  showing  part  of  a  very  small  bronchus 

cut  across.  (F.  E.  Sckultze.) 

a.  Fibrous  layer  containing  blood  vessels,    ft.  Layer  of  unstriated  muscle,    c.  Layer  of 
elastic  fibres,    d.  Ciliated  epithelium. 

of  completely  closing  the  glottis,  and  thus  shutting  off  the  lung 
cavities,  and  proper  air  passages  from  the  outer  air.  (Fig.  146.) 

In  the  trachea  a  special  muscle  exists  which  can  narrow  the 
windpipe  by  approximating  the  extremities  of  the  C-shaped 
springs  that  normally  preserve  its  patency. 

In  the  bronchial  tubes  a  large  quantity  of  smooth  muscle  cells 
exist,  for  the  most  part  arranged  as  a  circular  coat,  which  is  best 
developed  in  the  small  tubes  (Fig.  148,  b).  As  we  pass  from  the 
large  to  the  smaller  bronchi  the  walls  become  thinner  and  less 
rigid,  and  the  cartilaginous  plates  and  fibrous  tissue  gradually 


CONSTRUCTION   OF   THORAX. 


329 


FIG.  149. 


diminish,  while   on   the   other   hand  the  muscular  and  elastic 
elements  become  relatively  more  abundant. 

3.  The  thorax,  in  which  the  lungs  are  placed,  is  a  bony  frame- 
work, the  dimensions  of  which  can  be  altered  by  the  muscles 
which  close  in  and  complete  the  cavity. 

The  framework  is  a  rounded  blunt  cone,  composed  of  a  set  of 
bony  hoops,  the  ribs,  attached  by  joints  to  a  bent  pliable  pillar, 
the  vertebral  column,  and  held  together  in  front  by  the  sternum, 
to  which  they  are  attached  by  resilient  cartilaginous  springs. 
The  ribs  slope  downward  and  forward,  and 
are  more  or  less  twisted  on  themselves  about 
the  middle  of  the  shaft. 

The  first  pair  of  ribs,  which  encircles  the 
apex  of  the  thoracic  cone,  forms  part  of  a 
short  flattened  hoop.  It  slopes  downward 
in  front  to  reach  the  sternum.  Each  succeed- 
ing rib  from  above  downward  increases  in 
the  amount  of  its  slope  downward  and 
forward,  and  in  the  obliquity  of  its  shaft. 

The  floor  of  the  thorax  is  formed  by  a 
dome-shaped  muscle,  the  diaphragm,  which 
bulges  with  its  convex  side  into  the  cavity, 
and  separates  the  thoracic  from  the  abdom- 
inal viscera.  The  upper  outlet  is  closed 
around  the  trachea  by  several  muscles, 
which  pass  obliquely  upward  from  the  upper 
part  of  the  thorax  to  the  cervical  vertebrae, 
and  hold  that  part  of  the  chest  in  position. 
These  muscles  can  elevate  as  well  as  fix  the 
first  rib,  as  will  be  seen  when  speaking  of 
the  muscles  in  detail.  The  intervals  between 
the  ribs  are  filled  up  by  two  sets  of  muscular 
fibres,  which  cross  one  another  at  right  DrawiDg  of  the  lateral  view 
angles,  and  are  attached  to  the  margins  of 
the  neighboring  ribs. 

The  base  of  the  thorax  is  connected  by  a 

number  of  strong  muscles  with  the  pelvis  and  the  spine,  whence 
28 


330  MANUAL   OF   PHYSIOLOGY. 

they  pass  upward  to  the  lower  ribs.  The  anterior  muscles  pull 
down  the  sternum  and  anterior  part  of  the  ribs.  The  posterior 
fix  and  extend  the  last  rib. 

From  a  mechanical  point  of  view  the  thorax  may  be  regarded 
as  a  specially  arranged  bellows,  the  dimensions  of  which  can  be 
increased  in  all  directions. 

Within  this  bellows  are  the  lungs,  which  may  be  regarded  as 
an  elastic  bag,  the  interior  of  which  communicates  with  the  outer 
air  by  an  air-pipe,  the  only  way  by  which  the  atmosphere  can 
reach  the  interior  of  the  bellows.  When  the  framework  enlarges, 
the  pressure  of  the  atmosphere  forces  a  stream  of  air  into  the 
elastic  sac,  so  as  to  distend  it,  and  thus  fill  the  space  caused  by 
the  expansion  of  the  framework. 

By  the  motions  of  the  framework  a  stream  of  air  passes  in  or 
out  of  the  sac;  a  small  quantity  of  the  air  in  the  bronchi  is  thus 
changed  at  each  breath,  and  a  certain  standard  of  purity  kept 
up. 

In  order  to  fully  understand  the  motions  by  which  the  thorax 
is  enlarged,  a  more  detailed  knowledge  of  the  anatomy  of  the 
bony  case  and  its  muscles  than  can  be  given  here  must  be 
acquired. 

RESPIRATORY  MOVEMENTS. 

Physiologically  the  motions  are  divided  into  two  sets — (1)  those 
which  enlarge  the  thoracic  cavity,  and  cause  the  air  to  rush 
into  the  lungs,  called  inspiration;  and  (2),  those  which  diminish 
the  size  of  the  thorax  and  force  out  the  air,  called  expiration. 

No  action  of  life  is  more  familiar  than  the  rhythmical  move- 
ments of  respiration.  The  slow,  quiet  rise  and  fall  of  the  chest 
and  abdomen  are  the  signs  most  commonly  sought  as  indicative 
of  life ;  for  every  one,  knows  that  constant  ventilation  must  go 
on  in  order  that  the  blood  may  readily  obtain  the  necessary 
amount  of  oxygen,  and  get  rid  of  carbonic  acid  gas,  the  ordinary 
diffusion  that  takes  place  in  the  motionless  chest  being  quite 
insufficient  to  remove  the  heavy  carbonic  acid  gas  from  the 
lungs. 

The  rhythm  of  the  respiratory  movements  may  be  represented 
graphically  by  recording  the  changes  in  the  diameter  or  circum- 


RESPIRATORY   MOVEMENTS.  331 

ference  of  the  thorax,  the  movements  of  the  diaphragm,  or  the 
variations  of  the  pressure  in  the  air  passages.  These  methods, 
though  riot  quite  reliable,  give  curves  of  a  similar  character. 

The  rate  of  the  respiratory  movements  is  up  to  a  certain  point 
under  voluntary  control,  and  may  be  varied  by  the  will,  or 
stopped,  as  when  one  holds  one's  breath. 

The  voluntary  control  of  the  respiratory  movements  is,  how- 
ever, limited,  for  if  we  hold  our  breath  for  any  length  of  time,  a 
moment  soon  arrives  when  the  "  necessity  of  respiration  "  over- 
comes the  strongest  will.  The  usual  respiratory  movements  are 
carried  on  without  our  being  conscious  of  them,  and  are  strictly 
involuntary. 

The  rate  of  the  respiratory  movements  varies  according  to  cir- 
cumstances, being  in  an  adult  man  about  18  per  minute;  in 
most  of  the  lower  animals  it  is  much  more  rapid.  It  varies  with 
age,  being  very  rapid  at  birth,  decreasing  slowly  to  about  30, 
and  slightly  rising  toward  old  age.  The  following  table 
(Quetelet)  illustrates  this : — 

A  new-born  infant respires  44  times  per  minute. 


15-20 
20-25 
25-30 
30-60 


5  years. 


26 
20 

18.7 

16 

18.1 


Muscular  exercise  increases  the  rapidity  of  the  respiratory 
movements,  and,  consequently,  the  effort  of  standing  produces 
a  more  frequent  respiration  than  is  found  in  the  recumbent  pos- 
ture. Emotions  variously  affect  the  rate  and  rhythm  of  the 
inspiration  and  expiration  (e.  g.,  sighing) ;  and,  finally,  morbid 
conditions,  implicating  the  lungs,  usually  cause  a  greater  fre- 
quency of  respiration,  sometimes  attaining  a  rate  of  as  many  as 
60-70  respirations  per  minute. 

The  thorax  is  enlarged  in  all  directions  during  inspiration,  the 
motion  being  usually  referred  to  the  vertical,  transverse,  and 
antero-posterior  diameters  respectively. 

The  vertical  diameter  is  increased  by  the  descent  of  the  lateral 
portions  of  the  diaphragm  and  the  slight  elevation  of  the  parts 
about  the  apex. 


332  MANUAL   OF   PHYSIOLOGY. 

The  lateral  diameter  is  widened  by  the  side  droop  of  the  ribs 
being  lessened ;  each  rib  is  rotated  upon  the  line  uniting  its 
extremities,  and  at  the  same  time  is  moved  upward  and  outward. 

The  antero-posterior  diameter  is  enlarged  by  the  general  ele- 
vation of  the  ribs  and  sternum,  the  anterior  extremities  of  the  ribs 
being  drawn  up  from  their  general  downward  incline,  push  the 
sternum  forward. 

The  movements  of  the  diaphragm  depress  the  abdominal  vis- 
cera lying  beneath  it,  and  thereby  distend  the  elastic  abdominal 
wall  and  compress  the  gases  contained  in  the  intestines.  Thus 
the  diaphragmatic  movements  cause  a  rhythmical  heaving  of 
the  abdomen.  Respiration  depending  chiefly  on  the  action  of 
this  one  muscle  is  therefore  spoken  of  as  abdominal  respiration. 
On  the  other  hand,  when  the  ribs  are  the  chief  cause  of  expan- 
sion of  the  upper  parts  of  the  chest,  it  is  called  thoracic  or  costal 
respiration. 

These  two  types  of  respiratory  movements  may  be  imitated 
voluntarily,  and  are  variously  combined  in  different  individuals 
during  ordinary  respiration,  and  in  the  same  individual  under 
different  circumstances. 

In  men  the  general  character  of  the  ordinary  quiet  respiration 
is  abdominal,  the  movement  of  the  thorax  being  insignificant  in 
comparison  with  that  of  the  abdomen. 

In  women  the  reverse  is  the  case ;  the  abdominal  movements 
are  slight  when  compared  with  those  of  the  upper  part  of  the 
thorax.  This  difference  is  only  well  marked  during  quiet,  uncon- 
scious breathing;  any  forced  or  voluntary  respiratory  effort 
changes  the  typical  character  of  man's  breathing,  and  the  costal 
movements  become  more  prominent.  In  a  forced  deep  inspira- 
tion the  upper  part  of  the  chest  shows  the  greatest  increase  in  the 
antero-posterior  diameter  in  both  sexes. 

This  difference  in  type  between  male  and  female  respiratory 
movements  has  been  ascribed  to  different  causes.  The  most  com- 
mon of  these  is  the  change  brought  about  by  the  costume  ordi- 
narily adopted  by  females.  This  can  hardly  be  an  adequate 
explanation  of  the  phenomenon,  for  we  find  the  same  type  exist- 
ing when  the  tight  garments  are  removed,  and  it  is  apparent  in 


INSPIRATORY   MUSCLES.  333 

those  who  have  never  been  constricted  by  tight  clothing,  and  even 
in  cases  where  no  clothing  has  been  used,  as  among  the  inhabi- 
tants of  hot  countries ;  so  that,  though  the  corset  may  induce  an 
exaggeration  of  the  costal  respiration,  by  constricting  the  lower 
ribs  and  interfering  with  the  action  of  the  diaphragm,  it  would 
not  seem  sufficiently  to  account  for  the  normal  costal  type  of 
breathing  found  in  women. 

The  occasional  distention  of  the  abdomen  during  pregnancy 
has  also  been  assigned  as  a  cause  of  the  female  type  of  breathing, 
but  it  is  very  unlikely  that  pregnancy  is  the  sole  agency  in  pro- 
ducing it,  since  in  childhood  the  costal  type  is  marked  in  both 
sexes.  That  this  type  of  breathing  should  be  transmitted  more 
markedly  to  females  from  our  female  ancestors  is,  however,  quite 
possible.  It  is  probable  that  the  abdominal  breathing  of  the 
male  is  also  increased  by  hereditary  transmission,  but  is  origi- 
nally due  to  the  gradual  increase  in  the  development  of  the 
muscles  of  the  upper  extremities  in  males,  causing  a  greater 
fixedness  of  the  upper  ribs  from  which  they  take  origin. 

INSPIRATORY  MUSCLES. 

^The  act  of  inspiration  is  not  performed  by  any  single  muscle ; 
ladeed,  even  the  most  gentle  and  quiet  respiration  requires  the 
coordinated  action  of  many  sets  of  muscles.  Most  of  these  mus- 
cles have  other  duties  to  perform  besides  helping  to  produce 
respiratory  movements. 

Those  which  are  inspiratory  in  their  functions  are : — 

1.  The  Diaphragm  with  its  accessory  Quadratus  Lumborum 

to  fix  its  origin  from  the  last  rib. 

2.  Levatores  costarum  (including  the  scaleni)  with    their 

accessory  intercostals,  which  act  chiefly  as  regulators. 

3.  The  Serratus  posticus  superior. 

The  Diaphragm  is  the  most  important  inspiratory  muscle.  It 
is  the  only  one  which,  unaided,  can  keep  up  the  necessary  tho- 
racic ventilation,  and  in  injury  of  the  spinal  cord,  owing  to  its 
isolated  nervous  supply,  the  diaphragm  may  be  called  upon  to  do 
all  the  inspiratory  work. 


334 


MANUAL   OF    PHYSIOLOGY. 


FIG.  150. 


During  ordinary  quiet  breathing  in  the  male  it  does  the  greater 
part  of  inspiration. 

When  not  in  action,  one  part  of  the  muscular  sheets  of  the  dia- 
phragm lies  in  direct  contact  with 
the  inner  surface  of  the  lower  cos- 
tal part  of  the  thoracic  wall,  and 
the  other  (D)  is  higher  than  the 
central  tendon  which  forms  the 
floor  of  the  pericardium,  and  is 
fixed  in  one  position.  During  in- 
spiration these  lateral  parts  are 
separated  from  the  ribs  and  drawn 
below  the  level  of  the  central  ten- 
don by  the  contraction  of  the  mus- 
cular fibres.  The  separation  is 
aided  by  the  abduction  of  the  float- 
ing ribs,  which  is  accomplished  by 
the  quadratus  lumborum  and  the 

Diagram  of  a  section  made  vertically 
from  side  to  side  through  the  tho-     deep  dorsal 
racic  and  part  of  the  abdominal  cav- 
ities to  show  the  position  of  the 

diaphragm,  which  is  indicated  by  ITT 

the  dark  line  (DP)  placed  on  the     may    act    to    the     best     advantage, 
parts  of  the  muscle  that  descend  in 
inspiration. 

p.  Pericardial  cavity. 

L.  Liver. 

s.  Stomach. 

R.  Roots  of  lungs  cut  through. 


T  .  ,  ,  ,.       , 

In   order    that    the    diaphragm 


it  is  necessary  that  its  attachments 
be  fixed  by  the  other  muscles; 
for  when  the  quadratus  lumborum, 
levatores  and  other  fixing  muscles 
are  not  acting,  the  lower  floating  ribs  are  drawn  in  by  the 
diaphragm,  and  the  power  of  that  muscle  is  much  diminished 
by  the  approximation  of  its  attachments.  This  may  be  seen  in 
spinal  injuries  when  the  inspiration  is  carried  on  by  the  dia- 
phragm alone.  In  these  cases  a  circular  furrow  marks  the  line 
of  attachment  of  the  muscle  to  the  lower  ribs  and  their  carti- 
lages, which  are  drawn  inward  during  each  inspiration,  the 
breathing  being,  of  course,  purely  abdominal  in  type. 

The  Quadratus  Lumborum,  which  passes  from  the  pelvis  to  the 
last  rib,  has,  besides  the  action  in  aid  of  the  diaphragm  just 
mentioned,  the  power  of  drawing  down  the  lower  outlet  of  the 
thorax,  in  which  it  is  helped  by  other  abdominal  and  dorsal 


RESPIRATORY    MUSCLES. 


335 


muscles.  In  this  action  it  may  be  regarded  as  the  antagonist  of 
the  next  group. 

The  Sealeni  Muscles,  which  pass  down  from  the  lateral  aspects 
of  the  cervical  vertebrae  to  the  first  two  ribs,  which  they  raise  so 
as  to  draw  up  the  upper  outlet  of  the  thorax.  The  quadratus 
and  scaleni  muscles  thus  act  upon  the  thorax  in  the  same  way 
as  the  hands  in  extending  a  concertina. 

The  Levatores  Costarum  are  small  muscles,  but  on  account  of 
their  number,  their  aggregate  force  is  much  greater  than  is  com- 
monly thought.  They  are  short,  thick  muscles,  which  pass 


FIG.  151. 


FIG.  152. 


Diagram  showing  interval  between 
the  position  of  the  diaphragm  in 
expiration  (e,  e)  and  inspiration 
(i,  i).  The  increase  in  capacity 
is  shown  by  the  white  areas. 


View  from  behind  of  four  dorsal  vertebra 
and  three  attached  ribs,  showing  the 
attachment  of  the  elevator  muscles  of 
the  ribs  and  the  intercostals.  (Allen 
Thomson.) 

1.  Long  and  short  elevators.  2.  External 
intercostal.  3.  Internal  intercostal. 


obliquely  downward  and  outward  from  the  transverse  processes 
of  the  dorsal  vertebrae  to  the  angle  of  the  ribs.  Their  only  action 
is  to  raise  the  angle  of  the  ribs,  and  thus  diminish  their  anterior 
and  lateral  downward  slopes;  by  so  doing  they  increase  the 
intervals  between  the  ribs  and  enlarge  the  lateral  and  the  antero- 
posterior  diameters  of  the  chest.  Thus  they  are  purely  muscles 
of  inspiration,  and  probably,  acting  with  the  diaphragm  and  the 
scaleni,  are  the  chief  workers  in  ordinary  breathing. 

The  Intercostals  produce  various  effects  on  the  ribs  according 


336  MANUAL   OF    PHYSIOLOGY. 

to  the  different  sets  of  muscles  with  which  they  act  in  association. 
They  never  act  alone,  and  it  is  therefore  idle  to  try  to  ascribe 
to  them  any  constant  specific  inspiratory  or  expiratory  action. 
Generally  speaking,  the  intercostals  approximate  the  ribs,  and 
by  this  action  they  stiffen  the  thoracic  wall  and  help  to  elevate 
the  thorax  when  its  upper  part  is  fixed,  or,  when  its  lower  part 
is  fixed,  to  depress  it. 

Now,  if  both  the  upper  and  lower  margins  of  the  thorax  be 
held  firmly  by  strong  muscles,  as  really  occurs  in  inspiration — 
from  the  action  of  the  quadratus  and  scaleni — the  intercostals 
cannot  approximate  the  ribs.  Under  these  circumstances  the 
results  which  follow  their  contraction  will  be  twofold,  viz. :  (1) 
the  sternum  will  be  pushed  forward,  and  the  antero-posterior 
diameter  of  the  thorax  thus  increased;  and-  (2)  the  spaces 
between  the  ribs,  which  are  widened  by  the  other  muscles,  are 
kept  rigid  and  prevented  from  sinking  inward  when  the  intra- 
thoracic  pressure  falls.  When  acting  with  the  elevators  of  the 
ribs  both  intercostal  layers  of  muscle  have  an  inspiratory  effect. 
But  when  the  elevators  of  the  ribs  are  passive  the  intercostals, 
acting  with  the  anterior  abdominal  muscles,  draw  down  the  ribs, 
and  act  as  muscles  of  expiration. 

Extraordinary  Muscles  of  Inspiration. — For  forced  breathing  a 
great  number  of  muscles  are  called  into  play  during  the  inspi- 
ratory effort,  as  may  be  seen  during  pathological  occlusion  of  the 
air  passages,  where  all  the  thoracic,  cervical,  facial,  abdominal 
muscles,  and  even  the  muscles  of  the  extremities,  are  one  after 
another  thrown  into  a  recurring  spasm  before  suffocation  ends 
the  patient's  life. 

Among  the  muscles  which  lend  their  aid  when  more  ener- 
getic inspiratory  movements  are  required,  may  be  mentioned  the 
sterno-mastoid,  which  helps  the  scaleni  to  elevate  the  front  of  the 
thoracic  wall ;  the  pectoral  muscles  and  the  great  serratus,  which 
assist  when  the  arms  are  fixed. 

The  deep  muscles  of  the  back,  which  straighten  the  spine,  must 
thereby  act  upon  the  ribs  so  as  to  elevate  them  and  widen  the 
intervals  between  them.  This  straightening  of  the  dorsal  curve 
probably  helps  even  in  quiet  breathing,  and  no  doubt  has  an 


EXPIRATION.  337 

important  inspiratory  influence  in  forced  respiration.  Owing  to 
the  ribs  being  fixed  to  the  sternum  in  front,  they  can  only  be 
separated  laterally  when  the  dorsal  curve  is  lessened,  and  this 
tends  to  approximate  the  sternum  and  the  vertebrae,  thus  nar- 
rowing the  antero-posterior  diameter  of  the  thorax.  It  is  in 
preventing  this  flattening  of  the  chest  that  the  intercostals  are 
particularly  useful ;  by  holding  the  ribs  together  they  push  for- 
ward the  sternum,  when  the  dorsal  curve  is  extended. 

EXPIRATION. 

During  quiet  breathing  expiration  requires  no  muscular  effort, 
the  expulsion  of  the  air  from  the  FIG  lg3 

chest  being  accomplished  by  the 
elasticity  of  the  parts. 

A  powerful  expiratory  force  is 
the  elasticity  of  the  lungs,  which 
are  on  the  stretch  even  after  a 
forced  expiration,  and  when  dis- 
tended by  inspiration  are  capable 
of  exerting  considerable  traction 
on  the  thoracic  wall. 

The  ordinary  shape  of  the  elas- 
tic walls  of  the  thorax  when  the 
muscles  are  not  acting,  corresponds 
with  the  position  at  the  end  of 
gentle  expiration ;  therefore  the 
resiliency  of  the  muscles,  costal 
cartilages,  and  other  elastic  tissues 
which  are  stretched  during  inspi- 
ration, tends  to  restore  the  ribs  to 
the  position  of  expiration. 

The    Weight  of  the    thorax    itself,    Shows  the  position  of  the  Ribs  and  the 

!     v         ,  i  Spinal  Column  in  normal  form  of  the 

and  the  Clastic    gases    m    the  intes-       thorax,  i.e.,  that  assumed  in  expira- 

tinal  tract,  which  have  been  com- 
pressed by  the  diaphragm,  may  also  help  in  expiration. 

After  death,  when  the  elasticity  of  the  expiratory  muscles  is 
lost,  the  traction  exerted  by  the  lungs  on  the  thorax  reduces  it 
29 


338  MANUAL   OF   PHYSIOLOGY. 

below  the  size  its  own  elastic  equilibrium  would  tend  to  assume ; 
when,  therefore,  air  is  admitted  to  the  pleural  cavity  by  punc- 
ture, the  thorax  expands  slightly  as  the  lungs  shrink,  and  the 
pressure  on  the  pleural  surface  becomes  equal  to  that  within  the 
bronchi. 

In  forced  expiration,  or  when  the  air  is  used  during  expiration 
for  any  purpose,  such  as  the  production  of  voice,  or  any  blowing 
movement,  a  number  of  muscles  are  called  into  action.  The 
only  muscles  that  could  be  called  exclusively  special  muscles  of 
expiration  are  the  weak  triangularis  sterni,  serratus  posticus  infe- 
rior, and  parts  of  the  intercostals  ;  but  in  all  violent  and  forcible 
expiratory  efforts  these  are  aided  by  the  abdominal  muscles  form- 
ing the  anterior  wall  of  the  abdomen,  which,  associated  with  the 
intercostals  and  quadratus  lumborum,  are  the  most  powerful  agents 
in  drawing  down  the  thoracic  walls  and  expelling  the  air. 

FUNCTION  OF  THE  PLEURA. 

From  what  has  been  already  said,  it  is  obvious  that  by  far  the 
greatest  amount  of  enlargement  takes  place  in  the  lower  part  of 
the  thorax,  while  the  capacity  of  the  apex  changes  but  little. 
The  increase  of  capacity  in  the  chest  during  inspiration  takes 
place  practically  between  the  costal  wall  and  the  diaphragm 
(compare  Figs.  148,  149).  If  the  lungs  and  the  walls  of  the 
thorax  were  fused  together,  without  the  interposition  of  serous 
membranes,  the  different  parts  of  the  lungs  would  have  to  follow 
the  movements  of  that  part  of  the  thorax  to  which  they  are 
attached.  Thus  the  lower  parts  of  the  lung  would  be  much  dis- 
tended during  inspiration,  and  the  apices  would  receive  but  little 
addition  to  their  contained  air.  This  condition  is  often  found 
when  disease  of  the  pleura  leads  to  adhesion  of  the  visceral  and 
parietal  layers.  When  such  cases  live  for  some  time  after  the 
pleurisy  and  the  adhesions  persist,  the  air  cells  of  the  lower  mar- 
gins of  the  lungs  are  commonly  found  to  be  distended  and  blood- 
less (i.  e.}  local  emphysema  from  habitual  over  distention) ;  while, 
on  the  other  hand,  the  apices  become  abnormally  dense,  and  the 
alveoli  are  contracted  and  airless. 

The  surface  of  the  soft  elastic  lung  tissue  is  normally  quite 


FUNCTION   OF   THE   PLEURA.  339 

free,  being  encased  in  a  serous  membrane,  the  smooth  surface 
of  which  can  slide  uninterruptedly  and  freely  over  the  similar 
lining  of  the  costal  wall.  That  this  motion  of  the  lung  actually 
occurs  may  be  seen  from  watching  the  lung  through  the  exposed 
parietal  pleura,  or  recognized  by  studying  the  sounds  produced 
by  a  roughness  of  the  pleura,  such  as  occurs  in  inflammation, 
when  a  "  friction  sound  "  can  be  detected  by  the  ear. 

The  lungs  move  in  a  definite  direction.  From  the  least  mov- 
able points  of  the  thorax,  namely,  the  apex  and  vertebral  margin, 
they  pass  toward  the  more  movable  inferior  costal  and  sternal 
regions.  In  short,  the  anterior  part  of  the  lungs  passes  down- 
ward and  forward  to  fill  up  the  gap  made  by  the  descent  of  the 
diaphragm  and  by  the  passing  of  the  costal  wall  upward  and  for- 
ward. 

The  position  of  the  inferior  margin  of  the  lung  may  be  easily 
recognized  by  percussion  over  the  liver,  and  may  thus  be  shown 
to  move  up  and  down  with  the  expiration  and  inspiration  re- 
spectively. By  percussion  we  also  find  that  the  space  between 
the  two  lungs  in  front  is  increased  during  expiration  and  dimin- 
ished during  inspiration,  so  that  the  heart  is  more  or  less  covered 
by  lung,  and  the  prsecordial  dullness  is  altered  every  time  we 
draw  a  breath. 

By  means  of  this  free  movement  of  the  lungs  in  the  cavities 
lined  by  serous  membrane  the  air  exerts  equal  force  on  the  walls 
of  all  the  air  cells  whether  they  are  situated  in  the  apex  or  base 
of  the  lung,  and  the  alveoli  are  all  equally  filled  with  air. 

If  the  pleural  cavity  be  brought  into  contact  with  the  air, 
either  by  puncture  of  the  thoracic  walls  or  by  rupture  of  the 
visceral  pleura,  the  lung,  owing  to  the  great  elasticity  of  its  tis- 
sue, shrinks  to  very  small  dimensions,  and  the  pleural  cavity 
becomes  filled  with  air  (pneumothorax). 

If  air  be  admitted  to  both  pleural  cavities  so  as  to  produce 
double  pneumothorax,  death  must  ensue,  for  if  the  opening 
remain  free  the  motions  of  the  thorax  only  alter  the  quantity  of 
air  in  the  pleural  cavity,  and  cannot  ventilate  the  lungs.  This 
demonstrates  the  important  fact  that  it  is  the  atmospheric  pres- 
sure which,  having  access  to  them  only  through  the  trachea, 


340  MANUAL    OF    PHYSIOLOGY. 

maintains  the  distention  of  the  elastic  lungs,  and  keeps  them 
pressed  against  the  wall  of  the  thorax. 

The  power  with  which  the  lungs  can  contract  when  the  atmos- 
pheric pressure  is  admitted  to  the  pleura,  has  been  found  after 
death,  without  inflation,  to  be  six  millimetres  of  mercury,  which  is 
probably  below  the  pressure  exerted  during  life,  when  the  smooth 
muscle  of  the  bronchi  is  acting  and  the  tubes  are  free  from  mucus, 
for  this  rapidly  collects  in  the  minute  air  tubes  at  death,  and 
impedes  the  outflow  of  air. 

When  the  lungs  are  inflated  before  the  pleura  is  opened,  the 
pressure  can  easily  be  made  to  rise  to  nearly  li  inches  (30  mm. 
mercury). 

From  this  it  would  appear  probable  that  when  the  lungs  are 
stretched  by  inspiration  they  exert  a  negative  pressure  equal  to 
30  mm.,  and  when  the  lungs  are  in  a  position  of  expiration  they 
still  tend  to  contract  with  a  force  of  6  mm.  mercury. 

PRESSURE  DIFFERENCES  IN  THE  AIR. 

The  immediate  effect  of  the  increase  in  capacity  of  the  chest  is 
that  a  pressure  difference  is  established  between  the  interior  of 
the  thoracic  cavity  and  the  atmosphere. 

The  reduction  in  pressure  produced  in  the  lungs  and  air  pas- 
sages by  inspiratory  movements,  or  the  increase  of  pressure 
accompanying  expiration,  is  very  slight  during  ordinary  quiet 
breathing  with  free  air  passages.  But  the  least  impediment  to 
the  entrance  or  to  the  exit  of  the  air  at  once  makes  the  difference 
very  notable. 

It  is  difficult  to  obtain  an  accurate  experimental  estimate  of 
the  variations  in  the  pressure  in  different  parts  of  the  air  passages 
during  quiet  breathing,  because  even  the  most  careful  attempt 
to  measure  the  pressure  causes  an  increase  which  is  still  further 
magnified  by  the  sensitive  muscular  mechanism  of  the  air  pas- 


The  variations  in  pressure  occurring  in  the  pulmonary  air  are 
greatest  in  the  alveoli,  and  gradually  diminish  toward  the  larger 
air  tubes,  so  that  they  disappear  at  the  nasal  orifice,  where,  if  no 
impediment  be  placed  to  the  course  of  the  air,  the  pressure  will 


VOLUME   OF   AIR.  341 

remain  very  nearly  equal  to  that  of  the  atmosphere.  By  con- 
necting one  nostril  with  a  manometer  and  breathing  through  the 
nose  with  the  mouth  shut,  it  can  be  shown  that  inspiration  causes 
a  negative  pressure  of  about  1  mm.  mercury,  and  expiration  a 
positive  pressure  of  2  to  3  mm. ;  these  results  must  be  divided  by 
two,  since  by  plugging  one  nostril  they  shut  off  half  the  normal 
inlet.  Forced  inspiration  and  expiration  give  respectively  —  57 
and  -J-  87  mm. 

This  great  difference  depends  on  the  elastic  forces  against 
which  the  inspiratory  muscles  act  in  distending  the  thorax,  all 
of  which  assist  in  expiration. 

THE  VOLUME  OF   AIR. 

During  ordinary  respiration  the  volume  of  the  inspiratory  and 
expiratory  stream  of  air  is  surprisingly  small  when  compared 
with  the  volume  of  air  sojourning  in  the  lungs. 

After  an  ordinary  expiratory  act  we  can  force  out  a  great 
quantity  of  air  by  a  voluntary  effort;  but  even  after  this  is  got 
rid  of  the  lungs  are  still  well  filled.  Some  of  this  residual  air, 
which  never  leaves  the  chest  during  the  life  of  the  animal,  is 
pressed  out  by  the  elasticity  of  the  lungs  when  the  pleura  -is 
opened.  But  a  certain  amount  of  air  cannot  be  removed  in  any 
way  from  the  alveoli.  Even  when  the  lung  is  cut  out  of  the 
chest  and  divided  into  pieces,  enough  air  is  retained  in  the  air 
cells  to  render  it  buoyant.  This  fact  is  relied  on  by  medical 
jurists  as  an  evidence  that  an  infant  has  been  born  alive  and 
distended  the  lungs  with  air,  for  except  breathing  has  been  well 
established,  the  tolerably  fresh  lung  of  an  infant  will  sink  in 
water. 

In  order  to  have  a  clear  idea  of  the  volumes  of  air  at  rest  and 
in  motion  during  pulmonary  ventilation,  it  is  convenient  to  fol- 
low the  classification  from  which  the  nomenclature  in  common 
use  has  been  borrowed. 

Tidal  air  is  the  current  of  air  which  passes  in  and  out  of  the 
air  passages  in  quiet  natural  breathing.  It  amounts  to  about 
500  cc.  (30  cubic  inches). 

Reserve  air  is  that  volume  which  can  be  voluntarily  emitted 


342  MANUAL    OF   PHYSIOLOGY. 

after  the  end  of  a  normal  tidal  expiration,  and  which,  therefore, 
during  ordinary  respiration  remains  in  the  lungs ;  it  is  estimated 
at  about  1500  cc.  (or  nearly  100  cubic  inches). 

Complemental  air  is  that  which  can  be  voluntarily  taken  in 
after  an  ordinary  inspiration  by  a  forced  inspiration ;  it  also 
amounts  to  about  1500  cc.,  but  is  not  used  during  ordinary 
breathing. 

Residual  air  is  the  volume  which  remains  in  the  lungs  after  a 
forced  expiration,  that  is  to  say,  which  no  voluntary  effort  can 
remove  from  the  lungs ;  it  includes  the  air  which  leaves  the  lungs 
when  the  pleura  is  opened  after  death  and  the  air  which  persist- 
ently remains  in  them  after  they  have  collapsed.  This  amounts 
to  about  2000  cc.  (or  about  120  cubic  inches). 

Vital  capacity  is  a  term  meaning  the  greatest  amount  of  air 
that  can  be  emitted  by  a  forced  expiration  immediately  following 
a  forced  inspiration,  so  that  it  equals  the  sum  of  the  tidal, 
reserve  and  complemental  air.  The  vital  capacity  is  estimated 
by  spirometers  of  different  kinds,  and  gives  an  approximate 
measurement  of  (1)  The  capacity  of  the  chest.  (2)  The  power 
of  the  respiratory  muscles.  (3)  The  resistance  offered  by  the 
elasticity  or  rigidity  of  the  walls  of  the  thorax.  (4)  The  work- 
ing capacity  of  the  lungs,  i.  <?.,  their  extensibility  or  freedom  from 
disease.  It,  therefore,  varies  greatly  according  to  the  age,  sex, 
position  of  the  body,  the  occupation,  weight,  height,  the  fullness 
of  the  hollow  viscera  of  the  abdomen,  and  the  pathological  con- 
dition of  the  lungs.  It  can  be  much  increased  by  practice,  and 
this  fact,  apart  from  the  injury  forced  respirations  may  produce 
in  a  morbid  state  of  the  lung,  renders  it  inapplicable  as  a  gauge 
of  pulmonary  disease. 

DIFFUSION. 

From  the  foregoing  it  appears  that  the  volume  of  air  habitually 
sojourning  in  the  lungs  during  natural  respiration,  or  stationary 
air,  is  about  3500  cc.  (nearly  220  cubic  inches),  while  the  fresh 
air  introduced  by  each  inspiration  is  only  a  little  over  500  cc.  (30 
cubic  inches),  or,  in  other  words,  about  one-seventh  of  the  air  in 
the  lungs  is  changed  at  each  breath.  Indeed,  the  500  cc.  of  air 


NERVOUS   MECHANISM   OF    RESPIRATION.  343 

is  only  just  sufficient  to  fill  the  trachea  and  larger  bronchial  pas- 
sages, so  that  the  fresh  air  does  not  reach  the  pulmonary  alveoli, 
or  directly  replace  any  of  the  air  they  contain.  The  tidal  stream 
is,  however,  brought  into  immediate  relation  with  the  stationary 
air,  and  the  thoracic  movements  cause  them  to  mix  mechanically, 
so  that  rapid  diffusion  takes  place  in  the  minute  bronchi.  Dif- 
fusion is  also  constantly  occurring  between  the  air  of  the  small 
tubes  and  the  terminal  sacs,  and  it  alone  suffices  to  maintain  the 
necessary  standard  of  purity  in  the  air  of  the  alveoli.  If  the 
harmless  gas,  hydrogen,  be  inhaled  during  one  inspiration,  it 
requires  6  to  10  respirations  to  get  rid  of  the  impurity  from  the 
expired  air.  From  this  it  has  been  inferred  that  this  number  of 
respiratory  acts  would  be  necessary  to  render  the  air  in  the  alveoli 
quite  pure  even  if  no  fresh  impurities  were  allowed  to  enter  from 
the  blood. 

RESPIRATORY  SOUNDS. 

As  the  streams  of  air  enter  the  air  passages  and  lungs  they  pro- 
duce sounds  which  are  of  the  greatest  importance  to  the  physician, 
owing  to  the  manner  in  which  they  become  altered  by  disease. 

A  sound  called  "  bronchial  breathing  "  is  produced  in  the 
large- bronchi  and  trachea,  and  is  like  the  noise  of  air  blowing 
through  a  tube.  This  can  normally  be  heard  over  the  trachea, 
jr  at  the  back  between  the  shoulder  blades  over  the  entrance  of 
the  large  bronchi  into  the  root  of  the  lung. 

Another  sound  called  "  vesicular  "  can  be  heard  all  over  the 
chest,  being  most  distinct  where  the  lung  is  most  superficial,  and 
where  other  sounds  are  absent,  as  in  the  subaxillary  region.  It 
is  a  gentle  rustling  sound  caused  by  the  air  passing  into  the 
infundibuli.  It  varies  much  with  the  force  of  respiration  and 
many  other  circumstances.  In  children  up  to  ten  or  twelve 
years  of  age  it  is  remarkably  sharp  and  loud,  and  is  called  "  pue- 
rile breathing." 

NERVOUS  MECHANISM  OF  RESPIRATION. 

The  movements  of  respiration  go  on  rhythmically  without  any 
voluntary  effort,  and  even  when  we  are  quite  awake  they  occur 
almost  without  our  being  conscious  of  them,  and  repeated  varia- 


344  MANUAL   OP   PHYSIOLOGY. 

tions  take  place  in  the  rate,  depth  and  general  type  of  our  respi- 
rations without  our  knowledge.  Indeed,  if  this  self-regulating 
arrangement  did  not  exist,  we  should  have  to  devote  our  atten- 
tion to  adapting  our  respiratory  movements  to  the  ever-varying 
requirements  of  the  gas  interchange  of  the  blood. 

Like  all  other  groups  of  skeletal  muscles,  those  which  act  on 
the  thorax  are  regulated  by  nerves,  and  work  together  in  har- 
mony. The  coordinated  movements  are  so  far  under  the  con- 
trol of  the  will  that  any  of  the  groups  of  muscles  may  be 
employed  separately,  or  in  conjunction. 

But  the  respiratory  differ  from  the  other  skeletal  muscles,  in 
that  they  undergo  rhythmical  coordinated  contractions  which 
are  not  directed  by  our  will,  and  can  be  influenced  only  to  a 
certain  extent  by  it,  for  they  cannot  be  made  to  cease  altogether. 

RESPIRATORY  CENTRES. 

The  normal,  rhythmical,  coordinated  movements  of  respira- 
tion are  not  only  brought  about,  but  are  also  regulated  by  an 
involuntary  nervous  mechanism.  Since  we  are  unconscious  of 
its  action,  it  certainly  is  not  dependent  on  the  voluntary  centres. 
The  upper  parts  of  the  brain  cannot  be  needed  for  regular  breath- 
ing, since  (1)  animals  born  with  deficiently  developed  brains 
breathe  rhythmically ;  and  (2)  removal  of  the  brain  of  birds, 
etc.,  or  loss  of  voluntary  movement  in  man  (hemiplegia),  causes 
no  interruption  of  the  respiratory  movements.  Injury  to  the 
upper  part  of  the  spinal  cord  causes  death  by  stopping  respira- 
tion. The  regulating  centre  must  then  be  lower  than  the  cerebral 
centres,  and  higher  than  the  cervical  part  of  the  spinal  marrow. 
The  direct  evidence  of  the  seat  of  this  centre  was  found  by 
Flourens,  who  showed  that  a  localized  spot  exists  in  the  medulla 
oblongata,  injury  of  which  causes  instant  cessation  of  the  res- 
piratory movement. 

This  vital  point,  nceud  vital,  is  situated  in  the  floor  of  the 
fourth  ventricle,  near  the  point  of  the  calamus  scriptorius,  and  is 
now  commonly  spoken  of  as  the  respiratory  centre.  It  is  con- 
venient to  suppose  that  there  are  two  groups  of  cells,  one  presiding 
over  the  inspiratory,  and  the  other  over  the  expiratory,  muscles. 


EXCITATION   OF    RESPIRATORY   CENTRE.  345 

From  this  centre  the  impulses  which  give  rise  to  the  all- 
important  respiratory  movements  rhythmically  pass  down  the 
spinal  cord  and  nerves.  So  long  as  the  nervous  communication 
between  the  centre  and  the  muscles  is  intact  the  movements  go 
on  with  undisturbed  regularity ;  if  it  be  cut  off,  or  the  centre  be 
destroyed,  respiration  instantly  stops. 

Excitation  of  Respiratory  Centre. — What  keeps  this  centre 
active  ?  It  has  been  already  stated  that  all  the  conditions  of  the 
body  which  cause  an  increased  tissue  change,  use  up  a  greater 
amount  of  oxygen  and  give  off  more  carbonic  acid,  therefore  are 
accompanied  by  more  active  movements  of  the  respiratory  mus- 
cles. From  this  it  would  appear  that  there  exists  some  relation 
between  the  activity  of  the  respiratory  centre  and  the  condition 
of  the  blood,  a  deficiency  of  oxygen  or  an  excess  of  carbonic  acid 
gas  calling  forth  increased  action.  One  has  only  to  hold  one's 
breath  as  long  as  possible,  and  note  the  series  of  rapid  and  deep 
respirations  that  follow  such  a  temporary  impediment  to  the 
proper  oxygenation  of  the  blood,  in  order  to  see  that  this  invol- 
untary respiratory  centre  is  profoundly  influenced  by  a  deficiency 
of  oxygen.  Experimentally,  it  can  be  shown  that  the  centre  is 
excited,  in  a  great  measure,  at  least,  by  the  poorly  oxygenated 
blood  flowing  through  the  medulla,  and  possibly  also  by  the 
action  of  the  venous  blood  circulating  through  distant  organs, 
and  reflexly  affecting  the  centre.  It  has  also  been  shown  that 
the  temperature  of  the  blood  circulating  through  the  medulla 
affects  the  activity  of  the  centre,  for,  if  the  blood  in  the  carotids 
be  warmed,  the  respiratory  movements  become  more  rapid. 

The  respiratory  centre  is  a  good  example  of  what  is  called  an 
"automatic  nerve  centre,"  not  depending  upon  the  arrival  of 
nerve  impulses  from  afar  for  its  excitation,  nor  merely  reflecting 
the  influences  of  other  centres,  but  acquiring  its  energy  from  its 
nutritive  income  and  the  thermal  condition  of  the  warm  blood 
which  flows  through  it. 

So  long  as  the  amount  of  oxygen  flowing  through  the  centre 
is  kept  up  to  a  certain  standard,  the  normal  excitability  of  the 
centre  continues,  and  we  have  natural  quiet  breathing,  called 
Eupncea,  When  the  oxygen  falls  below  the  normal  standard, 


346  MANUAL    OS'     PHYSIOLOGY. 

the  respiratory  centre  becomes  more  excitable,  and  more  active 
movements  or  "  difficulty  of  breathing  "  called  Dyspnoea  is  pro- 
duced. 

If  the  theory  that  a  deficiency  of  oxygen  is  the  normal 
stimulus  to  action  of  the  respiratory  centre  be  correct,  a  super- 
abundant quantity  should  diminish  the  activity  of  the  centre, 
and  a  condition  the  opposite  of  dyspnoea  would  be  produced. 
This  is  difficult  to  show  in  natural  breathing,  though  every  one 
knows  the  efficiency  of  taking  a  few  deep  breaths  before  a  dive 
into  water  or  an  attempt  to  hold  the  breath.  With  artificial 
breathing,  if  the  movements  be  carried  on  very  energetically  for 
some  time,  and  then  be  stopped,  the  animal  will  not  at  first 
attempt  to  breathe,  but  after  a  short  time,  somewhat  less  than  a 
minute,  gentle  and  slow  respiratory  movements  commence.  This 
cessation  of  breathing,  called  apncea,  depends  upon  the  blood 
being  so  charged  with  oxygen  that  it  no  longer  acts  as  a  stimulus 
to  the  centre. 

It  is  probable  that  dyspoena  is  produced  by  a  deficiency  in 
oxygen  rather  than  by  an  excess  of  carbonic  acid  gas.  This  is 
proved  by  the  fact  that  it  occurs  when  the  carbon  dioxide  is 
removed  from  the  blood  by  breathing  air  which  is  free  from 
CO2,  and  is  only  deficient  in  oxygen,  and,  secondly,  because  an 
excess  of  CO2  gas  in  the  air  causes  a  drowsy  condition  rather 
than  an  active  dyspnoea. 

Regulation  of  Respiratory  Activity . — Although  the  respiratory 
centre  is  in  the  common  sense  automatic,  yet  it  is  constantly  affected 
by  many  influences  coming  from  other  parts,  which  reflexly  mod- 
ify the  respiratory  movements.  Thus  mental  emotions  variously 
influence  both  the  rate  and  depth  of  breathing,  sometimes  caus- 
ing more  rapid  and  sometimes  slower  respiratory  action.  The 
application  of  stimulus  to  almost  any  part  of  the  air  passages 
completely  changes  the  respiratory  rhythm,  as  may  be  seen  by 
irritating  the  nasal  mucous  membrane.  The  ordinary  sensory 
nerves  passing  from  the  skin  are  also  capable  of  exciting  respira- 
tory movements.  This. is  well  shown  by  the  gasping  that  follows 
the  sudden  application  of  cold  to  the  body.  It  is  along  these 
sensory  nerves  that  one  tries  to  transmit  impulses  by  applying 


REGULATION   OF    RESPIRATORY   ACTIVITY. 


347 


mechanical,  thermal  or  other  stimulus  to  the  skin  of  a  new-born 
infant,  whose  respiratory  centre  having  been  kept  long  in  the 
condition  of  apnoaa,  is  slow  to  respond  to  an  exciting  influence 
caused  by  a  deficiency  of  oxygen. 


FIG.  154. 


Diagram  of  the  Nervous  Mechanisms  of  Respiration.    (After  Fick.) 
Sc.  Centre  for  inspiratory  movements,  from  which  pass  efferent  channels,  represented 
by  the  continuous  white  line  (o)  to  the  inspiratory  muscles  represented  by  the  dia- 
phragm (D). 
EC.  Centre  for  expiratory  movements,  from  which  efferent  channels  (p)  pass  down  the 

cord  to  the  muscles  of  expiration,  represented  by  the  abdominal  muscles  (A). 
To  both  these  centres  afferent  impulses  of  two  kinds  come  from  the  cerebral  centres  (o, 
6,  c,  d)  to  check  or  excite  activity.  These  voluntary  impulses  may  be  called  affer- 
ent as  far  as  the  respiratory  centres  are  concerned.  From  the  cutaneous  surface 
(/,  g)  and  the  nose  (e),  impulses  arrive,  which  modify  the  action  of  the  inspiratory 
centre.  From  the  larynx  (G)  come  checking  impulses  (h)  to  the  inspiratory,  and 
exciting  impulses  (i)  to  the  expiratory  centre.  And,  finally,  from  the  lungs  come 
both  exciting  and  inhibiting  impulses  (k,  I,  TO,  n)  to  both  the  expiratory  and  inspi- 
ratory centres,  and  by  these  channels  the  rhythm  of  ordinary  breathing  is  regu- 
lated. 


348  MANUAL   OF   PHYSIOLOGY. 

Experiment  proves  that  most,  if  not  all,  afferent  nerves  can 
affect  the  respiratory  centre,  either  by  increasing  or  reducing  its 
activity  ;  but  there  is  one  special  nerve,  namely,  the  pneumogas- 
tric  or  vagus  and  its  branches,  which  have  both  these  capabilities 
developed  to  such  a  degree  that  they  must  be  regarded  as  the 
regulating  nerves  of  respiration. 

Though  section  of  one  vagus  has  little  or  no  effect  on  respira- 
tion, if  the  two  vagi  be  cut  a  marked  change  takes  place  in  the 
respiratory  rhythm.  The  rate  of  the  inspiration  is  reduced  to 
less  than  half  the  normal  rate,  while  each  breath  becomes  deep 
and  prolonged,  so  that  the  respiratory  function  of  the  lungs  goes 
on  for  some  time  unimpaired,  and  the  haemoglobin  of  the  blood 
receives  the  due  amount  of  oxygen.  Although  the  character  oi 
the  breathing  is  completely  changed,  from  the  rapid  gentle  motion 
of  natural  respiration  to  a  series  of  slow  deep  gasps,  the  air  vol- 
umes per  minute  and  the  chemical  changes  remain  the  same.  If 
the  central  end  of  the  cut  vagus  be  now  stimulated  gently,  the 
rate  of  the  respiratory  movements  may  again  be  quickened  to 
the  normal.  If  the  stimulus  be  very  strong,  respiratory  spasm 
can  be  produced.  On  the  other  hand,  if  the  central  end  of  the 
cut  superior  laryngeal  branch  of  the  vagus  be  stimulated,  breath- 
ing becomes  slow,  and  can  be  made  to  cease  in  the  position  of 
ordinary  expiration,  while  a  violent  spasm  of  the  laryngeal  and 
expiratory  muscles  is  caused. 

So  that  in  the  pneumogastric  nerve,  fibres  exist  which  convey 
impulses  of  two  kinds  to  the  inspiratory  centre ;  the  one  increases 
its  excitability  and  hastens  the  discharges  of  inspiratory  impulses, 
the  other  decreases  its  irritability  and  checks  the  inspiratory 
movement.  The  marked  change  just  described  as  occurring  when 
the  two  pneumogastrics  are  cut  proves  that  these  afferent  influ- 
ences are  constantly  at  work,  quickening  the  respiratory  rhythm. 
We  may  assume  that  the  slow,  deep  respirations  which  follow 
section  of  the  vagi  are  caused  by  the  unregulated  automatic 
action  of  the  inspiratory  centre.  No  impulse  is  discharged  until 
the  venosity  of  the  blood  in  the  centre  arrives  at  a  certain  point, 
and  then  the  accumulated  energy  is  sent  to  the  respiratory  mus- 
cles, and  a  deep  gasping  inspiration  occurs,  and  thus  each  respi- 


MODIFIED    MOVEMENTS    OF    RESPIRATORY    MUSCLES.        349 

ratory  act  is  called  forth  by  the  blood  becoming  so  venous  as  to 
act  as  a  powerful  stimulus. 

So  long  as  the  centre  is  stimulated  by  the  regulating  influence 
of  the  vagi  this  venous  condition  is  not  allowed  to  occur,  the 
intense  excitation  of  the  centre  is  thereby  prevented,  and  the 
necessary  movements  performed  with  a  minimum  of  muscle 
energy. 

The  exact  mode  of  stimulation  of  the  pulmonary  terminals  of 
the  afferent  fibres  of  the  pneumogastric  is  not  certain.  It  has 
been  suggested  that  distention  or  retraction  of  the  lungs  may  act 
as  a  mechanical  stimulus  to  fibres  inhibiting  and  exciting  respec- 
tively the  inspiratory  centre.  Each  expansion  of  the  lungs  calls 
forth  the  ensuing  relaxation,  and  the  relaxed  state,  in  its  turn, 
induces  a  new  inspiration,  and  thus  the  lungs  themselves  are  able 
to  guide  the  thoracic  movements  by  means  of  the  pneurnogas- 
trics. 

The  expiratory  part  of  the  centre  probably  takes  no  part  in 
ordinary  breathing,  but  is  called  into  play  in  dyspnoea,  vocal 
use  of  the  expiratory  blast  of  air,  and  in  various  modified  respi- 
ratory movements. 

MODIFIED  MOVEMENTS  OF  THE  RESPIRATORY  MUSCLES. 

Besides  the  ordinary  respiratory  motions  and  the  voluntary 
modifications  made  use  of  in  speaking,  singing,  etc.,  the  muscles 
of  respiration  perform  a  series  of  movements  of  an  involuntary 
reflex  nature  indicative  of  certain  emotions  and  mental  states. 

They  will  be  seen  to  resemble  each  other  in  the  mechanism  of 
their  production,  though  differing  essentially  in  expression.  The 
following  are  the  more  important : — 

Coughing  is  caused  by  a  stimulus  applied  to  certain  parts  of 
the  air  passages,  but  more  particularly  to  the  larynx ;  the  stimu- 
lus passing  along  the  superior  laryngeal  branch  of  the  pneumo- 
gastric. It  consists  of  a  deep  inspiration,  closure  of  the  glottis, 
and  then  a  more  or  less  violent  expiratory  effort,  accompanied 
by  two,  three,  or  more  sudden  openings  and  closures  of  the  glot- 
tis, so  that  rapidly  repeated  blasts  of  air  pass  through  the  upper 
air  passages  and  mouth,  which  is  generally  held  open. 


350  MANUAL   OF    PHYSIOLOGY. 

Sneezing  is  caused  by  a  stimulus  applied  to  the  nose  or  eyes, 
the  impulses  being  carried  to  the  respiratory  centre  by  the  nasal 
and  other  branches  of  the  5th  nerve.  It  consists  of  a  deep  inspi- 
ration and  closure  of  the  glottis,  followed  by  a  single  explosive 
expiration  and  sudden  opening  of  the  glottis  and  posterior  nares 
and  facial  distortion. 

Sneezing  is  a  purely  reflex  act,  since  it  is  impossible  to  produce 
it  voluntarily,  except  indirectly  by  the  stimulation  of  the  nasal 
mucous  membrane  with  some  irritating  substance. 

Laughing  consists  of  a  full  inspiration,  followed  by  a  long 
series  of  very  short,  rapid,  expiratory  efforts.  The  facial  muscles 
are  at  the  same  time  thrown  into  a  characteristic  set  of  move- 
ments. 

Crying  is  made  up  of  a  series  of  short,  sudden  expirations, 
accompanied  by  peculiar  facial  contortions,  lachrymal  secretion, 
and  usually  associated  with  the  following : — 

Sobbing  consists  of  a  rapid  series  of  convulsive  inspiratory 
efforts,  causing  but  little  air  to  enter  the  chest,  followed  by  one 
long  expiration. 

Sighing  is  a  long,  slow  inspiration,  quickly  followed  by  a  cor- 
responding expiration. 

Yawning  is  a  very  long,  deep  inspiration,  completely  filling 
the  chest.  It  is  accompanied  by  a  peculiar  depression  of  the 
lower  jaw,  wide  open  mouth,  facial  movements,  and  commonly 
stretching  of  the  limbs. 

Hiccough  is  an  unexpected  inspiratory  spasm,  chiefly  of  the 
diaphragm,  the  entrance  of  the  air  being  checked  by  the  sudden 
closure  of  the  glottis. 


CHEMISTRY   OF   RESPIRATION.  351 


CHAPTER  XIX. 

THE  CHEMISTRY  OF  RESPIRATION. 

The  simplest  way  to  investigate  the  study  of  the  gas  inter- 
change that  takes  place  in  the  lungs,  between  the  air  and  the 
blood,  is  to  compare  the  composition  of  the  expired  air  with  that 
of  the  atmosphere,  and  from  the  alteration  found  to  have  taken 
place  during  the  tidal  current  we  arrive  at  the  changes  which 
the  air  undergoes  during  its  journey  in  and  out  of  the  air  pas- 
sages, and  we  then  examine  the  venous  and  arterial  blood  in 
order  to  ascertain  the  changes  the  blood  undergoes  in  becoming 
arterial. 

The  atmosphere  is  made  up  of  a  mixture  of  nitrogen  and 
oxygen  with  a  variable  amount  of  moisture  and  a  minute  pro- 
portion of  carbonic  acid. 

The  following  table  gives  the  volume*  of  the  gases  in  dried 
air: — 

Oxygen 20.96  per  cent.,  or  about  21  per  cent. 

Nitrogen 79.02       "  "        79       " 

Carbonic  dioxide 0. 02-0. 06  4  parts  in  10, 000. 

The  amount  of  moisture  contained  in  the  air  is  very  variable, 
and  depends  in  a  great  measure  upon  the  temperature  and  the 
direction  of  the  wind.  The  dampness  of  the  air  depends  upon 
the  temperature,  so  that  air  containing  the  same  absolute  amount 
of  moisture  may  be  relatively  dry  or  damp,  according  as  the 
temperature  rises  or  falls.  As  a  general  rule  the  air  is  relatively 
dry,  that  is  to  say,  it  does  not  contain  so  much  moisture  as  it  is 
capable  of  taking  up  in  the  form  of  aqueous  vapor  at  its  ordi- 
nary temperature.  At  certain  times  of  the  day  the  air  may  be 
saturated,  owing  to  a  sudden  fall  of  temperature. 

The  temperature  of  the  air  which  we  breathe  varies  consider- 

*  On  account  of  the  difference  in  the  atomic  weights,  the  atmosphere  being  only  a 
mechanical  mixture  of  the  gases,  the  proportion  by  weight  is  slightly  different,  being 
about — Oxygen  23  per  cent.,  Nitrogen  77  per  cent. 


352  MANUAL   OF   PHYSIOLOGY. 

ably,  according  to  the  season  of  the  year,  etc.,  but  almost  always 
in  this  country  it  is  lower  than  that  of  our  bodies. 

EXPIRED  AIR. 

The  following  are  the  notable  characters  in  the  tidal  air  on 
its  leaving  the  air  passages  : — 

1.  It  is  rich  in  CO2,  containing  in  quiet  breathing  on  an  aver- 
age 4.38  per  cent,  instead  of  .04  per  cent. 

2.  It  is  poor  in  O,  containing  about  4.5  per  cent,  less  than  the 
atmosphere,  i.  e.,  16.46  per  cent. 

3.  A  slight  increase  in  the  N.  has  been  observed,  possibly  the 
outcome  of  nitrogenous  metabolism. 

4.  The  temperature  of  the  air  is  approximated  to  that  of  the 
body,  and  it  therefore  commonly  exceeds  the  temperature  of  the 
air  inspired.     The  air  on  leaving  the  air  passages  is  about  36.5° 
C.     This  is  not   much  influenced  by  the  temperature  of  the 
atmosphere,  as  may  be  seen  from  Valentine's  Table : — 

Temperature  of  the  Atmosphere  and  of  Expired  Air. 

—    6.3°C.  =  -f  29.8°  C. 

+  17.0°  C.  +  36.2°  C. 

+  44.0°C.  -f  38. 5°  C. 

It  can  be  seen  from  the  last  statement  that  very   hot  air 
(-f  44°  C.)  if  breathed  is  cooled  in  its  transit  through  the  air 


5.  In  quiet  breathing  the  expired  air  is  saturated  with  moist- 
ure ;  in  rapid  breathing  this  is  not  the  case.     It  must  be  remem- 
bered that  the  air  when  warm  is  capable  of  holding  a  greater 
quantity  of  vapor  than  when  it  was  inspired.     The  difference 
can  be  best  appreciated  in  cold  weather,  when  the  vapor  of  the 
warm  expired  air  is  condensed  on  meeting  the  cold  atmosphere. 
Great  quantities  of  water  and  heat  are  given  off  in  producing  this 
saturation. 

6.  If  the  tidal  air  be  dried  and  cooled  and  measured  at  a  certain 
pressure  before  and  after  respiration,  it  is  found  that  the  expired 
air  has  lost  about  ^  of  its  volume.    But  owing  to  the  expansion 
from  the  increased  temperature  and  the  presence  of  the  vapor, 
the  volume  of  air  when  expired  is  greater  than  that  inspired. 


EXPIRED   AIR. 


353 


If  the  oxygen  were  all  used  to  make  CO2,  these  volumes  ought 
to  be  the  same,  for  the  volume  of  CO2  is  equal  to  that  of  the  O  it 
contains,  if  set  free.  The  volume  CO2  given  off  is,  however,  only 
about  4.38  to  4.5  volumes  of  O  taken  in,  so  that  part  of  the  O 
must  be  used  in  some  other  way,  probably  in  forming  H2O  and 
urea. 

7.  The  expired  air  is  also  said  to  contain  traces  of  the  fol- 
lowing impurities:  (1)  ammonia,  (2)  hydrogen,  (3)  carburetted 
hydrogen  (CH4),  (4)  organic  'matter.  These,  and  probably 
other  impurities,  give  the  breath  its  peculiar  odor  and  noxious 
properties,  for  an  atmosphere  rendered  "  stuffy  "  by  expired  air 
is  much  more  injurious  to  health  than  an  atmosphere  in  which  a 
similar  deficiency  of  O  or  excess  of  CO2  had  been  artificially  pro- 
duced by  chemical  means;  this  fact  ought  to  be  remembered 
when  calculating  the  ventilation  required  for  hygienic  purposes. 
The  following  table  may  assist  in  comparing  the  atmosphere  with 
the  expired  air: — 


Atmosphere. 

Expired  Air. 

Difference.' 

CO2  . 

04  per  cent. 

4  38  per  cent. 

+4.34 

o  

20.81 

16.03        " 

—4.78 

N  

79  15        " 

79  55       " 

+  .40 

Temperature  
Moisture         

_6o  a  \-  25°  C. 
about  10  grms  to  1 

29  8°  C-38.50  C. 
about  40  grms.  to  1  cubic 

cubic  metre. 

metre, 
f  apparently  increased,  ab- 
(     solutely  reduced  ^ 

<  NH3,  H,  CH«,  and  poison- 
\     ous  organic  matter 

About  one-seventh  of  the  O  which  is  used  does  not  take  part  in 
the  production  of  the  CO2,  but  this  proportion  may  vary  greatly. 
Thus,  the  estimation  of  the  CO2  can  give  no  sure  guide  to  the 
amount  of  O  taken  up ;  and  each  gas  has  to  be  estimated  sepa- 
rately if  an  accurate  measurement  be  required. 

The  average  amount  per  diem  may  be  said  to  be : — 

Carbon  dioxide  given  off  about 800  grammes. 

Oxygen  consumed  "     700 

Water  given  off  "     500 

The  amounts  of  O  taken   up  and  of  CO2  given   off  differ  in 
30 


354  MANUAL   OF    PHYSIOLOGY. 

different  individuals  and  in  the  same  individuals  under  varying 
circumstances,  among  which  the  following  may  be  enumerated  :  — 

1.  Increase  in  the  rapidity  or  the  depth  of  respiratory  move- 
ments, accompanied  by  an  increase  in  the  tidal  stream,  produces 
an  increase  of  the  total  amount  of  CO2  given  off,  while  the  per- 
centage in  the  volume  of  expired  air  is  diminished. 

2.  It  varies  with  age.     The  amount  increases  with  age  up  to 
30  years,  and  then  remains  constant. 

3.  Sex ;  is  less  in  women  than'in  men,  but  it  increases  in  preg- 
nancy. 

4.  With  muscular  activity  it  is  notably  increased. 

5.  Change  of  temperature  of  the  air  has  a  marked  influence 
on  the  CO2  output  of  cold-blooded  animals,  which  is  increased  in 
direct  proportion  to  the  elevation  of  temperature.     The  effect  on 
warm-blooded  animals  is  the  opposite,  so  long  as  they  can  regulate 
their  temperature.     The  sustentation  of  the  body  temperature  in 
cold  weather  is  accompanied  by  a  distinct  increase  in  the  output 
of  carbon  dioxide. 

6.  The  time  of  day :  a  maximum  is  arrived  at  about  midday 
and  a  minimum,  about  midnight. 

7.  An  increase  in  the  amount  of  carbon  dioxide  in  the  atmos- 
phere diminishes  the  amount  given  off  from  the  lungs. 

CHANGES  THE  BLOOD  UNDERGOES  IN  THE  LUNGS. 

In  order  to  understand  how  the  oxygen  and  the  carbonic  acid 
pass  to  and  from  the  blood  in  the  pulmonary  capillaries  we  must 
know  the  relationship  of  these  gases  to  the  blood  in  the  arterial 
and  venous  sides  of  the  circulation. 

In  the  chapter  on  the  blood  (pp.  243,  244)  it  is  stated  that 
both  the  oxygen  and  the  carbon  dioxide  can  be  removed  from 
the  blood  by  the  mercurial  air  pump,  and  that  the  greater  part 
of  these  gases  are  chemically  united  with  some  of  the  constituents 
of  the  blood,  and  that  a  different  quantity  of  each  gas  is  found  in 
arterial  and  venous  blood.  Now  that  we  know  the  change  from 
the  venous  to  the  arterial  condition  to  take  place  during  the 
passage  of  the  blood  through  the  pulmonary  capillaries,  where 
it  is  exposed  to  the  air,  we  may  assume  that  the  acquisition  of 


CHANGES   OF   BLOOD   IN   LUNGS.  355 

oxygen  and  the  loss  of  C02  form  the  essential  difference  between 
venous  and  arterial  blood. 

From  either  kind  of  blood  about  60  volumes  of  gas  may  be 
extracted  from  every  100  volumes  of  blood  with  the  mercurial 
gas  pump.  The  composition  of  this  gas  varies  considerably  in 
venous,  but  not  very  much  in  arterial  blood.  An  average  is 
given  in  the  following  table: — 

O  vols.  0.  CO  vols.  %.  N  vols.  £, 

Arterial 20  39  1-2 

Venous  (about) 8-10  46-50  1-2 

The  more  rapidly,  after  bleeding,  the  gases  are  removed,  the 
greater  is  the  proportion  of  O  that  can  be  obtained,  as  delay 
allows  some  of  it  to  combine  with  easily  oxidized  substances  in 
the  blood  itself.  The  amount  of  oxygen  varies  in  different  parts 
of  the  venous  system.  In  the  blood  of  an  animal  dying  of  slow 
asphyxia  only  traces  of  oxygen  can  be  found,  and  these  soon  dis- 
appear after  death. 

The  proofs  that  O  is,  for  the  most  part,  in  chemical  combination 
with  the  haemoglobin  of  the  red  blood  corpuscles,  and  not  merely 
absorbed,  as  one  might  be  led  to  suppose  from  its  coming  away 
when  the  pressure  is  reduced,  are  numerous  and  satisfactory. 

1.  When  arterial  blood  is  submitted  to  gradual  diminution  of 
pressure  in  the  mercurial  air  pump  the  oxygen  does  not  come 
away  in  accordance  with  the  established  law  of  the  absorption  of 
gases  (Henry-Dalton)  by  coming  off  in  proportion  to  the  dimi- 
nution of  the  pressure.     At  first  only  traces  appear  (probably  the 
small  amount  really  dissolved),  and  when  the  pressure  has  been  re- 
duced to  a  certain  point,  about  one-fifth  of  that  of  the  atmosphere, 
the  oxygen  comes  off'  suddenly ;  after  which  little  more  can  be 
obtained  by  further  reduction  of  pressure.     Haemoglobin  com- 
bines with  O  in  the  same  way,  very  rapidly  at  first,  even  when  the 
pressure  is  low. 

2.  If  the  oxygen  were  only  in  a  state  of  absorption,  the  blood, 
while  passing  through  the  pulmonary  capillaries,  could  only  take 
up  about  0.4  volume  per  cent.,  which  would  be  inadequate  for 
life.     We  know  that  the  quantity  of  O  going  to  the  blood  from 
the  air  in  the  alveoli   cannot   well   be   explained   on  physical 


356  MANUAL   OF    PHYSIOLOGY. 

grounds  alone ;  and  when  an  animal  dies  of  asphyxia  from  want 
of  ventilation  in  a  limited  space,  all  the  O  of  the  air  in  the  space 
is  absorbed.  Since  the  partial  pressure  of  the  O  in  the  chamber 
falls  to  zero  while  some  still  exists  in  the  haemoglobin,  it  cannot 
be  the  pressure  which  makes  the  O  pass  into  the  blood. 

3.  Another  conclusive  proof  that  the  union  of  the  O  with  the 
haemoglobin  is  really  a  chemical  one,  is  given  by  the  spectroscopic 
examination  of  a  haemoglobin  solution.      When  deprived  of  its 
O,  and  after  the  admixture  of  the  air,  quite  dissimilar  spectra 
are  seen,  as  already  pointed  out  in  Chapter  xiv.     (Fig.  155,  p. 
357.) 

4.  The  amount  of  O  taken  up  by  the  blood  is  not  always  in 
proportion  to  the  pressure  of  that  gas,  but  rather  to  the  amount 
of  haemoglobin  in  the  blood  ;  and  we  therefore  find  the  adequacy 
of  the  respiratory  function  of  the  blood  going  hand  in  hand  with 
its  richness  in  haemoglobin,  and  thus  the  "  shortness  of  breath  " 
of  anaemic  and  chlorotic  individuals  is  explained. 

5.  The  oxygen  can  be  displaced  by  the  chemical  union  of  other 
gases  with  the  haemoglobin. 

Our  knowledge  concerning  the  relation  of  the  CO2  to  the  con- 
stituents of  the  blood  is  less  definite. 

It  does  not  all  exist  as  a  mere  physical  solution,  for  it  comes 
off  irregularly  under  the  air  pump,  and  does  not  exactly  obey 
the  Henry-Dalton  law  of  the  absorption  of  gases.  Part  comes 
off  easily  and  part  with  difficulty.  It  is  not  associated  with  the 
corpuscles,  for  more  of  this  gas  can  be  obtained  from  serum  than 
from  a  like  quantity  of  blood.  It  is  more  easily  removed  from 
the  blood  than  from  the  serum,  a  certain  proportion  (about  7  per 
cent,  of  the  whole)  remaining,  in  the  serum  in  vacuo,  until  dis- 
sociated by  the  addition  of  an  acid  or  a  piece  of  clot  containing 
corpuscles.  If  bicarbonate  of  soda  be  added  to  blood  from  which 
all  the  gas  has  been  removed,  still  more  CO2  can  be  pumped  out, 
from  which  it  would  appear  that  something  exists  in  the  blood 
capable  of  dissociating  CO2  from  sodium  bicarbonate. 

It  has  been  suggested  that  the  C02  is  in  some  way  associated 
(possibly  as  sodium  bicarbonate)  with  the  plasma  of  the  blood, 
and  that  the  corpuscles  have  the  power  of  acting  like  a  weak 


CHANGES  OF  BLOOD  IN  LUNGS. 


357 


0> 


FIG.  155.— Spectra  of  Oxyhsemoglobin,  reduced  haemoglobin, and  CO-hsemoglobin.  (Gam- 
gee.)    1,  2,  3,  and  4.    Oxyhsemoglobin  increasing  in  strength  or  thickness  of  solution. 
5.  Reduced  haemoglobin.  6.  CO-hseraoglobin. 


358  MANUAL   OF   PHYSIOLOGY. 

acid,  and  of  dissociating  it  from  the  soda,  and  thus  raising  its 
tension  in  the  blood. 

The  great  importance  of  the  chemical  nature  of  the  union 
between  the  O  and  hsemoglobin  for  external  respiration  becomes 
most  striking  when  the  actual  manner  in  which  the  entrance  of 
the  O  is  effected  is  taken  into  account. 

It  must  be  remembered  that  the  further  we  trace  the  air  down 
the  passages,  the  less  will  be  the  percentage  of  O  found  in  it,  and, 
therefore,  a  less  pressure  exerted  by  that  gas.  This  is  shown  by 
the  fact  that  the  air  given  out  by  the  latter  half  of  a  single  ex- 
piration has  less  O  and  more  CO2  than  that  of  the  first  half.  The 
most  impure  air  lies  in  the  alveoli  of  the  lungs,  for,  since  the 
tidal  air  scarcely  fills  the  larger  tubes,  the  air  in  the  alveoli  is 
only  changed  by  diffusion  with  the  impure  air  of  the  small 
bronchi.  Any  impediment  to  the  ordinary  ventilation  of  the 
alveoli  so  reduces  the  percentage,  and,  therefore,  the  tension  of 
the  O,  that  it  would  probably  sink  below  that  in  the  blood,  and 
in  that  case,  were  it  not  a  chemical  union,  the  O  would  escape 
more  readily  from  the  blood  in  proportion  as  its  tension  in  the 
blood  exceeded  that  of  the  air  of  the  alveoli.  We  know,  how- 
ever, that  the  blood  retains  a  considerable  quantity  of  oxygen 
even  in  the  intense  dyspnoea  of  suffocation. 

In  the  same  way  the  difference  of  tension  of  the  CO2  in  the 
alveolar  air  and  in  the  blood  hardly  explains  the  steady  manner 
in  which  the  CO2  escapes,  and  it  has,  therefore,  been  suggested 
that  this  escape  also  depends  in  some  way  upon  a  chemical  pro- 
cess, possibly  connected  with  the  union  of  the  O  and  haemoglobin  ; 
because  the  admission  of  O  to  the  blood  seems  to  facilitate  the 
exit  of  the  C02. 

The  following  table  gives  the  approximate  tension  of  the  two 
gases  in  the  different  steps  of  the  interchange  in  the  case  of  dogs 
with  a  bronchial  region  occluded  so  that  the  air  it  contained 
could  be  examined.  It  shows  that  the  tensions  are  such  as  to 
enable  physical  absorption  to  take  some  share  in  the  entrance  of 
the  O  as  well  as  in  the  escape  of  the  CO2.  A  separate  column 
gives  the  volumes  per  cent,  of  each  gas,  corresponding  to  these 
tensions  as  compared  with  the  atmospheric  standard.  Thephys- 


INTERNAL    RESPIRATION. 


359 


ical  process  must  occur  before  the  oxygen  and  the  haemoglobin 
meet,  since  the  latter  is  bathed  in  the  plasma,  and  further  sepa- 
rated from  the  alveolar  O  by  the  vessel  wall  and  epithelium. 


C 

02 

( 

) 

Tension 
in  mm.  Hg. 

Correspond- 
ing Volume 
per  cent. 

Tension 
in  mm.  Hg. 

Correspond- 
ing Volume 
per  cent. 

In  arterial  blood  
In  venous  blood  
In  air  of  alveoli  
In  expired  air  
In  atmosphere  

21. 
41. 
27. 
21. 
0  38 

2.8 
5.4 
3.56 
2.8 
0.04 

29.6 
22. 
27.44 
126.2 
158 

3.9 
2.9 
3.6 
16.6 
20.8 

INTERNAL  RESPIRATION. 

The  arterial  blood,  while  flowing  through  the  capillaries  of  the 
systemic  circulation  and  supplying  the  tissues  with  nutriment, 
undergoes  changes  which  are  called  internal  or  tissue  respiration, 
and  which  may  be  shortly  defined  to  be  the  converse  of  pul- 
monary or  external  respiration.  In  the  external  respiration  the 
blood  is  changed  from  venous  to  arterial ;  whereas  in  internal 
respiration  the  blood  is  again  rendered  venous. 

There  can  now  be  no  doubt  that  these  chemical  changes  take 
place  in  the  tissues  themselves,  and  not  in  the  blood  as  it  flows 
through  the  vessels.  The  amount  of  oxidation  that  takes  place 
in  the  blood  itself  is  indeed  very  small.  The  tissues,  however, 
along  with  the  substances  for  their  nutrition,  extract  a  certain 
part  of  the  O  from  the  blood.  In  the  chemical  changes  which 
take  place  in  the  tissues,  they  use  up  the  oxygen,  which  rapidly 
disappears,  the  tension  of  that  gas  becoming  very  low ;  at  the 
same  time  other  chemical  changes  are  indicated  by  the  appear- 
ance of  C02.  The  disappearance  of  the  O  and  the  manufacture 
of  CO2  do  not  exactly  correspond  in  amount,  and  they,  doubtless, 
often  vary  in  different  parts  and  under  different  circumstances. 
Of  the  intermediate  steps  in  the  tissue  chemistry  we  are  ignorant. 
We  do  not  know  the  way  in  which  the  oxygen  is  induced  by  the 
tissues  to  leave  the  haemoglobin ;  we  can  only  say  that  the  tissues 


360  MANUAL   OP   PHYSIOLOGY. 

have  a  greater  affinity  for  O  than  the  haemoglobin  has,  and  they 
at  once  convert  the  O  into  more  stable  compounds  than  oxy- 
hsemoglobin,  and  ultimately  manufacture  CO2,  which  exists  in  the 
tissues  and  fluids  of  the  body  at  a  higher  tension  than  even  in 
the  venous  blood. 

RESPIRATION  OF  ABNORMAL  AIR,  ETC. 

The  oxygen  income  and  carbonic  acid  output  are  the  essential 
changes  brought  about  by  respiration,  therefore  the  presence  of 
oxygen  in  a  certain  proportion  is  absolutely  necessary  for  life. 
The  21  per  cent,  of  O  of  the  atmosphere  suffices  to  saturate  the 
haemoglobin  of  the  blood,  and  14  per  cent,  of  O  has  been  found 
to  be  capable  of  sustaining  life  without  producing  any  marked 
change  in  respiration. 

Dyspnoea  is  produced  by  an  atmosphere  containing  only  7.5 
per  cent,  of  O.  This  dyspnoea  rapidly  increases  as  the  percent- 
age of  O  is  further  decreased,  and  when  it  gets  as  low  as  3  per 
cent,  suffocation  speedily  ensues. 

The  output  of  C02  can  be  accomplished  if  the  lungs  be  venti- 
lated by  any  harmless  or  indifferent  gas,  and  since  the  manufac- 
ture of  the  CO2  does  not  take  place  in  the  lungs,  its  elimination 
can  go  on  independently  of  the  quantity  of  O  in  them.  The  79 
per  cent,  of  N  contained  in  the  atmosphere  has  a  passive  duty  to 
perform  in  diluting  the  O  and  facilitating  the  escape  of  the  CO2 
from  the  lungs. 

Indifferent  gases  are  those  which  produce  no  unpleasant  effect 
of  themselves,  but  which,  in  the  absence  of  O,  are  incapable  of 
sustaining  life,  such  as  nitrogen,  hydrogen,  and  CH4. 

Irrespirable  gases  are  such  as,  owing  to  the  irritating  effect  on 
the  air  passages,  cannot  be  respired  in  quantity,  as  they  cause 
instant  closure  of  the  glottis.  In  small  quantities  they  irritate 
and  produce  cough,  and  if  persisted  in,  inflammation  of  the  air 
passages;  among  these  are  chlorine,  ammonia,  ozone,  nitrous, 
sulphurous,  hydrochloric,  and  hydrofluoric  acids. 

Poisonous  gases  are  those  which  can  be  breathed  without  much 
inconvenience,  but  when  brought  into  union  with  the  blood  cause 
death.  Of  these  there  are  many  varieties.  (1)  Those  which 


VENTILATION.  361 

permanently  usurp  the  place  of  oxygen  with  the  haemoglobin, 
viz. :  carbon  monoxide  (CO),  hydrocyanic  acid  (HCN).  (2) 
Narcotic :  (a)  Carbonic  dioxide  (CO2),  of  which  10  per  cent,  is 
rapidly  fatal,  1.0  per  cent,  is  poisonous,  and  over  0.1  per  cent, 
injurious.  (/5)  Nitrogen  monoxide  (N2O).  Both  of  these  gases 
lead  to  a  peculiar  asphyxia  without  convulsions.  (7)  Chloro- 
form, ether,  etc.  (3)  Sulphuretted  hydrogen  (H2S),  which  reduces 
the  oxy haemoglobin  and  produces  sulphur  and  water.  (4)  Phos- 
phuretted  hydrogen  (PH2),  arseniuretted  hydrogen  (AsH2),  and 
cyanogen  gas  (C,N2)  also  have  specially  poisonous  effects. 

VENTILATION. 

In  the  open  air  the  effects  of  respiration  on  the  atmosphere 
cannot  be  appreciated,  but  in  enclosed  spaces,  such  as  houses, 
rooms,  etc.,  which  are  occupied  by  many  persons,  the  air  soon 
becomes  appreciably  changed  by  their  breathing. 

The  most  important  changes  are  (1)  removal  of  oxygen,  (2) 
increase  in  carbonic  acid,  and  (3)  the  appearance  of  some  poison- 
ous materials  which,  though  highly  injurious,  cannot  be  deter- 
mined. The  deficiency  in  oxygen  never  -causes  any  inconvenience, 
as  it  is  never  reduced  below  what  is  sufficient  for  the  saturation 
of  the  haemoglobin.  The  excess  of  CO2  seldom  gives  any  incon- 
venience, since  the  air  becomes  disagreeably  fusty  or  stuffy  long 
before  the  amount  of  CO2  from  breathing  has  reached  0.1  per 
cent.,  which  amount  of  pure  CO2  can  be  inspired  without  any 
unpleasantness.  It  is,  then,  the  exhalations  coming  from  the 
lungs,  and  probably  skin,  some  of  which  must  have  a  poisonous 
character,  that  render  the  proper  supply  of  fresh  air  imperative. 

The  difficulty  of  determining  the  presence  of  the  poisonous 
organic  materials  makes  it  convenient  to  use  the  amount  of  CO2 
present  in  the  air  as  the  means  of  measuring  its  general  purity. 
For  this  we  must  suppose  that  the  relation  between  the  poisonous 
organic  ingredients  and  the  CO2  is  constant. 

Air  which  is  rendered  impure  by  breathing  becomes  disagree- 
able to  the  sense  of  smell  when  the  CO2  has  reached  the  low 
standard  of  .06  or  .08  per  cent.,  that  is  to  say,  scarcely  twice  as 
much  CO2  as  is  contained  in  the  pure  atmosphere.  Supposing 
31 


362  MANUAL   OP    PHYSIOLOGY. 

that  air  is  unwholesome  when  its  impurities  are  appreciable  by 
the  senses,  then,  if  the  animul  body  be  the  source  of  the  CO2,  .06 
per  cent,  of  this  gas  makes  the  air  unfit  for  use. 

An  adult  man  disengages  more  than  half  a  cubic  foot  of  CO.., 
in  one  hour  (.6,  Parkes),  and  consequently  in  that  time  he  renders 
quite  unfit  for  use  more  than  1000  cubic  feet  of  air,  by  raising 
the  percentage  of  CO2  to  .1  (0.4  being  initial,  and  .06  respiratory). 

It  is  obvious  that  the  smaller  the  space  and  the  more  confined, 
the  more  rapidly  will  the  air  become  vitiated  by  respiration.  It 
becomes  necessary  for  health,  therefore,  to  have  not  only  a  certain 
cubic  space  and  a  certain  change  of  air  for  each  individual,  but 
the  cubic  space  and  the  change  of  air  should  bear  to  each  other 
a  certain  proportion,  in  order  that  the  air  may  remain  sufficiently 
pure. 

The  space  allowed  in  public  institutions  varies  from  500  to 
1500  cubic  feet  per  head,  in  such  apartments  as  are  occupied  by 
the  individuals  day  and  night.  As  a  fair  average  1000  cubic 
feet  may  be  fixed  as  the  necessary  space  in  a  perfect  hygienic 
arrangement.  In  order  to  keep  this  perfectly  wholesome  and 
free  from  a  stuffy  smell,- and  the  CO.2  below  .06  per  cent.,  it  is 
necessary  to  supply  some  2000  cubic  feet  of  air  per  head  per 
hour. 

To  give  the  necessary  supply  of  fresh  air  without  introducing 
draughts  or  greatly  reducing  the  temperature  of  the  room 
is  no  easy  matter,  and  forms  the  special  study  of  the  hygienic 
engineer. 

ASPHYXIA. 

If  an  adequate  supply  of  oxygen  be  withheld  and  its  percent- 
age in  the  blood  is  reduced  to  a  certain  point,  the  death  of  the 
animal  follows  in  three  to  five  minutes,  accompanied  by  a  series  of 
phenomena  commonly  included  under  the  term  asphyxia.  This 
may  be  divided  into  four  stages.  1.  Dyspnoea.  2.  Convulsion. 
3.  Exhaustion.  4.  Inspiratory  spasm.  As  asphyxia  is  a  mode 
of  death  the  symptoms  of  which  the  physician  can  be  called  upon 
to  treat,  he  should  be  able  to  recognize  its  different  phases. 

If  the  air  passages  be  closed  completely  the  respirations  become 
deep,  labored  and  rapid.  The  respiratory  efforts  are  more  and 


ASPHYXIA.  363 

more  energetic,  and  the  various  supplementary  muscles  are  called 
into  play  one  after  the  other,  until  gradually  the  second  stage  is 
reached  in  about  one'  minute. 

As  the  struggles  for  air  become  more  severe,  the  inspiratory 
muscles  lose  their  power,  and  the  expiratory  efforts  become  more 
and  more  marked,  until  finally  the  entire  body  is  thrown  into  a 
general  convulsion,  in  which  the  traces  of  a  rhythm  are  hardly 
apparent.  This  stage  of  convulsion  is  short,  the  expiratory 
muscles  becoming  suddenly  relaxed  by  exhaustion. 

Then  the  longest  stage  arrives,  in  which  the  animal  lies  almost 
motionless,  making  some  quiet  inspiratory  attempts.  These  be- 
come gradually  deeper  and  slower,  until  they  are  nothing  more 
than  deep  gasps  separated  by  long  irregular  intervals. 

The  pupils  of  the  eyes  become  widely  dilated,  the  pulse  can 
hardly  be  felt,  and  the  animal  lies  apparently  dead,  when  often, 
after  a  surprisingly  long  interval,  one  or  more  respiratory  gasps 
follow,  and  with  a  gentle  tremor  the  animal  stretches  itself  in  a 
kind  of  tonic  inspiratory  spasm,  after  which  it  is  no  longer  capa- 
ble of  resuscitation.  This  last  pulseless  stage,  to  which  the  term 
asphyxia  is  more  properly  confined,  is  the  most  irregular  in  dura- 
tion, but  always  the  longest. 

The  blood  of  an  animal  which  has  died  of  asphyxia  is  nearly 
destitute  of  oxygen,  the  hsemoglobin  being  in  a  much  more 
reduced  condition  than  is  found  in  venous  blood.  The  first  and 
most  obvious  effect  produced  by  the  circulation  of  blood  so  defi- 
cient in  oxygen  is  excessive  stimulation  of  the  respiratory  centre, 
which  causes  the  extreme  and  varied  actions  just  described.  In 
the  first  stage  of  asphyxia,  the  venous  blood,  reaching  the  systemic 
arterioles,  affects  their  muscular  walls,  exciting  the  vaso-con- 
strictor  mechanism,  so  as  to  cause  a  rapid  and  considerable  rise 
in  blood  pressure  and  consequent  distention  of  the  left  ventricle. 
The  general  constriction  of  the  small  arteries  may  be  brought 
about  by  the  venous  blood  acting  as  a  stimulus  to  the  cells  of  the 
medullary  and  spinal  vasomotor  centres,  or  more  probably  it 
acts  as  a  direct  stimulant  to  the  muscle  cells  of  the  arterioles 
themselves.  The  centres  in  the  medulla  which  govern  the  inhib- 
itory fibres  of  the  pneumogastric  are  also  stimulated,  and  con- 


364  MANUAL    OF    PHYSIOLOGY. 

sequeutly  the  heart  beats  more  slowly.  The  increase  in  arterial 
tension  and  the  slow  beat  give  rise  to  distention  of  the  ventricle, 
which,  when  a  certain  point  is  reached,  impedes  the  working  of 
the  heart,  and  its  muscle  begins  to  beat  more  and  more  feebly,  so 
that  in  the  third  stage  the  pulse  can  hardly  be  felt.  The  mus- 
cular arterioles  then  become  exhausted  and  relax,  the  blood 
pressure  falls  rapidly,  and  with  the  death  of  the  animal  it  reaches 
the  level  of  atmospheric  pressure.  Both  sides  of  the  heart  and 
great  veins  are  engorged  with  blood  in  the  last  stage  of  asphyxia; 
the  cardiac  muscle  being  exhausted,  from  want  of  oxygen,  is 
unable  to  pump  the  blood  out  of  the  veins  or  empty  its  cavities. 
Owing  to  the  force  of  the  rigor  mortis  of  the  left  ventricle,  and 
the  greater  capacity  of  the  systemic  veins,  the  left  side  is  found 
comparatively  empty  some  time  after  death,  and  at  post-mortem 
examination  the  right  side  alone  is  found  over-filled. 


BLOOD-ELABORATING    GLANDS.  365 


CHAPTER   XX. 
BLOOD-ELABORATING  GLANDS. 

In  the  preceding  chapters  we  have  seen  that  the  blood  under- 
goes important  changes  as  it  courses  through  the  different  parts 
of  its  circuit.  Where  it  comes  in  contact  with  the  tissues  it  yields 
to  them  nutrient  material  for  assimilation,  and  oxygen  for  their 
metabolism,  and  carries  away  from  them  some  waste  products. 
In  the  lungs  it  receives  oxygen  and  gives  off  carbonic  acid. 
While  it  flows  through  the  minute  vessels  of  the  alimentary  tract, 
some  of  the  materials  elaborated  by  the  digestion  of  food  are 
absorbed,  and  directly  added  to  the  blood ;  at  the  confluence  of 
the  great  veins  in  the  neck  the  stream,  composed  of  lymph  and 
chyle,  is  poured  into  the  blood  before  it  enters  the  heart,  so  as  to 
be  thoroughly  mingled  with  it  on  its  return  from  the  general 
circulation.  Moreover,  in  various  glands,  different  substances 
are  used  in  the  manufacture  of  their  secretions. 

Thus  there  is  a  kind  of  material  circulation,  a  constant  income 
and  output  going  on  in  the  blood  itself  as  it  passes  through  the 
different  parts  of  the  body.  The  investigation  of  the  exact 
changes  which  take  place  in  the  blood  in  each  organ  or  part  is 
surrounded  with  difficulty,  and  in  many  cases  it  is  quite  impos- 
sible to  ascertain  what  changes  occur.  In  some  parts  it  may  be 
made  out  by  noting  the  results  produced,  or  the  substances  given 
off  or  taken  up  by  the  blood,  as  seen  in  the  changes  found  in  the 
air  after  its  exposure  to  the  blood  in.  the  lungs,  where  we  can 
definitely  state  that  the  blood  has  lost  or  gained  certain  materials, 
and  is  so  far  altered.  In  other  parts,  such  as  the  muscles  or  the 
ductless  glands,  where,  no  doubt,  profound  changes  in  the  blood 
occur,  we  have  no  separate  outcome  which  we  can  analyze,  and 
we  must  therefore  trust  altogether  for  the  elucidation  of  the 
change  going  on  in  them  to  the  differences  which  may  be  found 
to  exist  in  the  blood  flowing  to,  and  that  flowing  from,  such  an 
organ.  For  this  purpose  one  can  either  examine  samples  of  the 


•366 


MANUAL   OF   PHYSIOLOGY. 


Fm.  155. 


blood  from  the  artery  and  vein  of  the  organ,  while  the  ordinary 
circulation  is  going  on,  or,  immediately  after  the  removal  of  the 

organ,  by  causing  the  artificial 
stream  of  blood  to  flow  through 
it;  then  the  changes  brought 
about  in  the  blood  in  its  pas- 
sage through  the  organ  will 
give  the  required  information. 
It  can  be  seen,  from  the  fore- 
going enumeration  of  processes, 
that  some  organs  have  a  double 
function  as  regards  the  blood. 
Thus,  in  the  lung  there  is  both 
renovation  by  taking  in  oxy- 
gen, and  purification  by  getting 
rid  of  carbon  dioxide.  The 
textures  in  their  internal  respi- 
ration take  the  nutriment  and 
oxygen,  and  give  the  blood 
CO2  and  various  other  waste 
products  of  tissue  change. 

DUCTLESS  GLANDS. 

There  is  a  certain  set  of 
organs  which  have  but  slight 
traits  of  resemblance  to  "one 
another,  and  in  consequence  of 

the     Want     of     more     accurate 

knowledge    as  to"  their     exact 

/.                              ,  ,          n     t      ^ 

lUnctlOIl,     and  the     lact     that 

,  i           j  ,  i     • 

they  do  not  pour  their  pro- 
ducts  into  ducts,  but  probably 

into  the  blood  current,  are  commonly  grouped  together  as  duct- 

less or  blood  glands. 

It  has  been  shown  that  a  great  part  of  the  absorbed  nutrient 

material  passes  through  a  special  set  of  vessels  called  the  lacteals 


Vertical  section  of  jhejupra-renal  Capsule, 

1.  Cortex.  2.  Medulla,  a.  Fibrous  capsule, 
b.  External  cell  masses,  c.  Columnal 
layer,  d.  Internal  cell  masses,  e.  Medul- 
lary  substance,  in  which  lies  a  large  vein, 

partly  seen  in  section/. 


THYROID    BODY. 


367 


or  lymphatics,  and  in  so  doing  has  to  traverse  peculiar  organs 
called  lymphatic  glands,  where  it  is  no  doubt  modified,  and  has 
added  to  it  a  number  of  cells  (lymph  corpuscles)  which  sub- 
sequently are  poured  into  the  large  veins  with  the  lymph  and 
become  important  constituents  of  the  blood. 

Some  of  these  blood  glands  are  doubtless  nearly  akin  to  the 
lymphatic  glands  already  described  (Fig.  151),  their  duty  being 
the  further  elaboration  and  perfection  of  the  blood.  In  this 
group  are  commonly  placed  the  supra-renal  capsules,  the  thyroid 
the  thyrnus,  and  the  spleen. 


Section  of  the  Thyroid  Gland  of  a  child,  showing  two  complete  sacs  and  portions  of  others. 
The  homogeneous  colloid  substance  is  represented  as  occupying  the  central  part  of  the 
cavity  of  the  vesicles,  which  are  lined  by  even  cubical  epithelium.  (Schitfer.) 


SUPRA-RENAL    CAPSULE. 

With  regard  to  the  function  of  the  supra-renal  capsule  we 
may  say  that  nothing  definite  is  known.  The  cortical  part  is  said 
to  resemble  the  lymph  follicles  in  structure,  while  the  central 
part,  on  account  of  its  numerous  peculiar,  large  cells  and  great 
richness  in  nerves,  has  been  explained  as  belonging  to  the  nervous 
system. 

THYROID  BODY. 

The  thyroid  is  made  up  of  groups  of  minute  closed  sacs 
embedded  in  a  stroma  of  connective  tissue,  lined  with  a  single 


368 


MANUAL   OF    PHYSIOLOGY. 


row  of  epithelium  cells,  and  filled  with  a  clear  fluid  containing 
nmcin.  In  the  adult  the  sacs  are  commonly  much  distended 
with  a  colloid  substance  and  peculiar  crystals,  and  the  epithelium 
has  disappeared  from  their  walls.  Although  said  to  be  rich  in 
lymphatics  and  to  contain  follicular  tissue,  positive  proof  of  the 


FIG. 


Portion  of  Thymus  re- 
moved from  its  envel- 
ope and  unraveled 
so  as  to  show  the 
lobules  (6,  6)  attached 
to  a  central  band  of 
connective  tissue  (a). 


FIG.  159. 


Magnified  section  of  a  portion  of  injected  Thymus,  showing 
one  complete  lobule,  with  soft  central  part  (cavity)  (6),  anc 
parts  of  other  lobules.    (Cadiat.) 
(a)  Lymphoid  tissue,     (c)  Blood  vessels,    (d)  Fibrous  tissue. 

FIG.  160. 


Elements  of  Thymus  (high  power).    (Cadiat.)    (a)  Lymph 
corpuscles.     (6)  Epitheloid  nests  of  Hassall. 


relation  of  the   thyroid  body  to   the  lymphatic  system  is  still 
wanting. 

THYMUS  GLAND. 

The  functional  activity  of  the  thy m us  is  restricted  to  that 
period  of  life  when  growth  takes  place  most  rapidly.    It  is  well 


SPLEEN.  369 

developed  in  the  foetus,  and  increases  in  size  for  a  couple  of 
years  after  birth  ;  but  it  gradually  diminishes  in  bulk  and  loses 
its  original  structure  during  the  later  periods  of  childhood,  so  as 
to  become  completely  degenerated  and  fatty  in  the  adult.  It  is 
composed  of  numerous  little  follicles  of  lymphoid  tissue  collected 
into  groups  or  lobules  connected  to  a  kind  of  central  stalk.  The 
lymphoid  follicles  of  the  young  thymus  have  some  likeness  to 
those  of  the  intestinal  tract,  but  they  differ  from  these  agminate 
glands  not  only  in  arrangement  but  also  in  having  peculiar  small 
nests  of  large  cells  (corpuscles  of  Hassall)  in  the  midst  of  the 
adenoid  tissue  of  which  they  are  made  up.  On  account  of  the 
structure  of  the  lobules  being  so  nearly  identical  with  that  of  a 
lymphatic  gland,  and  from  its  great  richness  in  lymphatic  vessels, 
the  thymus  is  said  to  be  related  to  the  lymphatic  system,  and  is 
supposed  to  play  an  important  part  in  the  elaboration  of  the 
blood  during  the  earlier  stages  of  animal  life. 

SPLEEN. 

Structure. — The  spleen  also  resembles  a  lymphatic  organ  in 
structure,  but  differs  from  it  in  the  relation  borne  by  the  blood 

FIG.  161. 


(a)  Trabeculse  of  the  Spleen.         (6)  Artery  cut  obliquely.    (Cadiat.) 

to  the  elements  of  the  follicular  tissue.  It  is  encased  in  a  strong 
capsule  made  of  fibrous  tissue  and  unstriated  muscle  cells.  From 
this  many  branching  prolongations  pass  into  the  substance  of  the 


370 


MANUAL    OF    PHYSIOLOGY. 


FIG.  162. 


Reticulum  of  the  Spleen  Pulp  injected 
with  colorless  gelatine.  (Oadial.) 

(a)  Meshes  made  of  endotheliuin. 

(6)  Lacunar  spaces,  through  which  the 
blood  flows. 

(c)  Nuclei  of  endothelium. 


organ,  so  as  to  traverse  the  soft,  red,  spleen  pulp.     In  these  tra- 
beculse  or  prolongations  from  the  capsule  are  found  the  branches 

of  the  splenic  artery,  dividing 
into  smaller  twigs  without  anas- 
tomosis. On  leaving  the  tra- 
beculse  the  arteries  break  up  sud- 
denly into  a  brush-like  series  or 
small  branches,  ending  in  capil- 
laries, which  are  lost  in  the  pulp 
where  the  small  veins  may  be 
seen  to  commence. 

Between  the  trabeculse  are 
found  two  distinct  kinds  of  tissue: 
(1)  Rounded  masses  of  lymphoid 
tissue,  called  Malpighian  bodies, 
scattered  here  and  there  through 
the  organ ;  and  (2)  the  peculiar 
soft  splenic  pulp  making  up  its  bulk. 

The  small  rounded  masses  of  lymph  follicular  tissue  are  sit- 
uated on  the  course  of  the  fine  arterial  twigs.  The  delicate 
adenoid  reticulum  which  holds  the  lymph  cells  together  is  inti- 
mately connected  with  the  vessel  wall.  The  pale  appearance 
of  these  follicles,  which  distinguishes  them  from  the  surround- 
ing splenic  pulp,  depends  on  the  number  of  the  white  cells 
which  are  packed  in  the  meshes  of  this  peri-vascular  adenoid 
tissue. 

The  splenic  pulp  consists  of  a  system  of  communicating  lacunar 
spaces  lined  with  endothelium.  Into  these  spaces  the  blood  is 
poured  from  the  arteries,  and  thus  mingles  with  vast  numbers  of 
white  cells.  Besides  the  ordinary  blood  discs  and  the  white  cor- 
puscles or  lymph  cells,  many  peculiar  cells  are  found  in  the 
spleen  pulp.  Some  of  these  look  like  lymph  cells  containing 
little  masses  of  haemoglobin,  and  appear  to  be  transitions  from 
the  colorless  to  the  reef  corpuscles,  while  some  small,  misshapen, 
red  corpuscles  are  regarded  as  steps  in  a  retrograde  change  in 
the  discs.  But  few,  if  any,  lymph  channels  lead  from  the  spleen 
pulp,  and  only  a  relatively  small  number  pass  out  from  the  hilus, 


SPLEEN.  371 

so  that  the  splenic  artery  and  vein  must  be  regarded  as  taking 
the  places  of  the  afferent  and  efferent  lymph  channels. 

Chemical  Composition  of  the  Spleen  Pulp. — Chemical  examina- 
tion shows  the  splenic  pulp  to  have  remarkable  peculiarities.  Al- 
though so  full  of  blood,  which  is  generally  alkaline,  the  spleen  is 
acid  in  reaction,  and  contains  a  great  quantity  of  the  oxidation 
products  (so-called  extractives}  commonly  found  as  the  result  of 
active  tissue  change.  The  chief  of  these  are  uric  acid,  leucin, 
xanthin,  hypoxanthin,  inosit,  lactic,  formic,  succinic,  acetic  and 
butyric  acids.  It  also  contains  numerous  pigments,  rich  in  carbon, 
but  little  known,  which  are  probably  the  outcome  of  destroyed 
haemoglobin.  A  peculiarly  suggestive  constituent  is  an  albumi- 
nous body  containing  iron.  The  ash  is  found  to  contain  a  con- 
siderable quantity  of  oxide  of  iron,  to  be  rich  in  phosphates  and 
soda,  with  but  small  quantities  of  chlorides  and  potassium. 

Changes  in  the  Blood  in  the  Spleen. — If  the  blood  flowing  in  the 
artery  to  the  spleen  be  compared  with  that  in  the  vein,  the  dif- 
ference gives  us  the  changes  the  blood  has  undergone  in  the 
organ,  and  hence  is  of  great  importance.  In  the  blood  of  the 
vein  is  found  an  enormous  increase  in  the  number  of  white  cor- 
puscles (1  white  to  70  red  in  the  vein,  as  against  1  to  2000  in  the 
splenic  artery).  The  red  corpuscles  from  the  vein  are  smaller, 
brighter,  less  flattened  than  those  of  ordinary  blood ;  they  do 
not  form  rouleaux,  and  are  more  capable  of  resisting  the  injurious 
influence  of  water.  The  blood  of  the  splenic  vein  is  also  said  to 
have  a  greater  proportion  of  water,  and  to  contain  an  unusual 
quantity  of  uric  acid  and  other  products  of  tissue  waste.  The 
amount  of  blood  in  the  spleen  varies  greatly  at  different  times. 
Shortly  after  meals  the  organ  becomes  turgid,  and  remains 
enlarged  during  the  later  periods  of  digestion. 

Pathological  Changes. — The  size  of  the  spleen,  which  may  be 
taken  as  a  measure  of  its  blood  contents,  is  also  altered  by  many 
abnormal  conditions  of  the  blood.  Thus,  in  all  kinds  of  fever, 
particularly  ague  and  typhoid,  and  in  syphilis,  the  spleen  becomes 
turgid,  and  in  some  of  these  diseases  it  remains  swollen  for  some 
time.  In  a  remarkable  disease,  leucocythsemia,  in  which  the 
white  blood  cells  are  greatly  increased  in  number,  and  the  red 


372  MANUAL   OF    PHYSIOLOGY. 

ones  are  comparatively  diminished,  the  spleen,  in  company  with 
the  lymphatic  glands,  is  often  found  to  be  profoundly  altered  and 
diseased,  and  commonly  immensely  enlarged  ;  but,  on  the  other 
hand,  advanced  amyloid  degeneration  of  the  spleen  may  occur 
without  any  notable  alteration  taking  place  in  the  number  or 
properties  of  the  blood  corpuscles. 

Extirpation  of  the  Spleen. — The  spleen  may  be  removed  from 
the  body  without  any  marked  changes  taking  place  in  the  blood 
or  the  economy  generally.  It  is  said  that  if  an  animal  whose 
spleen  is  extirpated  be  allowed  to  live  for  a  certain  time,  the  lym- 
phatic glands  increase  in  size,  or  become  swollen. 

In  attempting  to  assign  a  definite  function  to  the  spleen  all  the 
foregoing  facts  must  be  carefully  reviewed,  and  the  peculiarity  of 
its  (1)  structure,  (2)  chemical  composition,  (3)  the  changes  the  blood 
undergoes  while  flowing  through  it,  (4)  the  variations  in  blood 
supply  which  follow  normal  and  pathological  changes  in  the 
economy,  and  (5)  the  absence  of  effect  following  its  extirpation, 
must  all  be  borne  in  mind. 

Its  structure  teaches  us  that  it  is  intimately  related  to  lymphatic 
glands.  The  Malpighian  bodies  are  simply  lymph  follicles,  and 
the  pulp  may  be  regarded  as  a  sinus  like  that  of  a  lymph  gland, 
with  this  difference,  that  it  is  traversed  by  blood  instead  of  lymph. 
The  cell  elements  found  in  it  indicate  that  not  only  white  cells 
are  rapidly  generated,  but  also  that  these  cells  have  some  peculiar 
relationship  to  hsemoglobin,  as  they  are  often  found  to  contain 
some.  The  varieties  in  size,  form,  and  general  appearance  of  the 
red  corpuscles  can  be  accounted  for  by  either  their  destruction  or 
their  formation  occurring  in  this  organ. 

Its  chemical  composition  also  shows  that  certain  special  changes 
go  on  in  the  pulp,  and  that  probably  stages  of  the  construction 
or  destruction  of  haemoglobin  are  here  accomplished  may  be 
inferred  from  the  peculiar  association  of  iron  with  albuminous 
bodies. 

From  the  characters  of  the  blood  flowing  from  the  spleen  it 
has  been  argued  that,  besides  an  enormous  production  of  white 
corpuscles,  the  destruction  of  the  red  discs  goes  on,  while  some 
new  discs  are  formed,  probably  by  means  of  the  white  cells 


GLYCOGENIC    FUNCTION    OF    THE    LIVER.  373 

making  haemoglobin  in  their  protoplasm,  which,  gradually  dis- 
appearing, leaves  only  the  red  mass  of  haemoglobin. 

The  increased  activity  of  the  spleen  after  meals,  and  in  certain 
abnormal  states  of  the  blood,  as  shown  by  its  containing  more 
blood,  distinctly  points  out  that  some  form  of  blood  elaboration 
goes  on  in  it,  which  is  nearly  related  to,  or  associated  with,  nutri- 
tion. 

Fio.  163. 


Section  of  Spleen  through  a  lymph  follicle  (Malpighiau  body)  (a)  injected  to  show  the 
vessel  (c)  entering  the  follicle,  the  lymphoid  tissue  of  which  is  pale  in  comparison 
with  the  pulp  (6),  the  meshes  of  which  are  filled  with  injection.  (Cadiat.) 

The  swelling  of  the  lymphatic  glands  after  extirpation  of  the 
spleen  confirms  its  relation  to  these  organs,  and  the  fact  is  un- 
disputed that  it  is  a  source  of  the  white  corpuscles  of  the  blood ; 
but  the  paucity  of  evidence  after  this  operation  as  to  changes  in 
the  number  or  character  of  the  red  discs  proves  that  if  the  spleen 
be  either  the  place  of  origin  or  destruction  of  the  red  corpuscles 
it  cannot  be  the  only  organ  in  which  they  are  produced  or 
destroyed. 

GLYCOGENIC   FUNCTION   OF   THE  LIVER. 

Of  all  the  organs  that  modify  the  composition  of  the  blood 
flowing  through  them,  the  liver  plays  the  most  important  part  in 
elaborating  the  circulating  fluid.  The  elimination  of  the  various 


374 


MANUAL    OF    PHYSIOLOGY. 


FIG.  164. 


constituents  of  the  bile,  which  has  already  been  mentioned  as 
necessary  for  the  purification  of  the  blood,  and  useful  in  aiding 
absorption,  is  probably  but  a  secondary  function  of  this  great 
gland.  The  production  of  a  special  material — animal  starch — 

essential  to  the  nutrition 
and  growth  of  the  textures 
is  probably  the  most  im- 
portant duty  of  the  liver 
cells,  and  possibly  the  con- 
stituents of  the  bile  are 
but  the  by-products,  which 
must  be  got  rid  of,  result- 
ing from  this  and  other 
unknown  chemical  pro- 
cesses. 

In  the  chapter  on  the 
digestive  secretions  the 
structure  of  the  liver  was 
mentioned,  and  attention 
was  directed  to  the  peculi- 
arities of  its  double  blood 
supply.  A  relatively  small 
arterial  twig  carries  blood 
to  it  from  the  aorta,  while 
the  great  portal  veins  dis- 
tribute to  it  all  that  large 
ood  which 

flows  through  the  intestinal 
tract  and  the  spleen. 

The  blood  in  the  vena  porta  during  digestion  can  hardly  be 
called  venous  blood,  for  much  more  passes  through  the  intestinal 
capillaries  when  digestion  is  going  on  than  is  necessary  for  the 
nutrition  of  the  tissue  of  the  intestinal  wall.  The  portal  blood 
is  also  to  be  distinguished  from  ordinary  venous  blood  from 
the  fact  that  it  has  just  been  enriched  with  a  quantity  of  the 
soluble  materials  taken  from  the  intestinal  canal,  namely,  pro- 


Diagram  of  the  Portal  Vein  (p  v)  arising  in  the     o,inn]v    of 
alimentary  tract  and    spleen  (s),  and  carrying     bUPP17     ' 
the  blood  from  these  organs  to  the  liver. 


GLYCOGEN.  375 

teids,  sugar,  salts,  and  possibly  some  fats  ;  and  it  has  been  further 
modified  by  the  changes  taking  place  in  the  spleen. 

It  is  from  this  blood  that  the  liver  cells  manufacture  the  starch- 
like  substance  above  mentioned.  Animal  starch  was  discovered 
by  Claude  Bernard,  and  called  by  him  Glycogen,  on  account  of 
the  great  facility  with  which  it  is  converted  into  sugar  in  the 
presence  of  certain  ferments  which  exist  in  the  liver  itself  and  in 
most  tissues  after  death.  Shortly  after  the  death  of  an  animal 
the  tissue  of  the  liver,  and  also  the  blood  contained  in  the 
hepatic  veins,  are  extremely  rich  in  sugar,  which  has  been 
formed  by  the  fermentation  of  the  hepatic  glycogen.  The  quan- 
tity of  sugar  increases  with  the  length  of  time  that  has  elasped 
since  the  death  of  the  animal,  and  is  minimal,  if  not  nil,  if  the 
liver  or  hepatic  blood  be  taken  for  examination  while  the  tissue 
elements  are  still  alive. 

The  peculiar  blood  of  the  great  portal  vein  coming  from  the 
stomach,  intestines,  and  the  spleen  has  then  to  pass  through  a 
second  set  of  capillaries  in  the  liver,  and  undergoes  such 
important  changes  that  this  organ  must  be  regarded  as 
occupying  a  foremost  position  among  the.blood  glands.  Differ- 
ences of  the  utmost  importance  have  long  been  thought  to  exist 
between  the  blood  going  to  and  that  coming  from  the  liver,  and 
to  it  has  even  been  attributed  paramount  utility  as  a  blood  elab- 
orator  ;  but  the  scientific  knowledge  of  its  power  in  this  respect 
must  date  from  the  discovery  of  its  glycogenic  function. 

GLYCOGEN. 

Glycogen  is  a  substance  nearly  allied  to  starch  in  its  chemical 
composition,  and  is  converted  with  great  readiness  into  grape 
sugar  by  the  action  of  certain  ferments  and  acids.  Many  of  the 
animal  textures  contain  these  ferments,  among  others  the  liver 
itself,  at  least  when  its  tissue  is  dying;  and  consequently  the 
liver  with  the  blood  coming  from  it  (if  examined  in  an  animal 
some  time  dead)  does  not  contain  glyoogen,  but  sugar  which  has 
been  formed  from  it.  If  a  piece  of  liver  taken  from  an  animal 
immediately  after  it  is  killed  be  plunge  into  boiling  water,  so  as 
to  check  the  action  of  the  ferment,  no  trace  of  sugar  is  found  in 


376  MANUAL   OF   PHYSIOLOGY. 

it,  but  only  glycogen.  After  the  lapse  of  a  little  time  another 
piece  of  the  same  liver,  which  has  lain  at  the  ordinary  room 
temperature,  will  give  abundance  of  sugar. 

The  mode  of  preparation  of  glycogen  depends  upon  the  fore- 
going facts.  The  perfectly  fresh  liver  taken  from  an  animal 
killed  during  digestion  is  rapidly  subdivided  in  boiling  water. 
When  the  ferment  has  been  destroyed  by  heat  the  pieces  of  liver 
are  rubbed  up  to  a  pulp  in  a  mortar,  and  then  reboiled  in  the 
same  fluid.  The  liquor  is  then  filtered,  and  from  the  filtrate 
the  albuminous  substances  are  precipitated  with  potassio-mercuric 
iodide  and  hydrochloric  acid,  and  removed  on  a  filter.  From 
this  filtrate  the  glycogen  may  be  precipitated  by  alcohol,  caught 
on  a  filter,  washed  with  ether  to  remove  fat,  and  dried. 

Glycogen  thus  prepared  has  the  following  properties.  It  is  a 
white  powder,  forming  an  opalescent  solution  in  water,  which 
becomes  clear  on  the  addition  of  caustic  alkalies.  It  is  insoluble 
in  alcohol  and  ether.  With  a  solution  of  iodine  it  gives  a  wine- 
red  color,  and  not  blue,  like  starch,  which  it  otherwise  much 
resembles  in  chemical  relationship. 

Glycogen  is  widely  distributed  among  many  other  parts  besides 
the  liver,  namely,  in  all  the  tissues  of  the  embryo,  and  in  the 
muscles,  testicles,  inflamed  organs,  and  pus  of  adults;  in  short, 
where  any  very  active  tissue  change  or  growth  is  going  on,  some 
traces  of  glycogen  can  be  found. 

Some  light  is  thrown  upon  its  origin  by  the  fact  that  the 
amount  of  glycogen  in  the  liver  depends  in  a  great  measure  on 
the  kind  and  quantity  of  food  used.  It  rapidly  increases  with  a 
full,  and  decreases  with  a  spare  diet,  though  it  never  disappears 
even  in  prolonged  starvation.  The  formation  of  glycogen  is 
much  more  dependent  on  the  carbohydrate  food  than  on  the 
proteid,  for  it  rapidly  rises  with  increase  in  the  quantity  of  sugar 
taken,  and  falls,  as  in  starvation,  when  pure  proteid  (fibrin) 
without  any  carbohydrate  is  used  either  with  or  without  fat. 
Although  the  large  supply  of  glycogen  normally  manufactured 
in  the  liver  is  probably  derived  from  the  sugar  of  the  food,  we 
must  not  conclude  from  this  that  the  liver  cells  cannot  make 
glycogen  from  other  materials.  Possibly  anything  that  suffices 


GLYCOG'EN.  377 

for  the  nutrition  of  their  own  protoplasm  enables  the  cells  to 
produce  glycogen.  The  slowness  with  which  glycogen  disap- 
pears in  starvation  would  seem  to  point  to  this. 

The  ultimate  destiny  and  physiological  application  of  glycogen 
have  been  for  some  time  vexed  questions.  Whether  it  is  con- 
verted into  sugar,  and  as  such  carried  off  by  the  blood  of  the 
tissues,  or  whether  it  is  simply  distributed  as  glycogen,  is  dis- 
puted by  different  observers,  while  others  say  glycogen  is  a  step 
in  the  formation  of  fat  out  of  carbohydrate. 

The  want  of  clear  evidence  on  the  subject,  together  with  the 
obvious  chemical  difficulties,  force  us  to  abandon  the  theory  that 
fat  can  be  an  immediate  outcome  of  liver  glycogen,  though  we 
must  admit  that  carbohydrates,  or  any  form  of  nutriment,  may 
indirectly  aid  in  the  ultimate  formation  of  fat  by  protoplasm. 

The  difficulty  of  determining  the  exact  amount  of  sugar  or 
glycogen  in  the  blood  makes  this  a  very  unsatisfactory  means  of 
determining  the  physiological  application  of  liver  glycogen.  It 
seems  probable  that  glycogen  forms  the  general  carbohydrate 
nutriment  of  the  textures — the  diffusible  sugar  being  trans- 
formed in  the  liver,  into  indiffusible  glycogen,  in  order  that  it 
may  be  distributed  throughout  the  various  tissues  without  being 
lost  in  the  excretions. 


32 


378  MANUAL   OF    PHYSIOLOGY. 


CHAPTER  XXI. 
SECRETIONS. 

The  secretions  which  are  poured  into  the  alimentary  tract 
have  been  already  described  in  the  chapter  on  digestion.  There 
are  other  glands  which  can  now  be  conveniently  considered,  since 
they  more  or  less  alter  the  blood  flowing  through  them,  and  thus 
may  be  said  to  aid  slightly  in  the  perfect  elaboration  of  that 
fluid.  They  are,  however,  subservient  to  very  different  func- 
tions, some  having  merely  local  offices  to  perform,  and  others 
having  duties  allotted  to  them  of  the  greatest  general  importance 
to  the  economy.  This  becomes  obvious  from  a  glance  at  the 
following  enumeration  of  the  remaining  glandular  organs. 

Secreting  glands  (other  than  those  forming  special  digestive 
juices) : — 

Lachrymal.  Mammary. 

Mucous.  Sebaceous. 

Excreting  glands : — 

Sudorific.  Urinary. 

SURFACE   GLANDS. 
LACHRYMAL  GLANDS. 

Most  vertebrate  animals  that  live  in  air  have  a  gland  in  con- 
nection with  the  surface  of  their  eyes,  which  secretes  a  thin  fluid, 
to  moisten  the  conjunctiva.  This  fluid  commonly  passes  from 
the  eye  into  the  nasal  cavity,  and  supplies  the  inspired  air  with 
moisture. 

The  lachrymal  fluid  is  clear  and  colorless,  with  a  distinctly 
salty  taste  and  alkaline  reaction.  It  contains  only  about  1  per 
cent,  of  solids,  in  which  can  be  detected  some  albumin,  mucus, 
and  fat  (1  per  cent.),  epithelium  (1  per  cent.),  as  well  as  sodium 
chloride  and  other  salts  (.8  per  cent.). 

The  secretion  is  produced  continuously  in  small  amount,  but 
is  subject  to  such  considerable  and  sudden  increase,  that  at  times 


MUCOUS    GLANDS.  379 

,  cannot  all  escape  by  the  nasal  duct,  but  is  accumulated  in  the 
eyes  until  it  overflows  to  the  cheek  as  tears.  This  excessive 
secretion  may  be  induced  by  the  application  of  stimuli  to  the 
conjunctiva,  the  lining  membrane  of  the  nose,  or  the  skin  of  the 
face,  or  by  strong  stimulation  of  the  retina,  as  when  one  looks  at 
the  sun.  A  similar  increase  of  secretion  follows  certain  emotional 
states  consequent  on  grief  or  joy.  These  facts  show  that  the 
secretion  of  the  gland  is  under  nervous  control,  the  impulses 
stimulating  secretion  commonly  starting  either  from  the  periph- 
ery, and  passing  along  the  sensory  branches  of  the  fifth  or 
along  the  optic  nerve,  or  from  the  emotional  centres  in  the  brain, 
and  arriving  at  the  gland  in  a  reflex  manner.  The  amount  of 
secretion  can  also  be  augmented  by  direct  stimulation  of  the 
lachrymal  nerves,  so  that  in  all  probability  these  are  the  efferent 
channels  for  the  impulse. 

MUCOUS   GLANDS. 

In  connection  with  mouth  and  stomach  secretions,  mention  has 
been  made  of  glands  which  are  elongated  saccules  lined  with 
clear  cells  with  highly  refracting  contents  (Fig.  165).  They  are 
distributed  over  all  mucous  membranes,  and  are  the  chief  source 
of  the  thick,  tenacious,  clear,  alkaline,  and  tasteless  secretion 
called  mucus. 

This  material  contains  about  five  per  cent,  of  solid  matters,  of 
which  the  chief  is  mucin,  the  characteristic  material  of  mucus, 
which  swells  up  in  water  and  gives  the  peculiar  tenacity  to  the 
fluid.  It  is  precipitated  by  weak  mineral  and  acetic  acids;  and, 
as  the  precipitate  with  the  latter  does  not  redissolve  in  an  excess, 
this  acid  becomes  a  good  test  to  distinguish  it  from  its  chemical 
allies.  Mucin  is  not  precipitated  by  boiling.  Mucus  also  con- 
tains traces  of  fat  and  albumin,  and  inorganic  salts,  viz.,  sodium 
chloride,  phosphates  and  sulphates,  and  traces  of  iron. 

The  fluid  is  secreted  either  by  the  special  mucous  glands,  or  it 
may  be  produced  by  the  epithelium  of  the  mucous  surfaces.  The 
cells  produce  in  their  protoplasm  a  quantity  of  the  secretion, 
which  may  often  be  seen  to  swell  them  out  to  a  considerable 
extent.  This  clear  fluid  is  then  expelled,  and  the  altered  cells 


380 


MANUAL   OF    PHYSIOLOGY. 


are  repaired  or  replaced.  Many  form  elements,  like  the  remains 
of  epithelial  cells,  are  found  in  the  secretion  ;  and  also  round 
nucleated  masses  of  protoplasm  similar  to  white  blood  corpuscles 
after  the  imbibition  of  water.  In  the  abnormal  secretion  of  a 
mucous  surface  during  inflammation  these  mucous  corpuscles  are, 
as  well  as  the  general  amount  of  secretion,  greatly  increased,  so 
that  the  secretion  may  become  opaque,  and  may  appear  to  be 
purulent. 

The  chief  object  of  the  secretion  seems  to  be  to  protect  the 
mucous  surfaces,  which  are  rich  in  delicate  nerves  and  vessels, 


FIG.  165. 


Section  of  the  Mucous  Membrane  of  the  upper  part  of  nasal  cavity  showing  numerous 
Mucous  Glands  cut  in  various  directions,  o,  Surface  epithelium ;  b,  gland  saccule  lined 
with  secreting  cells ;  c,  connective  tissue.  (Cadiat.) 

and  are  subjected  to  many  injurious  influences  of  a  chemical  or 
mechanical  nature.  It  is  analogous  to  the  keratin  of  the  epi- 
dermis, and  may  be  regarded  as  an  excretion,  since  it  is  not 
absorbed,  but  is  cast  out  from  the  mucous  passages,  and  passes 

i  o      "  Jr 

from  the  intestinal  tract  with  the  faeces,  and  from  the  air  passages 
as  sputum,  etc. 


SEBACEOUS    GLANDS. 


381 


SEBACEOUS  GLANDS. 

These  belong  to  the  outer  skin,  and  commonly  open  into  the 
follicles  of  the  hairs,  but  also  appear  on  the  free  surface  of  the 
lips  and  prepuce,  etc.,  where  no  hairs  exist. 

The  secretion  cannot  be  collected  in  great  quantity  in  a  normal 
condition,  but,  as  far  as  can  be  made  out,  it  is  composed  of  neutral 
fat,  soap,  and  an  albuminous  body  allied  to  casein,  and  organic 
salts  and  water,  about  60  per  cent. 

The  secretion  contains  the   remains  of  numerous  epithelial 


FIG. 


Section  of  Skin  showing  the  roots  of  three  hairs  and  two  large  sebaceous  glands  (d). 

(Cadiat.) 

cells  which  are  thrown  off  from  the  inner  surface  of  the  glands, 
while  they  are  undergoing  a  peculiar  kind  of  fatty  change. 
These  cells  gradually  get  quite  broken  down  during  their  so- 
journ in  the  gland  alveoli,  and  the  secretion  is  finally  pressed 
out  by  the  band  of  smooth  muscle  which  usually  embraces  the 
gland  and  squeezes  it  against  the  hair  follicle. 

This  secretion,  the  use  of  which  is  to  lubricate  the  surface 
with  a  fatty  material,  is  cast  off  with  the  desquamated  epithelium 
and  the  hairs.  The  Meibomian  glands  of  the  eyelids  are  analo- 


382  MANUAL   OF    PHYSIOLOGY. 

gous  structures,  and  are  specially  elaborated  for  the  lubrication 
of  the  ciliary  margin.  The  glands  about  the  prepuce  and  clito- 
ris are  also  analogous  to  the  sebaceous  glands  ;  in  some  animals 
(Castor)  they  secrete  a  peculiarly  odoriferous  material. 

MAMMARY  GLANDS. 

The  secretion  of  milk  only  takes  place  under  certain  circum- 
stances and  continues  for  a  limited  period.  As  the  name  of  the 
glands  implies,  they  are  present  in  all  mammalian  animals.  The 
activity  of  the  gland  commences  in  the  later  stages  of  pregnancy, 

FIG.  167. 


Section  of  Mammary  Gland  during  active  lactation  (human),    (a)  Saccules  lined  with 
regular  epithelium.    (&)  Connective  tissue  between  the  alveoli.     (Cadiat.) 

and  then  continues,  if  the  secretion  be  regularly  withdrawn  from 
the  gland,  for  some  9  to  12  months. 

During  pregnancy  the  breasts  undergo  certain  preparatory 
changes  prior  to  the  appearance  of  the  milk.  They  increase  in 
bulk,  owing  to  the  greater  blood  supply,  and  to  certain  changes 
in  the  cell  elements  of  the  glands,  which  are  compound  saccular 
glands.  Each  breast  contains  a  series  of  some  ten  to  twelve 
glands,  with  distinct  ducts  ;  upon  these  there  are  dilatations  that 
act  as  reservoirs,  in  which,  during  active  lactation,  the  secretion 
is  stored  until  needed. 


MILK.  383 

The  alveoli  are  chiefly  saccular  in  form,  and  are  lined  with  a 
single  layer  of  glandular  epithelium,  and,  during  active  lacta- 
tion, contain  but  little  fat,  though  in  the  later  stages  of  preg- 
nancy, before  the  secretion  is  established,  the  cells  contain 
quantities  of  large  fat  globules. 

MILK. 

Milk  is  a  yellowish-white,  perfectly  opaque,  sweetish  fluid,  with 
an  alkaline  reaction  and  a  specific  gravity  of  about  1030.  When 
exposed  to  the  air,  particularly  in  warm  weather,  the  milk  soon 
loses  its  alkalinity,  first  becoming  neutral,  and  subsequently 
acid ;  the  milk  is  then  said  to  have  "  turned  sour,"  but  its  ap- 
pearance is  not  greatly  changed.  When  it  has  stood  a  very  long 
time  it  may  crack  or  curdle,  and  separate  into  two  parts — one  a 
thick,  white  curd  and  the  other  a  thin,  yellowish  fluid.  This 
turning  sour  and  ultimate  curdling  depends  upon  a  change 
brought  about  in  one  of  its  most  important  constituents,  namely, 
milk  sugar,  by  means  of  a  process  of  fermentation.  The  milk 
sugar,  in  the  presence  of  certain  forms  of  bacteria,  ferments  and 
gives  rise  to  lactic  acid.  When  the  quantity  of  lactic  acid  is 
sufficient,  it  not  only  makes  the  milk  sour,  but  also  precipitates 
another  of  its  important  constituents,  namely,  casein.  This  albu- 
minous body  in  its  coagulation  entangles  the  fat  of  the  milk, 
and  we  have  thus  formed  the  curd  of  cracked  milk,  while  the 
whey  consists  of  the  acid,  salts,  and  remaining  milk  sugar. 

Although  the  curdling  of  milk  depends  on  the  coagulation  of 
an  albuminous  body,  it  is  never  produced  by  boiling  fresh  milk, 
because  the  chief  proteid  is  casein,  a  form  of  derived  albumin 
(alkali-albumin),  which  does  not  coagulate  by  heat. 

When  milk  is  preserved  from  impurities  and  kept  in  a  cool 
place,  a  thick  yellow  film  soon  collects  on  the  top  of  the  fluid  ;  the 
thickness  of  this  layer — the  cream — may  be  taken  as  a  rough 
gauge  of  the  richness  of  the  milk.  Milk  consists  of  a  fine  emul- 
sion of  fat,  the  suspended  particles  of  which  are  kept  from  run- 
ning together  by  a  superficial  coating  of  dissolved  casein.  When 
left  at  rest,  the  light  fatty  particles  float  on  the  top  and  form 
cream. 


384  MANUAL   OF    PHYSIOLOGY. 

When  the  mammary  glands  commence  to  secrete,  the  milk 
contains  numerous  peculiar  structural  elements,  which  subse- 
quently disappear  from  the  secretion,  but  which  are  of  consider- 
able interest  in  relation  to  the  physiological  process  of  the  secre- 
tion. These  are  the  colostrum  corpuscles,  which  consist  of  large 
spherical  masses  of  fine  fat  globules  held  together  by  the  remains 
of  a  gland  cell,  which  encloses  the  fat  globules  as  a  kind  of  sac 
or  case,  and  in  which  at  times  a  nucleus  can  be  made  out. 

CHEMICAL  COMPOSITION. 

The  most  remarkable  point  about  the  chemical  composition  of 
milk  is  the  large  proportion  of  proteid  and  fat  it  contains  ;  this 
renders  it  unique  among  the  secretions.  There  are  two  distinct 
albuminous  bodies  present,  viz. :  casein,  which  appears  identical 
with  alkali-albumin,  and  another  form  of  albumin  allied  to 
serum-albumin.  The  fats  are  present  in  the  shape  of  globules  of 
various  sizes,  being  in  the  condition  of  a  perfect  emulsion,  as 
above  stated.  They  consist  of  glycerides  of  palmitic,  stearic  and 
oleic  acids. 

The  milk  sugar  is  very  like  glucose  or  grape  sugar,  but  not  so 
soluble.  It  has  the  peculiarity  of  undergoing  lactic  fermenta- 
tion. 

Of  the  inorganic  constituents  of  milk  the  most  important  are 
phosphates  and  carbonates  of  the  alkalies,  i.  e.,  the  salts  required 
to  form  bone.  It  is  a  remarkable  fact  that  the  potash  compounds, 
which  are  the  most  abundant  in  the  red  blood  corpuscles,  are 
present  in  greater  quantity  than  those  of  soda. 

The  following  table  shows  the  composition  of  human  milk,  a 
comparison  of  which  with  that  of  some  domestic  animals  will  be 
found  on  page  103  :— 

Casein 39.24 

Fat 26.66 

Milk  sugar 43.64 

Salts 1.38 


110.92 
Water ..  889.08 


1000.00 


COMPOSITION  OF  MILK. 


385 


The  relative  quantity  of  the  several  ingredients  of  milk  varies 
with  the  kind  of  diet  used.  A  vegetable  diet  increases  the  per- 
centage of  sugar,  but  diminishes  that  of  the  other  constituents, 
and  also  the  general  quantity  of  milk.  A  rich  meat  diet 
increases  both  the  general  quantity  and  the  percentage  of  fats 
and  proteids. 

The  quantity  of  milk  secreted  in  the  twenty-four  hours  is 
extremely  variable  in  different  individuals,  and  under  different 
circumstances  in  the  same  individual — the  average  in  general 
being  about  two  pints. 

The  amount  of  the  different  materials  in  milk  varies  under  the 


FIG.  168. 


FIG.  169. 


Section  of  I  he  Mammary  Gland  of  a  Cat  in 
the  early  stages  of  Lactation.  (A)  Cavity 
of  alveoli  filled  with  granules  and  globules 
of  fat.  1, 2, 3.  Epithelium  in  various  stages 
of  milk  formation. 


Cells  of  Mammary  Gland  during  lac- 
tation, stained  with  osmic  acid  so 
as  to  show  the  various  sized  oil 
globules  as  black  masses.  (Cadiat.) 


following  rules.  The  proportion  of  albumin  increases  as  the  milk 
sugar  decreases,  and  the  fat  remains  the  same  as  the  period  of 
lactation  advances.  The  portions  of  milk  last  drawn  are  much 
richer  in  fats  than  that  which  is  first  taken  from  the  gland.  In 
the  evening  the  milk  is  richer  in  fat  than  in  the  morning.  The 
general  amount  of  solid  constituents  falls  up  to  the  age  of  thirty 
years,  then  gains  slightly  until  thirty-five,  after  which  age  the 
milk  becomes  decidedly  thinner.  These  points  should  be  borne 
in  mind,  as  a  knowledge  of  them  may  prove  useful  in  the  selec- 
tion of  a  wet-nurse. 

Mode  of  Secretion. — Although  the  blood    contains   albumins 
33 


386  MANUAL    OF    PHYSIOLOGY. 

fats,  etc.,  very  similar  to  tho.se  which  form  the  solid  parts  of  the 
milk,  we  have  good  reason  for  thinking  that  the  constituents  of 
milk  are  not  merely  extracted  from  the  blood,  but  that  the  man- 
ufacture of  this  valuable  secretion  is  due  to  the  activity  of  the 
protoplasm  of  the  gland  cells,  which  construct  the  various  ingre- 
dients out  of  their  substance. 

It  has  been  suggested,  as  a  simple  explanation  of  the  forma- 
tion of  milk,  that  the  cells  undergo  fatty  degeneration,  and  the 
secretion  is  then  only  the  debris  of  the  degenerated  cells. 

Some  facts  support  this  view.  In  the  first  place,  the  ingredi- 
ents one  finds  in  milk  are  suggestive  of,  though  not  identical 
with,  the  chemical  materials  which  can  be  obtained  from  proto- 
plasm by  chemical  disintegration  rather  than  of  any  group  of 
substances  found  in  the  blood.  Further,  we  find  that  the  so- 
called  colostrum  corpuscles,  which  appear  to  be  secreting  cells 
filled  with  fat  particles,  are  thrown  off  from  the  gland  in  the 
early  stages  of  the  secretion,  and  appear  in  numbers  in  the  milk. 

But  these  colostrum  corpuscles  soon  cease  to  be  thrown  off  in 
the  secretion,  and  the  saccules  of  the  glands  during  active  lacta- 
tion do  not  contain  any  signs  of  the  debris  of  cast-off  cells,  or  any 
gradations  in  degeneration.  Only  one  row  of  finely-granular 
cells  is  found  lining  the  saccules,  and  the  cavities  are  filled  with 
globules  of  various  sizes.  From  this  it  would  appear  that  in  the 
earlier  stages  of  the  production  of  the  secretion,  the  mammary 
cells,  after  a  long  period  of  inactivity,  are  so  unaccustomed  to  the 
duty  they  are  called  upon  to  perform,  that  they  succumb  in  the 
effort,  and,  being  unable  to  produce  the  rich  secretion  and  retain 
their  vitality,  they  are  cast  off.  Their  offspring,  however,  after 
a  generation  or  two,  acquire  the  necessary  faculty  of  making 
within  their  protoplasm  all  the  necessary  ingredients  of  the  milk, 
and  discharge  them  into  the  lumen  of  the  saccules  without  them- 
selves undergoing  any  destructive  change. 

The  composition  of  the  milk  teaches  us  that  the  cells  of  this 
gland  can  manufacture  from  their  own  protoplasm  casein,  fat, 
milk  sugar,  etc.,  which  fact  shows  beyond  doubt  that  these  com- 
plex materials  may  be  made  in  the  body. 

The  influence  of  the  nervous  system  on  the  secretion  of  the  mam- 


SUDORIFEROUS    GLANDS.  387 

mary  glands  is  distinctly  shown  by  the  wonderful  sympathy 
between  the  action  of  these  glands  and  the  conditions  of  the 
generative  apparatus.  Further,  different  emotions  have  an 
effect,  not  only  on  the  quantity,  but  also  on  the  quality  of  the 
secretion.  Local  stimulation  also  promotes  the  secretion,  for  the 
application  of  the  child  to  the  breast  at  once  produces  this  effect, 
partly,  it  may  be,  through  mental  influences,  but  chiefly,  no 
doubt,  by  reflex  excitation  of  the  gland  following  the  local  stimu- 
lation. 

For  the  details  of  the  dietetic  value  of  milk,  see  Chapter  v,  on 
Food,  p.  102. 

EXCRETIONS. 

The  term  excretion  is  commonly  used  to  denote  a  gland  fluid 
the  chief  constituents  of  which  are  manufactured  by  other  tissues, 
and  are  of  no  use  in  the  economy,  but,  on  the  contrary,  require 
to  be  continually  removed  in  order  that  their  accumulation  in 
the  blood  may  not  give  rise  to  injurious  consequences.  These 
effete  matters  are  the  outcome  of  the  various  chemical  changes 
in  the  tissues,  whence  they  are  collected  by  the  blood  and  carried 
to  the  glands  which  preside  over  their  elimination. 

The  next  group  of  cutaneous  glands  is  commonly  arranged 
among  the  excretory  organs,  though  its  more  important  func- 
tion, as  will  hereafter  appear,  is  to  supply  surface  moisture  for 
the  purpose  of  regulating  the  temperature. 

SUDORIFEROUS  GLANDS. 

The  sweat  glands  are  distributed  all  over  the  cutaneous  surface, 
but  in  some  parts,  such  as  the  axilla,  perineum,  etc.,  they  are 
both  more  abundant  and  larger  than  elsewhere.  They  are  simple 
tubes  extending  in  a  more  or  less  wavy  manner  through  the  skin, 
and  ending  in  a  rounded  knot  formed  of  several  coils  of  the  tube 
some  way  beneath  the  corium,  where  they  are  surrounded  by  a 
capillary  plexus.  The  tube  is  lined  with  glandular  epithelium, 
and  its  basement  membrane  is  beset  with  longitudinally  arranged 
smooth  muscle  fibres. 

The  secretion  of  sweat  is  always  going  on,  though  it  does  not 
constantly  appear  as  a  moisture  on  the  surface,  because  the 


388  MANUAL    OF    PHYSIOLOGY. 

amount  produced  is  only  just  equal  to  the  amount  of  evaporation 
that  takes  place.  In  this  case  it  is  spoken  of  as  insensible  per- 
spiration. Under  certain  circumstances  the  sweat  collects  on  the 
surface  and  becomes  obvious  as  liquid — sensible  perspiration— 
which  bathes  the  skin,  being  produced  more  rapidly  than  it  can 
be  evaporated.  The  quantity  of  secretion  necessary  to  become 
sensible  varies  with  the  dryness  and  heat  of  the  air,  that  is,  with 
the  rapidity  with  which  evaporation  takes  place.  It  happens, 
however,  that  the  very  circumstances  which  tend  to  assist  evapo- 
ration also  promote  the  secretion  of  sweat.  Indeed,  the  effect  of 
great  heat  and  dryness  of  the  air  is  to  increase  the  cutaneous 
secretion  more  rapidly  than  they  increase  the  capability  of  evapo- 
ration, and  therefore,  when  the  air  is  hot  and  dry  and  evaporation 
is  going  on  very  actively,  we  have  the  secretion  of  sweat  made 
sensible  to  our  feelings.  When  dampness  is  associated  with 
warmth  of  the  atmosphere  the  sweat  collects  in  large  quantities 
on  the  skin,  for  the  heat,  as  we  shall  see  hereafter,  aids  the 
secretion,  and  the  damp  air  impedes  the  evaporation. 

The  quantity  of  perspiration  given  off  is  considerable,  but  the 
wide  limits  within  which  the  amount  may  vary  render  an  attempt 
to  express  an  average  in  numbers  useless.  The  amount  will  depend 
on  (1)  the  temperature  of  the  air,  (2)  the  quantity  and  quality  of 
fluids  imbibed,  (3)  the  amount  of  heat  generated  in  the  body, 
and  it  therefore  varies  directly  with  muscular  exercise.  The 
amount  that  becomes  perceptible  to  our  senses  depends  on  the 
impediments  to  evaporation  that  may  exist,  as  well  as  on  -the 
amount  of  fluid  produced. 

The  chemical  composition  of  sweat  varies  with  the  amount 
secreted.  When  collected  as  a  fluid  by  enclosing  a  part  of  the 
body  in, an  impervious  sac,  it  is  found  to  have  about  two  per  cent, 
of  solid  matters,  the  greater  quantity  of  which  is  made  up  of 
inorganic  salts,  sodium  chloride  being  by  far  the  most  abundant. 
It  also  contains  some  epithelial  debris,  traces  of  neutral  fats,  and 
several  volatile  and  fatty  acids  (butyric,  proprionic,  caproic),  to 
which  it  owes  its  peculiar  smell.  It  is  said  to  contain  urea,  but 
this  has  been  denied ;  and  since  all  the  nitrogenous  income  is 
accounted  for  in  the  urea  excreted  by  the  kidneys,  it  is  probable 


DESQUAMATION.  389 

that  the  cutaneous  elimination  of  urea  is  minimal,  if  not  exclu- 
sively pathological.  It  is  also  said  to  contain  salts  of  ammonia, 
and  it  affords  a  means  of  escape  to  many  drugs.  In  certain  parts 
of  the  body,  especially  in  some  individuals,  it  contains  a  consid- 
erable amount  of  pigments,  varying  in  color  from  brick-red  to 
bluish-black,  which  need  not  be  here  further  described. 

The  effect  of  nervous  influence  on  the  secretion  of  sweat  is  so 
associated  with  the  nervous  mechanisms  of  the  cutaneous  vessels 
that  under  ordinary  circumstances  it  is  a  difficult  matter  to  sepa- 
rate them.  There  can  be  no  donbt,  however,  that  a  special  ner- 
vous control  is  exerted  over  the  production  of  sweat.  This 
appears  to  be  observable  in  some  diseases,  the  poisons  of  which 
variously  affect  the  two  sets  of  nerves.  Thus,  in  fever,  we  observe 
a  dry  red  skin,  accompanied  by  an  increased  supply  of  blood  and 
a  suppression  of  the  secretion  of  the  sweat  glands  ;  while  in  cer- 
tain stages  of  acute  rheumatism  the  exact  opposite  is  seen,  i.  e.,  a 
profuse  sweat  drips  from  the  pale,  bloodless  skin.  It  has,  more- 
over, been  recently  shown  that  in  some  animals  (cats)  the  stimu- 
lation of  the  sciatic  nerve,  causing  contraction  of  the  blood  ves- 
sels, produces  at  the  same  time  a  copious  secretion  of  sweat ;  and 
a  warm  atmosphere  is  said  to  have  no  effect  on  the  secretion  of  a 
limb  the  nerve  of  which  has  been  cut,  although  the  warmth  be 
so  great  as  to  make  the  rest  of  the  animal's  body  sweat  profusely. 

The  effect  of  drugs  upon  the  cutaneous  secretion  is  well  known. 
There  is  a  large  group  of  medicines,  especially  pilocarpin,  which 
produce  an  increased  flow,  while  many  others,  notably  atropin, 
have  a  contrary  effect. 

CUTANEOUS  DESQUAMATION. 

Together  with  cutaneous  excretion  should  be  mentioned  the 
continuous  and  extensive  loss  all  over  the  surface  of  the  body, 
from  the  casting  off  of  the  superficial  layers,  of  the  dried  horny 
cells  of  which  the  outer  part  of  the  skin  is  composed. 

The  way  in  which  the  cells  of  the  mammary  gland  produce 
their  important  secretion  is  by  their  protoplasm  adopting  a  pecu- 
liar method  of  fat  manufacture,  while  all  the  strength  of  its 
nutritive  powers  is  devoted  to  the  elaboration  of  the  constituents 


390  MANUAL    OF    PHYSIOLOGY. 

of  milk.  In  a  similar  way  the  cells  of  the  epidermis  devote 
their  nutritive  activity  to  the  production  of  a  certain  material- 
keratin — which  cannot  be  called  a  secretion  in  the  ordinary  accep- 
tation of  the  term,  but  which  is  certainly  elaborated  as  the  result 
of  the  nutritive  changes  going  on  in  the  protoplasm  of  the  cell 
during  its  life  history,  just  as  many  other  substances  are  produced 
as  the  result  of  the  nutritive  activity  of  gland  cells. 

The  work  of  the  epidermal  cells  supplies — not  a  peculiar 
chemical  reagent,  as  do  some  of  the  gland  cells  of  the  digestive 
tract,  nor  yet  a  nutrient  fluid,  like  milk — but  an  insoluble,  imper- 
vious, tough  coating,  for  the  exterior  of  the  body,  which,  though 
thin  and  elastic,  is  very  strong  and  resisting. 

The  nearest  analogy  among  the  secretions  to  the  keratin  in 
the  epidermal  cells  is  the  production  of  mucin  in  the  cells  of  the 
epithelial  lining  of  the  mucous  membranes.  Both  substances 
may  be  looked  upon  as  excretions,  as  they  do  not  reeuter  the  sys- 
tem, being  cast  off,  but  each  of  them  performs  a  definite  function, 
and  is  produced  by  special  protoplasmic  elements,  like  the  secre- 
tions more  generally  recognized  as  such. 

The  amount  of  nitrogenous  substances  thus  excreted  cannot 
well  be  reckoned,  but  having  regard  to  the  great  extent  of  sur- 
face from  which  they  are  derived,  it  must  be  considerable. 


STRUCTURE  OF  THE  KIDNEYS. 


391 


FIG.  170. 


CHAPTER  XXII. 
URINARY  EXCRETION. 

The  urine  is  the  most  important  fluid  excretion,  for  by  it,  in 
mammalia,  nearly  all  the  nitrogen  of  the  used-up  proteid  leaves 
the  body  in  the  form  of 
urea.  The  construction  of 
the  urinary  glands  is  pecu- 
liar, and  requires  special  /. 
notice.  f|| 

STRUCTURE   OF   THE  U; 

KIDNEYS.  \    i 

The  kidneys  may  be 
called  complex  tubular 
glands,  because  the  tubes 
of  which  they  are  com- 
posed are  made  up  of  a 
number  of  parts  essentially 
differing  from  one  another 
both  in  their  structure  and 
in  their  relation  to  the 
blood  vessels. 

The  tubes  begin  by  a 
small  rounded  dilatation 
(Malpighian  capsule), 
which  is  lined  by  thin  flat- 
tened epithelium.  Opening 
from  this  capsule,  Fig.  171 
(#),  is  found  a  tortuous 
tubule  (/),  lined  by  pecu- 
liar large,  rod-beset,  epithe- 
lial cells,  which  occupy  the 
greater  portion  of  its  diameter. 


Section  of  Kidney  of  Man. 

a.  Cortical  substance  composed  chiefly  of  convo- 

luted 4ubules;  the  portions  between  the 
medullary  pyramids  form  the  columns  of 
Bertin  (e). 

b.  Pyramids  of  medullary  substance,  composed 

of  straight  tubes,  eic.,  radiating  toward  cor- 
tex. 

d.  Commencement  of  ureter  leading  from  central 
sac  or  pelvis. 

c.  Papillae,    where  the  tubes  open  into    pelvis. 

(Cadiai.) 


This  convoluted   tubule  (/) 


392 


MANUAL   OF   PHYSIOLOGY. 


leads  into  a  tube  (e)  of  much  less  external  diameter,  but  about 
equal  lumen,  owing  to  the  thinness  of  its  lining  epithelium,  the 
cells  of  which  are  more  flattened  and  much  thinner  than  those  in 


FIG.  171. 


FIG.  172. 


Diagram  of  the  Tubules  of  the  Kidney. 

(Cadiat.) 

a.  Large  duct  opening  at  papillae. 
6  and  c.  Straight  collecting  tubes. 
d  and  e.  Looped  tubule  of  llenle. 

f.  Convoluted  tubules  of  cortex. 

g.  Capsule  from  which  the  latter  spring. 


Portions  of  various  Tubules  highly  magni- 
fied, showing  the  relation  of  the  lining 
epithelium  to  the  wall  of  the  tube.  (Ca- 
diat.) 

a.  Large  duct  near  the  papilla. 

b.  Commencement  of  Henle's  loop 

c.  Thin  part  of  Henle's  loop. 


the  tortuous  tubes.  This  thin  tube  forms  a  loop  extending  down 
to  the  medullary  pyramid  and  returning  to  the  cortex,  where  it 
can  be  seen  to  become  again  convoluted  (d)  and  then  to  open 


BLOOD    VESSELS.  393 

into  a  straight  collecting  tube.  The  collecting  tubes  (c,  b)  receive 
many  similar  tributary  tubes  on  their  way  toward  the  apex  of 
the  medullary  pyramid,  where  they  pour  their  contents  into  the 
pelvis  of  the  kidney.  The  epithelial  lining  of  these  collecting 
tubes  is  of  the  ordinary  cylindrical  type. 

We  thus  find  four  kinds  of  epithelial  cells  in  the  various  parts 
of  the  urinary  tubules,  viz.,  scaly  cells  in  the  capsule;  peculiar 
rod-beset  glandular  cells  in  the  convoluted  tubes  ;  flattened  cells 
in  a  great  part  of  the  loop ;  and  ordinary  cylindrical  cells  in  the 
large  straight  tubes.  (Figs.  172  and  173.) 

BLOOD  VESSELS. 

The  renal  artery,  on  its  way  from  the  hilus  to  the  boundary 
between  the  cortical  and  medullary  portions  of  the  kidney,  breaks 

FIG.  173. 


Portion  of  the  Convoluted  Tubule,  showing  peculiar  fibrillated  epithelial  cells. 
(Heidenhain.) 

up  suddenly  into  numerous  small  branches;  these  vessels  then 
form  arches,  which  run  along  the  base  of  the  pyramids.  From 
the  latter,  straight  branches,  called  interlobular  arteries,  pass 
toward  the  surface,  and  give  off  lateral  branchlets,  which  form 
the  afferent  vessels  to  the  neighboring  Malpighian  capsules. 
Within  the  capsules  the  afferent  arteries  at  once  break  up  into 
a  series  of  capillary  loops,  forming  a  kind  of  tuft  of  fine  vessels 
— the  glomerulus,  which  fills  the  cavity  at  the  beginning  of  the 
tubules,  and  is  only  covered  by  thin,  scaly  epithelial  cells,  and 
thus  separated  from  the  urine.  It  is  a  singular  fact  that  in  the 
renal  circulation  the  efferent  vessel,  on  leaving  the  glomerulus, 
does  not,  like  most  veinlets,  unite  with  others  to  form  a  large 
vein,  but  again  breaks  up  into  capillaries,  which  form  a  dense 


394  MANUAL    OF    PHYSIOLOC4Y. 

mesh  work  around  the  convoluted  tubules.  The  blood  is  thence 
conveyed  to  small  straight  veins  corresponding  to  the  intra- 
lobular  arteries. 

Another  striking  peculiarity  of  the  renal  vessels  is  that  a  dis- 
tinct set  of  arteries,  starting  from  the  same  point  as  the  inter- 
lobular  (between  the  cortex  and  medulla),  pass  toward  the  centre 
of  the  gland  into  the  pyramids.  They  consist  of  bunches  of 
straight  arterioles,  which  lie  between  the  straight  and  the  looped 
tubules.  Corresponding  with  these  straight  arteries  are  minute 

FIG. 174. 


Glomerulus,  treated  with  silver  nitrate,  showing  the  endothelium. 

straight  veins,  which  carry  the  blood  back  to  the  vessels  at  the 
base  of  the  pyramids. 

In  the  kidney,  then,  we  have  three  sets  of  capillary  vessels, 
which  differ  in  their  position,  the  form  of  their  meshes,  and  their 
relation  to  their  parent  artery.  Probably  the  pressure  exerted 
by  the  blood  in  them,  and  the  rapidity  of  its  flow  through  them, 
differ  also : — 

1.  The  capillaries  in  the  glomeruli  are  loops  collected  into  a 


THE    URINE. 


395 


tuft  by  their  covering  of  delicate  epithelium.  On  account  of 
their  relation  to  the  afferent  artery  which  ends  abruptly  in  these 
capillaries,  and  to  the  smaller  efferent  vessel  that  leads  to  a 
secondary  plexus  of  capillaries,  the  pressure  within  the  glome- 
rulus  must  be  very  great  compared  with  that  of  the  general  capil- 
laries of  the  body,  and  must  vary  much  with  changes  in  local 
blood  pressure. 

2.  The  secondary  capillary  plexus,  with  its  narrow  meshwork 
closely  investing  the  tubules,  can  only  be  under  comparatively 
trifling  pressure  which  varies  but  little,  on  account  of  the  blood 
having  first  to  pass  through  the  capillaries  of  the  glomerulus. 
Their    current    of    blood 

i  i       i  FIG.  175. 

must  also  move  slowly, 
since  the  bed  of  the  stream 
is  here  very  great. 

3.  The  straight  vessels, 
with    long    meshed    capil- 
laries, in  the  pyramids  be- 
tween    the     looped     and 
straight   tubules,  are    un- 
like   the    two    preceding. 
In   these   straight    vessels 
the  blood   probably  flows 
with  greater  velocity  than 
in  those  around  the  convo- 
luted   tubes  ;     and     their 
blood  pressure  is  less  than 
that  in  the  glomeruli,  but 
greater   than   that  in   the 
intertubular  capillaries. 

THE   URINE. 

When  freshly  voided, 
the  urine  of  man  in  health 
is  a  clear  straw-colored 
fluid,  with  a  peculiar  aro- 
matic odor.  The  intensity 
of  the  color  varies  with  the  amount  of  solids — the  color  being  a 


Diagram  showing  the  relation  borne  by  the  blood 
vessels  to  the  tubules  of  the  kidney.  The  upper 
half  corresponds  to  the  cortical,  the  lower  to 
the  medullary  part  of  the  organ.  The  plain 
tubes  are  shown  separately  on  the  right,  and 
the  vessels  on  the  left.  The  darkly-shaded 
arteries  send  off  straight  branches  to  the  pyra- 
mid and  larger  interlobular  branches  to  the 
glomeruli,  the  efferent  vessels  of  which  form 
the  plexus  around  the  convoluted  tubes. 


396  MANUAL    OF    PHYSIO  LOO  Y. 

rough  indication  of  the  degree  of  concentration.  On  standing 
and  cooling,  a  slight  cloud  of  mucus  often  appears  floating  in  the 
fluid.  This  comes  from  the  lining  membrane  of  the  bladder, 
and  it  usually  entangles  a  few  flattened  epithelial  cells,  which 
are  the  only  organized  structural  elements  found  in  it  in  health. 

The  fresh  urine  has  a  distinctly  acid  reaction.  This  does  not 
depend  upon  the  presence  of  free  acid,  but  upon  the  large  amount 
of  acid  salts,  particularly  acid  sodium  phosphate,  which  it  inva- 
riably contains.  A  strictly  vegetable  diet  renders  man's  urine 
alkaline,  and  it  is  said  to  become  less  acid  after  meals.  In  the 
herbivorous  mammalia  the  urine  is  normally  alkaline  so  long  as 
their  digestion  is  going  on,  but  when  they  are  deprived  of  food 
for  some  time,  it  becomes  acid,  showing  that  the  alkalinity 
depends  upon  their  diet. 

The  specific  gravity  of  urine  varies  greatly  at  different  times, 
commonly,  however,  ranging  between  the  figures  1015-1020. 
After  copious  drinking,  abstinence  from  proteid  food,  and  in  cool 
weather,  it  may  fall  as  low  as  1003  ;  and  after  prolonged  absti- 
nence from  liquids,  much  animal  food,  and  very  active  sweating, 
it  may  attain  1040. 

The  quantity  of  urine  secreted  is  also  very  variable,  that  pro- 
duced by  an  adult  usually  amounting  to  about  2  pints  per  diem 
(1000-1500  cc.).  The  amount  is  increased  by — (1)  elevation 
of  the  general  blood  pressure,  or  the  pressure  in  the  arteries  from 
any  cause  whatever;  (2)  contraction  of  the  cutaneous  vessels 
from  cold  ;  (3)  copious  drinking;  (4)  excess  of  nitrogenous  diet; 
(5)  the  presence  of  soluble  matter  in  the  blood,  such  as  sugar, 
salt,  etc. ;  and,  (6),  the  presence  of  urea  as  well  as  various  medi- 
caments, has  a  special  action  on  the  renal  secretion,  greatly 
increasing  the  amount  of  urine  passed. 

Although  the  quantity  of  urine  differs  so  much  under  different 
circumstances,  the  amount  of  solids  excreted  by  the  kidneys  in 
the  24  hours  remains  pretty  much  the  same,  being  on  an  average 
over  II  oz.  (50  grammes)  for  an  adult  man. 

From  this  it  is  obvious  that  the  height  of  the  sp.  gr.  must  vary 
inversely  with  the  amount  secreted,  so  that  the  more  scanty  the 
urine  the  higher  we  expect  to  find  the  percentage  of  solids. 


SECRETION    OF    THE    URINE.  397 

SECRETION  OF  THE  URINE. 

We  have  just  seen  that  the  arterial  twig,  or  afferent  vessel, 
which  enters  the  capsule  of  Malpighi,  breaks  up  into  a  set  ot 
capillary  loops,  which  are  only  covered  by  a  single  layer  of  ex- 
tremely thin  epithelial  cells  separating  them  from  the  lumen  ot 
the  urinary  tubule,  and  that  the  pressure  in  the  vessels  of 
the  glomerulus  is  habitually  higher  than  that  in  most  capillaries, 
and  constantly  greater  than  that  of  the  second  capillary  network 
around  the  convoluted  tubules. 

The  general  arrangement  of  these  vessels,  and  the  high  pres- 
sure in  the  glomerulus,  give  the  impression  that  it  is  simply  a 
filtering  apparatus  by  means  of  which  the  fluid  parts  of  the  blood 
pass  into  the  urinary  tubules.  This  view  seems  supported  by  the 
fact  that  the  quantity  of  urine  secreted  bears  a  direct  proportion 
to  the  blood  pressure  in  the  minute  renal  vessels,  whether  the 
change  in  pressure  depends  on  local  vascular  mechanisms  or  on 
changes  in  the  general  blood  pressure. 

Such  a  theory,  however,  cannot  adequately  explain  the  forma- 
tion of  urine,  because  the  urine  differs  so  materially  from  the 
fluid  one  could  obtain  as  a  filtrate  from  the  blood.  In  health  it 
contains  no  albumin,  a  substance  in  which  the  blood  is  very 
rich  ;  and  it  is  much  richer  in  urea  and  salts  than  the  blood. 
There  is,  therefore,  both  a  quantitative  and  qualitative  difference, 
which  implies  a  distinct  process  of  selection,  and  although  filtra- 
tion may  not  be  altogether  excluded  from  the  process,  it  must  be 
completely  modified  by  other  forces. 

In  the  general  description  of  the  structure  of  the  organ  it  was 
seen  that  in  a  great  part  of  the  tubules,  both  the  epithelial  and 
vascular  supply  give  the  idea  of  actively  secreting  gland  tubes. 
From  the  mere  construction  of  the  different  portions  of  the  gland, 
it  has  been  concluded  that  there  are  two  distinct  departments, 
each  of  which  plays  a  different  part  in  the  production  of  the  urine. 
One  is  said  to  be  a  simple  filtering  mechanism,  and  the  other  a 
definitely  secreting  glandular  tubule. 

It  is  not  surprising  that,  with  such  a  complex  arrangement  as 
the  tubules  above  mentioned,  there  should  exist  different  views 
as  to  the  exact  mode  in  which  the  urine  is  secreted.  As  these 


398  MANUAL    OF    PHYSlOLOfJY. 

are  more  or  less  at  variance  in  their  explanation  of  the  method 
of  secretion,  and  as  it  is  difficult  to  put  any  of  them  aside  as 
quite  erroneous,  it  becomes  necessary  to  enumerate  each  some- 
what in  detail. 

Feeling  convinced  of  the  filter-like  function  of  the  glomerulus, 
and  recognizing  the  fact  that  some  other  agency  was  also  at 
work  in  the  formation  of  urine,  Bowman  explained  the  process 
thus :  From  the  glomerulus  the  watery  parts  of  the  fluid  are 
filtered,  while  the  glandular  epithelium  selects  the  important 
solid  constituents  which  it  is  necessary  to  remove  from  the 
blood. 

Ludwig  takes  a  different  view.  He  believes  that  the  watery 
part  of  the  plasma,  bearing  with  it  the  salts,  etc.,  is  filtered  from 
the  glomerulus.  As  this  fluid  passes  through  the  tortuous  uri- 
nary tubules,  a  large  portion  of  the  water  is  reabsorbed  into  the 
capillary  networks  surrounding  them.  This  reabsorption  is 
assisted  by  the  high  specific  gravity  of  the  blood  and  the  low 
pressure  in  these  capillaries  as  compared  with  those  of  the  glome- 
ruli,  where  the  filtration  of  the  liquid  occurs.  The  role  of  the 
epithelium  is  not  then  selection  from  the  blood  of  specific  mate- 
rials, but  possibly  the  prevention  of  the  return  of  the  solids  with 
the  water  back  to  the  blood  vessels. 

Heidenhain  attempted  to  settle  the  question  as  to  the  function 
of  the  renal  epithelium,  by  introducing  into  the  blood  a  blue 
coloring  matter — pure  sodium  sulphindigotate — which  he  found 
to  be  eliminated  by  the  kidneys,  giving  rise  to  blue  urine.  On 
examining  the  organ  with  the  microscope  at  a  suitable  time  after 
the  injection  of  the  color  into  the  blood,  the  tubules  are  found  to 
be  filled  with  the  pigment,  and  in  some  cases  the  peculiar  epithe- 
lium of  the  convoluted  tubules  is  stained  with  the  blue  substance, 
while  the  glomerulus  and  capsule  are  entirely  free  from  the  color. 
If  the  stream  of  fluid  from  the  glomerulus  be  stopped  in  any 
way — tying  the  ureter,  section  of  the  spinal  cord,  or  local  destruc- 
tion of  the  glomeruli — the  blue  color  is  only  to  be  found  in  the 
convoluted  tubes  and  their  epithelium,  and  hence  it  has  been 
concluded  that  its  presence  in  the  looped  and  collecting  tubes  of 
the  kidneys  and  urinary  bladder  depends  upon  its  being  washed 


SECRETION    OF    THE    URINK. 


399 


out  of  the  convoluted  tubes  by  the  stream  of  fluid  filtered  from 
the  blood  at  the  glomerulus. 

The  following  facts  may  also  be  adduced  in  further  support  of 
the  view  that  the  glandular  epithelium  has  a  considerable  share 
in  the  removal  of  the  more  important  solid  constituents  of  the 
urine. 

The  epithelium  in  the  tubules  of  the  kidney  of  birds  is  found 
impregnated  with  acid  urates,  which  form  the  chief  constituents 
with  the  solid  urine  of  birds. 

The  amount  of  liquid  passing  out  at  the  kidneys  is  in  direct 
proportion  to  the  blood  pressure,  whereas  the  excretion  of  the 
specific  constituents  of  urine  is  independent  of  the  pressure,  but 
is  related  to  the  amount  existing  in  the  blood,  and  the  condition 
of  the  epithelium.  This  is  shown  by  the  increased  elimination 
of  urea  when  that  substance  is  artificially  introduced  into  the  cir- 
culation, even  after  the  flow  of  the  fluid  has  been  checked  by 
section  of  the  spinal  cord. 

Another  view  has  been  put  forward,  which,  with  some  modifica- 
tion, appears  plausible,  or  at  least  worthy  of  mention.  Paying 
attention  to  the  fact  that  where  vascular  filtration — i.  e.,  the  pass- 
age of  liquid  under  pressure  through  the  capillary  wall — occurs 
elsewhere  in  the  body  it  is  not  only  water  and  salts,  but  plasma 
that  passes  out  of  the  vessels  into  the  interstices  of  the  tissues, 
we  may  then  assume  that  the  fluid  part  of  the  blood,  as  such, 
and  not  merely  its  watery  part,  escapes  at  the  glomerulus.  That 
is  to  say,  the  solid  ingredients  of  the  urine  in  a  diluted  form, 
plus  serum-albumin,  pass  into  the  tubules.  But  on  its  way  down 
the  long  and  circuitous  route  through  the  tubules  the  albumin 
with  much  water  is  reabsorbed  by  the  capillaries  of  the  convo- 
luted tubes.  The  first  step  in  this  case  is  a  mechanical  filtration  ; 
the  second  is  a  vital  process  of  reabsorption  of  a  solution  of 
serum-albumin  carried  on  by  the  gland  cells  in  the  tubules, 
aided  by  the  low  pressure  in  the  peri-tubular  capillary  plexus. 
This  view  seems  supported  by  pathological  experience,  which 
teaches  that  the  removal  of  the  epithelium  of  the  tubes  (the 
glomeruli  remaining  perfect),  is  followed  by  the  appearance  of 
albumin  in  the  urine,  and  cysts  formed  by  the  destruction  of  the 


400  MANUAL   OF   PHYSIOLOGY. 

epithelium  and  occlusion  of  the  tubules,  commonly  contain  a 
fluid  somewhat  like  plasma. 

Doubtless  much  remains  to  be  found  out  as  to  the  exact 
method  of  secretion  of  the  urine,  and  possibly  future  research 
may  show  us  that  all  the  views  here  enumerated  have  some  truth 
in  them.  That  a  filtration,  not  mere  osmosis,  takes  place, 
seems  probable  from  the  special  vascular  mechanism  of  the 
glomerules.  Why  simply  water  and  salts  without  albumin 
should  pass  through  the  capillaries  of  the  glomerulus  and  not 
through  any  other  capillaries,  is  not  sufficiently  explained  to 
make  it  sure  that  such  a  filtration  really  occurs.  That  the 
glandular  epithelium  does  take  an  active  part  in  the  elimination 
of  the  urea  is  rendered  almost  indisputable  from  the  researches 
of  Heidenhain.  And  yet  there  remain  other  parts,  e.  g.,  the 
loops  of  Henle,  which  are  invariably  found  in  the  kidney,  and 
have  a  special  vascular  mechanism,  to  which  none  of  the  fore- 
going theories  assign  any  special  or  peculiar  function. 

From  the  foregoing  evidence  we  may  fairly  suppose  that  most 
of  the  urea,  and  possibly  some  other  solid  constituents  of  the 
urine,  are  selected  from  the  blood  by  the  epithelial  cells  of  the 
convoluted  tubules,  that  the  fluid  part  of  the  blood  escapes  at 
the  glomerulus,  and  flows  along  the  varied  and  circuitous  route 
of  the  tubules,  carrying  with  it  the  matters  poured  into  the  tubes 
by  the  cells,  and  that  in  some  part  of  the  tubules  the  dilute 
filtrate  loses  much  of  its  water  and  all  its  albumin. 

CHEMICAL  COMPOSITION   OF   URINE. 

The  percentage  of  the  solid  and  liquid  materials  in  urine 
varies  as  the  secretion  alters  in  strength,  but  on  an  average  it 
may  be  said  to  contain  about  4  per  cent,  of  solids  and  96  per 
cent,  water. 

The  following  are  the  more  important  solid  matters  : — 
Urea  is  the  most  important,  and  at  the  same  time  most  abun- 
dant solid  constituent,  commonly  forming  about  2  per  cent,  of  the 
urine.  It  is  regarded  as  the  chief  end-product  of  the  oxidation 
of  the  nitrogenous  matter  in  the  body,  so  that  the  amount 
excreted  per  diem  gives  us  the  best  estimate  of  the  amount  of 


CHEMICAL   COMPOSITION    OF    URINE.  401 

chemical  change  taking  place  in  the  nutrition  of  the  tissues.  It 
is  readily  soluble  in  alcohol  and  water,  but  insoluble  in  ether. 
It  forms  acicular  crystals  with  a  silky  lustre.  From  a  chemical 
point  of  view  it  may  be  regarded  as  the  monamide  of  carbamic 

acid,  with  the  formula  CO  j  ^jj2-  It  is  isomeric  with  ammo- 
nium cyan  ate  ^TT  [•  O,  from  which  it  was  first  prepared  arti- 
ficially. 

On  exposure  to  the  air  bacteria  develop  in  the  urine,  and, 
acting  as  a  ferment,  change  the  urea  into  ammonium  carbonate, 
two  molecules  of  water  being  at  the  same  time  taken  up  thus : — 

CO(NH2)2  +  2H20  =  (NH4)2C03. 

This  gives  rise  to  a  change  in  the  reaction  of  the  urine,  which, 
after  a  time,  becomes  increasingly  alkaline,  and  the  change  is 
commonly  spoken  of  as  the  alkaline  fermentation  of  the  urine. 
This  change  is  extremely  slow  in  solutions  of  pure  urea,  which 
do  not  support  bacterial  life. 

With  nitric  and  oxalic  acids,  urea  forms  sparingly  soluble 
salts — a  fact  made  use  of  in  its  preparation  from  urine. 

The  amount  of  urea  eliminated  in  the  24  hours  is  about  500 
grains  (35  grammes).  The  amount  varies  (1)  in  some  degree 
with  the  amount  of  urine  secreted  ;  an  increase  in  the  amount  of 
water  being  accompanied  by  a  slight  increase  in  the  urea  elimi- 
nated. Some  materials,  such  as  common  salt,  increase  the  water, 
and  thereby  also  increase  the  urea.  (2)  The  character  and 
quantity  of  the  diet  influences  most  remarkably  the  quantity  of 
urea  given  off,  the  amount  increasing  in  direct  proportion  to  the 
quantity  of  proteid  consumed.  Fasting  causes  a  rapid  fall 
in  the  amount  of  urea ;  even  in  the  later  days  of  starvation  it 
continues  to  fall,  but  very  slowly.  (3)  The  amount  differs  with 
age,  being  relatively  greater  in  childhood  than  in  the  adult 
(about  half  as  much  again  in  proportion  to  the  body  weight). 
(4)  Many  diseases  have  a  marked  influence  on  the  amount  of 
urea.  In  most  febrile  affections  it  increases  with  the  intensity  of 
the  fever,  while  in  disease  of  the  liver  it  often  notably  decreases. 
In  diabetes,  if  the  consumption  of  food  be  very  great,  the  daily 
34 


402  MANUAL   OF    PHYSIOLOGY. 

excretion  of  urea  may  reach  nearly  4  oz.  (100  grammes)  or  three 
times  as  much  as  normal. 

Preparation. — To  obtain  urea  from  human  urine  it  is  evapo- 
rated to  one-sixth  of  its  bulk,  an  excess  of  nitric  acid  is  added, 
and  it  is  left  to  stand  in  a  cool  place.  Impure  nitrate  of  urea 
separates  from  the  fluid  as  a  yellow  crystallized  precipitate.  This 
sparingly  soluble  salt  is  caught  on  a  filter,  dried,  dissolved  in 
boiling  water,  mixed  with  animal  charcoal  to  remove  the  color- 
ing matter,  and  filtered  while  hot ;  when  the  filtrate  cools,  color- 
less crystals  of  nitrate  of  urea  are  deposited.  The  precipitate  is 
dissolved  in  boiling  water,  and  barium  carbonate  added  as  long 
as  effervescence  takes  place,  barium  nitrate  and  urea  being  pro- 
duced. This  is  evaporated  to  dryness,  and  the  urea  extracted 
with  absolute  alcohol,  which,  on  evaporation,  leaves  crystals  of 
pure  urea. 

Estimation. — Urea  can  be  estimated  volumetrically  by  the 
method  of  Liebig,  which  depends  on  the  power  of  mercuric 
nitrate  to  give  a  precipitate  with  it.  The  sulphates  and  phos- 
phates must  be  first  removed  by  the  addition  of  40  cc.  of  a  mix- 
ture of  1  volume  saturated  barium  nitrate  and  2  volumes  satu- 
rated solution  of  caustic  baryta,  to  40  cc.  of  urine.  This  is 
filtered,  and  from  the  filtrate  an  amount  corresponding  to  10  cc. 
urine  is  taken.  Into  this  known  volume  of  urine  a  standard 
solution  of  mercuric  nitrate  (of  which  1  cc.  corresponds  to  1 
centigramme  of  urea)  is  dropped  until  a  sample  drop  of  the 
liquid,  mingled  on  a  watch  glass  with  a  drop  of  concentrated 
sodium  carbonate  solution,  gives  a  yellow  color,  which  indicates 
that  some  free  mercuric  nitrate  is  present.  For  every  cubic 
centimetre  of  the  standard  mercuric  solution  used,  there  is  one 
centigramme  of  urea  in  the  sample  of  urine ;  a  reduction  of  2  cc. 
should  be  made  from  the  mercuric  solution  used  in  the  experi- 
ment, on  account  of  the  chlorides,  which  are  present  in  tolerably 
constant  amount. 

Another  simple  and  more  accurate  method  consists  in  mixing 
known  quantities  of  urine  and  sodium  hypobromite  (NaBrO) 
with  excess  of  caustic  soda.     The  urea  is  decomposed  in  the 
presence  of  this  salt,  and  free  nitrogen  evolved— 
CON2H,  +  3(NaBrO)  +  2(NaOH)  =  3NaBr  +  Na2C03+3H20  +  2N. 


CHEMICAL    COMPOSITION    OF    URINE.  403 

The  quantity  of  urea  may  be  determined  by  ascertaining  the 
volume  of  nitrogen,  which  can  be  measured  directly  in  a  gradu- 
ated tube.  37.5  cc.  of  N  represents  0.1  gramme  of  urea  at 
ordinary  temperature  and  pressure. 

Uric  acid,  of  which  the  formula  is  C5H4N4O3  or  C3H2O3 
(NH.CN)2,  is  only  present  in  extremely  small  quantities  in  the 
normal  urine  of  mammalia,  but  in  birds,  reptiles  and  insects  it 
forms  the  chief  ingredient  of  the  renal  secretion.  It  is  sparingly 
soluble  in  water,  and  insoluble  in  alcohol  and  ether.  However, 
in  solutions  of  the  neutral  phosphates  and  carbonates  of  the 
alkalies  it  combines  with  some  of  the  base,  so  as  to  form  acid 
salts,  and  at  the  same  time  converts  the  neutral  into  acid  phos- 
phates, to  which,  as  has  been  already  stated,  the  urine  owes  its 
acid  reaction.  These  salts  are  more  soluble  in  warm  than  in 
cold  water,  and  hence  generally  fall  as  a  sediment  when  the 
urine  cools.  Uric  acid  is  readily  converted  into  urea  by  oxida- 
tion, and  is  probably  one  of  the  steps  in  the  formation  of  urea 
generally  occurring  in  the  body  during  the  gradual  oxidation  of 
the  proteid  bodies. 

The  presence  of  uric  acid  may  be  recognized  by  the  rnurexide 
test.  The  substance  to  be  tested  is  gently  heated  in  a  flat  cap- 
sule with  some  nitric  acid.  A  decomposition  occurs,  N  and  CO2 
going  off,  urea  and  alloxan  remaining  as  a  layer  of  yellow  fluid. 
If  this  be  cautiously  evaporated,  and  a  drop  of  ammonia  added, 
a  striking  purple  red  color  is  produced,  which  the  addition  of 
potash  turns  violet. 

The  amount  of  uric  acid  normally  follows  pretty  closely  the 
variations  in  urea,  but  is  usually  only  about  8  grains  (.5  gramme) 
per  diem.  In  certain  diseases  the  quantity  may  be  much  in- 
creased. For  the  quantitative  estimation,  which  is  seldom  de- 
cided by  the  practitioner,  the  student  must  consult  the  text-books 
of  physiological  chemistry. 

Kreatinin  (CJH7N3O)  is  always  present  in  urine,  probably 
being  formed  from  kreatin  by  the  loss  of  one  molecule  of  water. 
About  15  grains  (1  gramme)  is  excreted  per  diem. 

Xanihin  (C5H4N4O2)  also  occurs  in  urine,  but  in  extremely 
small  quantities. 


404  MANUAL   OF   PHYSIOLOGY. 

Hippuric  acid  (C9H9NO3)  is  a  normal  constituent  of  human 
urine,  occurring,  however,  in  very  small  quantities.  On  the 
other  hand,  it  is  one  of  the  most  important  nitrogenous  constitu- 
ents of  the  urine  of  the  herbivora,  where  it  takes  the  place  of  uric 
acid.  Its  presence  depends  on  the  existence  of  certain  ingredi- 
ents (benzoic  acid,  etc.)  in  the  food,  which  are  capable  of  com- 
bining with  glycin,  and  forming  a  conjugated  acid,  a  molecule 
of  water  being  formed  at  the  same  time,  thus — 

Benzoic  Acid.  Glycin.  Hippuric  Acid.        Water. 

C7H602    +    C2H5N02    =    C9H9N03    +    H20. 

The  amount  of  hippuric  acid  increases  with  increased  consump- 
tion of  vegetable  food,  in  the  cellulose  of  which  the  materials 
exist  that  are  required  for  its  formation.  The  union  of  glycin 
and  benzoic  acid  may  take  place  in  the  liver,  for,  after  removal 
of  that  organ,  benzoic  acid  injected  into  the  veins  appears  un- 
changed in  the  urine ;  but  the  extirpated  kidney  is  also  said  to 
be  capable  of  effecting  this  synthesis. 

Oxalic  acid  (C2H2O4)  occurs  often,  but  not  constantly,  in  the 
urine.  It  is  generally  united  with  lime.  It  is  said  to  appear  in 
greater  quantity,  together  with  an  excess  of  uric  acid,  after  meals, 
and  therefore  to  be  related  to  the  production  of  the  latter  in  the 
body ;  but  it  probably  is  chiefly  derived  from  oxalates  being 
contained  in  some  materials  taken  with  the  food. 

COLORING  MATTERS. 

It  appears  probable  that  the  color  of  the  urine  depends  on  the 
presence  of  small  quantities  of  distinct  substances  which  have 
different  origins  in  the  body.  Three  such  have  been  described, 
and  may  be  taken  provisionally  to  represent  our  knowledge  of 
the  subject : — 

1.  Urobilin,  which  is  an  outcome  of  the  coloring  matter  of 
the  bile,  and  therefore  a  remote  derivative  of  the  coloring  mat- 
ter of  the  blood,  is  frequently  present  in  the  urine.     It  is  proba- 
bly the  same  as  hydrobilirubin,  some  of  which  is  occasionally 
absorbed  from  the  intestinal  tract  and  eliminated  by  the  kid- 
neys. 

2.  Urochrom  is  said  to  be  the  special  pigment  of  the  urine.     It 


INORGANIC   SALTS.  405 

oxidizes  on  exposure,  forming  a  reddish  substance  that  gives  the 
dark  color  to  some  urinary  sediments  (  Uroerythrin). 

3.  A  certain  material  (Indicari)  capable  of  producing  Indigo, 
is  commonly  present  in  the  urine  of  man,  and  in  greater  quantity 
in  that  of  some  animals,  particularly  the  horse.  It  is  supposed 
to  be  formed  from  the  indol  that  arises  from  the  putrefactive 
changes  consequent  on  the  pancreatic  digestion.  The  indol  is 
absorbed  and  unites  with  sulphuric  acid  to  form  Indican,  which 
is  a  yellow  substance.  Under  certain  conditions  it  can  be  con- 
verted by  oxidation  into  indigo-blue. 

INORGANIC  SALTS. 

The  urine  is  the  great  outlet  for  all  inorganic  salts.  The  most 
important  of  these  are — 

Common  salt  (NaCl),  of  which  a  very  variable  but  always 
considerable  amount  passes  away  in  the  urine.  The  average 
quantity  excreted  per  diem  may  be  said  to  be  about  half  an  oz. 
(15  grammes).  It  depends  greatly  on  the  quantity  taken  with 
the  food,  and  falls  during  starvation,  but  does  not  completely 
disappear.  It  is  said  that  if  absolutely  no  common  salt  be  taken 
with  the  food  the  quantity  of  NaCl  excreted  diminishes  greatly, 
and  albumin  appears  in  the  urine  about  the  third  day.  The 
amount  of  salt  eliminated  follows,  with  striking  accuracy,  the 
changes  that  take  place  at  different  times  and  under  different 
circumstances,  in  the  quantity  of  urea  excreted.  These  facts 
seem  to  indicate  that  there  is  some  relationship  between  the  secre- 
tion of  the  two  bodies,  or  that  sodium  chloride  participates  in 
the  chemical  changes  of  the  nitrogenous  tissues.  In  many 
diseases  there  occur  variations  in  the  quantity  of  common  salt  in 
the  urine  which  can  hardly  be  explained  by  the  change  in  or 
absence  of  food. 

Phosphates. — About  60  grains  (3  to  4  grammes)  of  phosphoric 
acid  is  excreted  daily  in  the  urine,  being  combined  with  alkalies 
to  form  salts,  viz.,  potassium,  sodium,  calcium,  and  magnesium 
phosphates. 

Sulphates. — Nearly  40  grains  (2  to  3  grammes)  of  sulphuric 
acid,  as  sulphates  of  alkalies,  are  daily  got  rid  of  in  the  urine. 


406  MANUAL   OF   PHYSIOLOGY. 

The  acid  comes  partly  from  the  food,  but  chiefly  from  the  oxida- 
tion of  the  sulphur  contained  in  the  proteids  of  the  tissues. 

A  considerable  quantity  of  potassium,  sodium,  calcium,  and  mag- 
nesium, combined  as  already  mentioned,  or  with  chlorine,  is  con- 
tained in  the  urine. 

Small  traces  of  iron  are  also  always  present  in  the  urine. 

Gases. — The  urine  also  contains  free  CO2,  N,  and  some  O.  100 
volumes  of  gas  pumped  out  of  fresh  urine  have  been  found  to 

consist  of— 

CO.,  =  65.40  per  cent. 
N    =31.86        u 
0   =    2.74        " 

ABNORMAL  CONSTITUENTS. 

Different  kinds  of  substances  occur  in  urine  under  circum- 
stances of  special  physiological  interest,  and  therefore  may  be 
here  enumerated,  although  their  accurate  study  belongs  rather 
to  pathology.  First  among  these  to  be  named  is — 

Albumin,  which  occurs  from  (1)  any  great  increase  in  the  blood 
pressure  in  the  renal  vessels,  whether  caused  by  increased  inflow 
or  impeded  outflow.  (2)  Excess  of  albumin  in  the  blood,  and, 
strange  to  say,  some  forms  of  albumin  escape  much  more  readily 
than  others.  Thus,  egg  albumin,  globulin,  or  peptone,  if  intro- 
duced artificially  into  the  blood,  are  soon  found  in  the  urine. 
(3)  A  watery  condition  of  the  blood,  such  as  would  give  rise  to 
oedema  elsewhere.  (4)  Total  abstinence  from  NaCl  for  some  time. 
(5)  Destruction  of  some  of  the  epithelium  of  the  urinary  tubes. 

Next  in  importance  to  albumin  are  the  following: — 

Grape  sugar,  of  which  normally  only  the  merest  trace  occurs 
in  the  urine,  although  there  is  always  a  certain  quantity  in  the 
blood.  It  is  present  in  large  quantities  in  (1)  the  disease  known 
as  diabetes,  when  a  great  quantity  of  pale  urine  with  a  very  high 
specific  gravity  is  passed.  (2)  After  injury  of  a  certain  part  of 
the  floor  of  the  4th  ventricle  of  the  brain.  (3)  After  poisoning 
by  curara,  carbonic  oxide,  and  nitrate  of  amyl.  In  short,  any 
disturbance  of  the  circulation  of  the  liver  gives  rise  to  an  increase 
of  sugar  in  the  blood,  and  when  the  amount  reaches  6  per  cent, 
it  appears  in  the  urine. 


URINARY    CALCULI.  407 

Bile  Acids  and  Pigments  appear  in  the  urine  when,  from  occlu- 
sion of  the  bile  ducts,  they  find  their  way  into  the  blood. 

Leucin  and  Tyrosin  also  occur  in  the  urine,  but  only  after  inter- 
ference with  the  functions  of  the  liver. 

The  urine  undergoes  important  changes  after  being  voided,  the 
explanation  of  which  is  of  much  interest  to  the  practitioner,  and 
must  be  understood  by  the  student  of  medicine.  The  urine  often 
loses  its  transparency  as  soon  as  it  gets  cold,  though  perfectly 
clear  when  passed,  or  when  again  heated  to  the  body  temperature, 
for  the  urates  are  soluble  in  warm  but  almost  insoluble  in  cold 
water.  The  "  muddiness,"  which  soon  settles  down,  as  a  more  or 
less  brightly  colored  sediment,  is  chiefly  caused  by  the  precipita- 
tion of  acid  sodium  urate,  stained  with  a  coloring  matter  derived 
from  the  urochrome.  When  this  occurs  the  urine  will  always  be 
found  to  be  distinctly  acid,  and  if  it  be  left  standing  for  some 
time  in  a  cool  place,  the  acidity  will  be  found  to  increase,  owing 
to  the  presence  of  a  peculiar  fungus  which  sets  up  acid  fermen- 
tation. This  is  said  to  depend  on  the  formation  of  lactic  and 
acetic  acids,  and  crystals  of  uric  acid,  amorphous  sodium  urate, 
and  crystals  of  lime  oxalate  are  deposited. 

After  a  certain  time  (which  is  shorter  when  the  urine  is  not 
very  acid  and  is  exposed  to  a  warm  atmosphere)  the  develop- 
ment of  bacteria  occurs  in  it,  and  causes  the  urea  to  unite  with 
water  and  to  change  in  the  manner  already  mentioned  (p.  75) 
into  ammonium  carbonate.  This  gradually  neutralizes  the  acid- 
ity, and  finally  renders  the  urine  alkaline.  At  the  same  time  an 
amorphous  precipitate  of  lime  phosphate  appears,  and  crystals 
of  ammonio-magnesium  phosphate  and  of  ammonium  urate  are 
are  produced. 

URINARY  CALCULI. 

Various  ingredients  of  the  urine,  which  are  difficult  of  solu- 
tion, sometimes  become  massed  together  as  concretions,  particu- 
larly if  there  exist  any  small  foreign  body  in  the  bladder,  which, 
by  acting  as  a  nucleus,  lays  the  foundation  of  a  stone.  Some- 
times small  concretions  are  formed  in  the  tubes  or  pelvic  recesses 
of  the  kidney,  and,  when  these  make  their  way  into  the  bladder, 
they  commonly  grow  larger  and  larger.  The  structure  and  com- 


408  MANUAL   OF    PHYSIOLOGY. 

position  of  a  calculus  often  gives  the  history  of  its  own  transit 
from  the  kidney,  and  also  of  various  changes  in  the  metabolism 
of  the  individual,  for  successive  layers  of  different  substances  are 
generally  found  in  a  stone  that  has  attained  any  great  size.  The 
chief  materials  found  in  calculi  are — uric  acid,  ammonium  urate, 
calcium  oxalate  and  carbonate,  ammonio-magnesium  phosphate, 
etc. 

SOURCE  OF  UREA,  ETC. 

The  question  as  to  whether  the  chief  materials  of  the  urine  pre- 
exist in  the  blood  and  are  therefore  merely  removed  by  the  kid- 
ney, or  are  manufactured  by  the  special  powers  of  the  renal  cells, 
has  been  widely  discussed,  and  though  the  great  weight  of  evi- 
dence is  in  favor  of  the  former  view,  some  of  the  experimental 
results  on  the  subject  are  rather  conflicting. 

The  following  are  the  more  important  points  in  the  argu- 
ment : — 

1.  The  blood  normally  contains  most  of  the  important  sub- 
stances found  in  the  urine ;  so  they  need  not  necessarily  be  made 
in  the  kidney. 

2.  The  blood  in  the  vessel  leading  to  the  kidney — the  renal 
artery — is  said  to  contain  more  urea  than  the  vessel  leading  from 
it — the  renal  vein — so  that  the  blood  appears  to  lose  urea  in  pass- 
ing through  the  kidney. 

3.  If  the  ureters  be  tied  and  the  elimination  be  thus  prevented, 
urea  accumulates  in  the  blood.     This  can  hardly  be  made  by  the 
kidney,  because — 

4.  If  the  renal  arteries  be  tied  so  that  no  blood  goes  to  the 
kidneys  to  affect  the  elaboration  of  urea  in  those  organs,  then  the 
same  accumulation  results,  showing  that  the  kidneys  are  certainly 
not  the  only  organs  where  urea  is  made. 

5.  Extirpation  of  the  kidneys  also  gives  rise  to  a  great  increase 
of  the  urea  in  the  blood.     The  amount  of  urea  in  the  blood  after 
nephrotomy   is  said  to  increase  steadily  with  the  time  which 
elapses  after  the  operation,  and  the  amount  accumulated  corre- 
sponds to  the  amount  that  would  normally  have  been  excreted 
in  the  same  time,  had  the  animal  not  been  operated  upon. 


SOURCE   OF   UREA,  ETC.  409 

6.  Iii  some  diseases  which  interfere  with  or  suppress  the  secre- 
tion of  the  kidneys,  an  accumulation  in  the  blood  of  certain  poi- 
sonous or  injurious  materials  takes  place,  and  gives  rise  to  the 
gravest  symptoms  called  ursemic  poisoning,  which  closely  coincide 
with  those  observed  in  experimental  annihilation  of  the  renal 
function. 

From  the  foregoing  it  would  appear  to  be  satisfactorily  settled 
that  the  urea,  which  is  by  far  the  most  important  ingredient  of 
the  secretion  of  the  kidney,  is  probably  made  elsewhere  and  not 
in  that  organ,  whose  duty  seems  to  be  chiefly  to  remove  it  from 
the  blood.  This  is  most  probably  also  true  of  all  the  other 
organic  constituents  of  the  urine.  The  question  then  arises, 
where  is  the  urea  formed? 

We  naturally  turn  for  an  answer  to  the  most  widespread  and 
most  actively  changing  nitrogenous  tissue,  namely,  muscle.  Here 
we  find  only  a  partial  explanation  of  the  source  of  urea,  for 
neither  does  muscle  contain  much  urea,  nor  does  active  muscular 
work  perceptibly  increase  the  general  urea  elimination.  In  mus- 
cle, however,  a  material  closely  allied  to  and  readily  convertible 
into  urea,  namely,  kreatin,  occurs,  and  it  has  been  suggested  that 
this  substance  is  changed  into  urea  in  the  kidney.  This  cannot 
explain  the  origin  of  all  the  urea,  for,  as  already  remarked,  the 
amount  of  urea  excreted  does  not  correspond  with  the  muscle 
metabolism. 

A  considerable  quantity  of  urea  no  doubt  comes  from  muscle, 
which  tissue  forms  so  large  a  part  of  our  bodies,  but  we  must 
conclude  that  there  are  many  other  sources  of  urea,  because  there 
are  many  other  organs  where  nitrogenous  substances  are  under- 
going chemical  changes  and  gradual  waste. 

The  liver  is  specially  worthy  of  note  as  a  source  of  urea,  since 
it  helps  to  explain  the  striking  relation  between  the  amount  of 
albuminous  food  and  the  quantity  of  urea  eliminated.  There  can 
be  no  doubt  that  most  people  consume  much  more  albuminous 
food  than  is  necessary  for  the  adequate  nutrition  and  preserva- 
tion of  the  nitrogenous  tissues,  and  therefore  must  have  a  surplus 
of  nitrogenous  material.  It  may  be  remembered,  as  was  pointed 
out  in  the  chapter  on  Digestion,  that  in  all  parts  of  the  aliment- 
35 


410  MANUAL    OF    PHYSIOLOGY. 

ary  tract  there  is  a  limit  to  the  absorption  of  peptones,  and  that 
in  the  small  intestine  when  delay  in  absorption  occurs  the  decom- 
position of  peptones  results,  because  in  prolonged  pancreatic 
digestion  these  peptones  are  changed  into  leucin  (C6H13NO2)  and 
tyrosin  (C9HUNO3),  and  as  such  pass  into  the  portal  circulation 
to  be  borne  to  the  liver.  In  the  liver  it  is  highly  probable  that 
these  bodies  are  converted  into  urea,  for,  when  they  are  intro- 
duced into  the  intestinal  tract,  they  are  absorbed  and  an  excess 
of  urea  appears  in  the  urine.  Thus  the  surplus  of  the  proteid 
food,  before  it  really  enters  the  system,  is  broken  up  in  the  intes- 
tine into  bodies  which,  notwithstanding  the  difficulty  of  explain- 
ing the  chemical  process,  may  be  regarded  as  a  step  toward  the 
formation  of  urea.  This  view  is  supported  by  the  facts  that  (1) 
in  disease  of  the  liver  tyrosin  and  leucin  appear  in  the  urine;  (2) 
if  these  bodies  be  introduced  into  the  general  circulation,  by  the 
jugular  instead  of  the  portal  vein,  they  are  excreted  unchanged 
by  the  kidneys. 

NERVOUS  MECHANISM  OF  THE  URINARY  SECRETION. 

With  regard  to  the  influence  exerted  by  the  nervous  system 
on  the  renal  secretion,  we  have  but  little  satisfactory  information, 
although  there  can  be  no  doubt  that  here,  as  in  other  glands,  the 
process  is  under  the  control  of  the  nerves.  Many  of  the  circum- 
stances which  cause  greater  activity  of  secretion,  such  as  taking 
large  quantities  of  water,  etc.,  have  no  effect  on  the  general  blood 
pressure,  so  that,  if  the  increased  flow  be  brought  about  by  the 
vasomotor  mechanisms,  it  must  be  by  means  of  nervous  chan- 
nels altering  the  blood  flow  in  the  special  arteries  of  the  glands. 
Further,  some  emotional  conditions  exist,  such  as  hysteria,  in 
which  an  unaccountably  great  quantity  of  urine  of  very  low 
specific  gravity  is  evacuated. 

With  regard  to  the  effects  of  the  vasomotor  nerves,  we  know 
that  section  of  all  the  nervous  twigs  going  to  the  kidneys  causes 
great  congestion  and  an  immense  increase  in  the  secretion,  which 
commonly  contains  albumin.  This  no  doubt  depends  on  the 
sudden  rise  in  pressure  in  the  glomeruli,  owing  to  the  dilatation 
of  the  arterioles.  If  the  spl  an  clinics,  in  which  the  renal  vaso- 


PASSAGE    OF   THE    URINE   TO   THE    BLADDER.  411 

motor  nerves  run,  be  cut,  a  great  quantity  of  urine  is  produced 
from  the  same  cause — vasomotor  paralysis — but,  on  account  of 
the  large  area  of  vessels  injured,  the  general  blood  pressure  falls, 
and  therefore  the  effect  is  not  so  much  marked.  If  the  peripheral 
end  of  the  cut  nerves  be  stimulated,  the  secretion  is  diminished, 
and,  owing  to  spasm  of  the  renal  arterioles  and  fall  of  blood 
pressure  in  the  glomerular  capillaries,  may  be  brought  to  a  stand- 
still. Section  of  the  spinal  cord  at  the  7th  cervical  vertebra 
stops  the  flow,  because  it  so  reduces  the  general  blood  pressure 
that  the  pressure  in  the  renal  vessels  falls  below  that  necessary 
for  the  filtration  of  the  urine. 

The  introduction  of  various  substances  into  the  blood  causes 
a  marked  change  in  the  blood  supply  of  the  kidney  and  the 
amount  of  urine  secreted.  These  changes  do  not  correspond 
with  the  changes  in  general  blood  pressure  occurring  during  the 
experiments. 

PASSAGE  OF  THE  URINE  TO  THE  BLADDER. 

The  pressure  exerted  by  the  blood  in  the  glomerular  capilla- 
ries is  quite  sufficient  to  make  the  urine  flow  from  the  pelvis  of 
the  kidneys  into  the  bladder,  because  when  the  ureters  are  tied 
they  become  distended  above  the  ligature  by  the  urine  flowing 
from  the  pelvis,  where  a  pressure  may  be  produced  of  some  forty 
millimetres  of  Hg,  at  which  pressure  the  secretion  stops  and 
becomes  somewhat  changed  in  chemical  composition  (kreatin 
appearing  in  greater  quantity). 

Normally,  however,  the  passage  of  the  urine  along  the  ureters 
is  accomplished  by  the  peristaltic  motion  of  the  ducts,  which  goes 
on  alternately  in  the  two  ureters,  so  that  the  urine  flows  into  the 
bladder  at  different  periods  from  the  right  and  left  kidney. 

The  ureters  have  a  strong  middle  coat  of  encircling  fibres  of 
smooth  muscle,  along  which  a  wave  of  contraction,  lasting  about 
one-third  of  a  second,  passes  rhythmically  in  about  6  to  10 
seconds  from  the  pelvis  of  the  kidney  to  the  bladder. 

Having  reached  the  bladder,  the  urine  cannot  return  into  the 
ureters  on  account  of  the  oblique  way  in  which  these  ducts  pass 
through  the  walls  of  the  bladder.  When  the  pressure  in  the 


412  MANUAL    OF    PHYSIOLOGY. 

bladder  increases,  the  opening  of  the  ducts  becomes  closed  and 
acts  as  a  kind  of  valve. 

RETENTION  OF  URINE  IN  THE  BLADDER. 

The  urine,  which  is  continuously  secreted  and  rhythmically 
conveyed  to  the  bladder,  is  only  voided  at  convenient  times ; 
therefore  special  arrangements  exist  for  its  retention  and  expul- 
sion. 

The  retention  of  urine  in  the  bladder  up  to  a  certain  point 
depends  on  the  elasticity  of  the  parts  concerned,  the  dense  elastic 
tissues  around  its  outlet  being  able  to  resist  the  elastic  force 
exerted  upon  its  contents  by  the  walls  of  the  bladder  and  the 
viscera.  Thus,  where  no  active  muscular  forces  can  possibly 
come  into  play,  as  in  the  case  of  the  dead  subject,  or  in  complete 
paralysis  following  destruction  of  the  spinal  cord,  a  considerable 
amount  of  urine  is  retained.  But  when  a  certain  pressure  is 
arrived  at  by  the  gradual  accumulation  of  urine  within  the  blad- 
der, the  elasticity  of  the  sphincter  and  the  other  tissues  around 
the  outlet  is  overcome  by  the  elasticity  of  the  bladder  wall,  and 
the  urine  slowly  dribbles  away. 

In  the  normal  condition,  however,  the  urine  is  retained  by  a 
muscular  mechanism  over  which  we  have  acquired  considerable 
voluntary  control. 

This  is  the  sphincter  muscle,  which,  by  contracting,  helps  the 
elastic  power  of  the  tissues  around  the  urethra  and  retains  the 
urine.  The  accumulation  of  urine  after  a  certain  time  gives  the 
sensation  known  as  a  full  bladder,  but  this  feeling  is  not  neces- 
sarily accompanied  by  any  irresistible  call  to  make  water,  though 
it  soon  produces  a  desire  in  that  direction.  We  suppose  that  the 
stimulus  given  to  the  afferent  nerves  by  filling  the  bladder 
reflexly  causes  a  constriction  of  the  sphincter  muscle,  so  that,  in 
proportion  as  the  pressure  within  the  bladder  increases,  the 
resistance  to  its  outflow  is  also  augmented.  This  does  not  imply 
any  automatic  action  of  the  sphincter  vesicse,  but  merely  a  con- 
stant reflex  excitation  of  that  muscle,  which  secures  its  contrac- 
tion and  the  retention  of  a  considerable  amount  of  urine  without 
the  intervention  of  voluntary  influences  or  attention.  When 


EVACUATION    OF    THE    BLA1XDEE.  413 

the  bladder  becomes  very  full,  the  reflex  mechanism  may  require 
the  assistance  of  the  voluntary  centres  to  augment  this  power 
and  prevent  the  urine  being  evacuated. 

EVACUATION   OF   THE    BLADDER. 

Micturition,  or  the  expulsion  of  the  urine,  does  not  normally 
depend  on  elastic  forces  alone,  as  in  the  case  mentioned  of  para- 
lytic incontinence,  when  the  urine  commences  to  dribble  away  as 
soon  as  a  certain  pressure  is  attained  within  the  bladder,  but  is 
accomplished  by  the  detrusor  urince  muscle  which  lies  in  the  wall 
of  the  bladder. 

Under  ordinary  circumstances  there  is  a  relationship  between 
the  expelling  and  retaining  powers  (neither  the  muscle  in  the 
wall  of  the  bladder  nor  voluntary  effort,  however,  coming  into 
action),  in  which  the  retaining  power  of  the  sphincter  is  just 
able  to  resist  the  elastic  pressure.  If  the  urine  be  retained  for  a 
considerable  time,  the  reflex  stimulation  of  the  sphincter  no 
longer  suffices  to  keep  back  the  fluid,  and  the  voluntary  effort 
has  to  be  called  to  the  aid  of  the  reflex  action  of  the  sphincter. 
If  a  drop  of  urine  happen  now  to  make  its  way  into  the  sensitive 
urethra,  matters  are  altered.  Even  voluntary  effort  does  not 
suffice  to  keep  back  the  stream,  and  an  irresistible  call  to  empty 
the  bladder  is  made  upon  the  spinal  nerve  mechanisms.  This  is 
accomplished  by  the  contraction  of  the  muscular  coat  of  the 
bladder,  which  is  excited  reflexly  by  the  stimulus  starting  from 
the  mucous  membrane  lining  the  urethra. 

When  the  urine  once  commences  to  flow,  it  continues  until  the 
bladder  is  quite  empty,  the  last  drops  of  urine  being  expelled 
from  the  urethra  by  rhythmical  spasms  of  the  muscles  around 
the  bulbous  portion  of  that  canal.  The  sequence  of  events  will 
then  be — (1)  stimulation  of  the  mucous  membrane  of  the  urethra 
by  a  drop  of  urine ;  (2)  contraction  of  the  detrusor  urinse ;  (3) 
relaxation  of  the  sphincter ;  (4)  rhythmical  contraction  of  the 
ejaculator  urinse,  and,  finally,  a  twitch  of  the  levator  ani  and 
neighboring  muscles. 

The  evacuation  of  the  bladder  is,  under  these  circumstances, 


114 


MANUAL   OF   PHYSIOLOGY. 


accomplished  independently  of  the  will  by  a  reflex  act,  of  which 
we  may  even  be  unconscious. 

This  reflex  micturition  may  occur  during 
the  sleep,  as  the  result  of  slight  local  ex- 
citations. In  infants  this  is  the  normal  mode 
of  emptying  the  bladder,  and  the  gradual 
education  of  the  centres  controlling  the  re- 
tention mechanisms  is  watched  with  interest 
in  young  children. 

At  an  early  age,  generally,  we  learn  to 
control  the  acts  of  these  centres  by  our  will. 
We  feel    a    desire   to   empty   the  bladder 
before  it  becomes  so  distended  that  the  reflex 
contraction  of  the  sphincter  is  insufficient 
to  retain  the  urine.     But  the  volition  serves 
to  call  into  activity  the  reflex  mechanism 
just  described.     Almost  at  any  time  we  can 
call  forth  the  reflex  act  by  increasing  the 
Mictvuri-  pressure  on  the  bladder  by  voluntary  con- 
traction  of   the    abdominal    muscles ;    the 
diaphragm  being  depressed   and  fixed,  the 
fromputhe|  muscles  of  expiration  are  put   into   action 
so  as  to  press  upon  the  pelvic  viscera.     At 
the    same     time   the     contraction    of   the 
re-  sphincter  muscle   is   probably  checked   by 
rhen  the  bladder  is  dis-  the  will,  and  thus  the  power  of  retention  is 

tended,  impulses  pass  to 

overcome. 

The  moment  the  balance  of  power  is  thus 


tl°un'Biadder. 


fhe^nafcord^whence 
' 


When 


the  brain  by  1,  and  when 
we  will,  the  tonus  of  the 
spinal  centre  stimulat- 
ing the  sphincter  is  ,  .  „  - 

checked,  and  the  abdom-  turned  in  tavor  oi  the  expelling  agencies    a 

inal  muscles    are   made     •,  ,,  .        , 

by  2  to  force  some  urine  drop  oi  urine  reaches  the  begrinninsr  of  the 

into   the    neck    of    the  iL  , 

bladder,    whence    im-   urethra    and    excites    reflexly    the    spinal 

Rulses  pass  by  3  to  in-  j      i          i     •  i 

iMt  the  sphincter  cen-  centres,  and  thus  brings  about  the  complete 

tfusornthreoXughV  °     '"  evacuation  of  the  bladder  without  further 

voluntary  effort. 

The  nervous  mechanism  that  controls  the  act  of  micturition  con- 
sists essentially  of  ganglionic  centres  which  are  situated  in  the 


EVACUATION    OF   THE    BLADDER.  415 

lumbar  enlargement  of  the  spinal  cord,  and  of  two  sets  of  nerve 
channels  passing  to  and  from  these  centres.  The  centres  may  be 
said  to  be  composed  of  functionally  distinct  parts — a  retaining 
and  evacuating  part.  The  retaining  centre  causes  the  sphincter 
muscle  to  contract.  The  evacuating  centre  can  excite  the  detru- 
sor  to  action.  One  set  of  nerve  channels  (3,  4,  R,  T)  communi- 
cates between  these  centres  and  the  urinary  organs  (B),  and  the 
other  (1,  2)  between  the  cord  centres  and  the  cerebral  hemi- 
spheres (c).  That  which  connects  the  special  lumbar  centres  with 
the  bladder,  contains  efferent  fibres  of  two  kinds,  going  to  the 
antagonistic  muscles,  the  sphincter  vesicse  (T),  and  the  detrusor 
urinse  (4)  respectively,  and  afferent  fibres  of  different  kinds  ; 
those  (R)  going  from  the  bladder  to  the  nerve  cells  in  the  cord 
which  stimulate  them  and  cause  the  sphincter  to  remain  tonically 
contracted ;  those  passing  from  the  mucous  membrane  of  the 
urinary  passages  to  the  ganglionic  cells  in  the  cord  have  two 
functions  ;  one  (4)  excites  the  contractions  of  the  detrusor  urinse 
and  the  other  (3)  inhibits  the  tonic  action  of  the  retaining  centre. 

The  action  of  the  ganglionic  cells  that  stimulate  the  sphincter 
muscle  can  to  a  certain  extent  be  either  aided  or  checked  by 
means  of  voluntary  or  other  cerebral  influences,  so  that  two 
kinds  of  fibres — a  stimulating  and  inhibitory  one — must  pass 
from  the  hemispheres  to  the  micturating  centre  in  the  cord. 

Those  cells  which  govern  the  motions  of  the  detrusor  seem  to 
be  least  under  voluntary  control,  and  are  probably  only  stimu- 
lated to  action  by  the  impulses  arising  from  the  urinary  passages, 
and  hence  are  simply  reflex  centres. 

The  effect  of  certain  emotions  on  the  act  of  micturition  seems 
to  show  that  those  ganglion  cells  in  the  cord  which  cause  the 
bladder  to  contract  are  connected  with  the  higher  centres.  Thus, 
extreme  terror  (in  a  dog  at  least)  often  causes  a  forcible  expul- 
sion of  urine,  and  great  anxiety  or  impatience  seems  in  man  often 
to  have  a  checking  influence,  causing  great  delay  in  initiating 
micturition. 


416  MANUAL   OF    PHYSIOLOGY. 


CHAPTER  XXIII. 
NUTRITION. 

We  can  compare  the  incomings  and  outgoings  of  the  economy, 
and  should  now  be  in  a  position  to  see  what  light  can  be  thrown 
by  this  comparison  upon  the  actual  changes  which  take  place  in 
the  textures  of  the  body. 

We  have  seen  that  the  income  is  made  up  of  substances  be- 
longing to  the  same  groups  of  materials  as  are  found  in  the  body, 
viz.,  albumins,  fats,  carbohydrates,  salts,  and  water,  introduced 
by  the  alimentary  canal,  and  oxygen,  which  is  acquired  by  the 
respiratory  apparatus ;  while  the  outgoings  consist  of  urea  from 
the  kidneys,  carbonic  acid  from  the  lungs,  certain  excrement 
from  the  intestine  and  other  mucous  passages,  sweat,  sebaceous 
secretion,  epidermal  scales,  from  the  skin  ;  together  with  a  quan- 
tity of  water  from  all  these  ways  of  exit.  The  milk,  ova,  and 
semen  may  be  here  omitted,  being  regarded  as  exceptional 
losses  devoted  to  special  objects. 

In  order  that  the  body  may  be  kept  in  its  normal  condition, 
it  is  necessary  that  the  income  should  at  least  be  equal  to  the 
outgoings  of  all  kinds,  and,  except  where  growth  is  going  on 
rapidly,  an  income  equal  to  the  expenditure  ought  not  only  to 
suffice,  but  ought  to  be  the  most  satisfactory  for  the  economy. 

We  know  that  animals  can  live  for  some  considerable  time 
without  food,  in  which  case  a  certain  expenditure  of  material 
derived  from  the  body  itself  is  necessary  to  sustain  life,  and 
therefore  the  outgoings  continue.  We  ought  thus  to  be  able  to 
arrive  in  a  very  simple  manner  at  the  minimal  expenditure  ne- 
cessary for  the  sustentation  of  the  body.  We  shall  find,  however, 
that  (I)  an  income  equal  to  this  minimal  expenditure  (that  of 
starvation)  does  not  at  all  suffice  to  keep  up  the  body  weight, 
and  that  (2)  a  considerable  margin  over  and  above  this  mini- 
mum is  necessary  in  order  to  establish  the  nutritive  equilibrium  ; 
(3)  further,  that  the  proportion  of  material  eliminated  and  stored 


TISSUE   CHANGES   IN   STARVATION.  417 

up  in  the  body  respectively  varies  as  the  income  is  increased  ;  (4) 
and  finally,  that  the  quality  of  the  food — i.  e.,  the  proportion  of 
each  group  of  food  stuff  present  in  the  diet — has  an  important 
influence  on  the  quantity  required  to  establish  the  equilibrium, 
and  that  best  suited  to  cause  increase  of  weight  or  to  fatten. 

It  will  be  convenient  to  consider  the  following  different  cases 
in  succession. 

1.  No  income,  except  oxygen,  i.  e.,  starvation. 

2.  An  income  only  equal  to  the  expenditure  found  during 
starvation. 

3.  Establishment  of  perfect  nutritive  equilibrium. 

4.  Excessive  consumption. 

TISSUE  CHANGES  IN  STARVATION. 

As  is  well  known,  deprivation  of  oxygen — by  cessation  of  the 
respiratory  function — almost  immediately  puts  an  end  to  the 
tissue  changes  necessary  for  life,  so  that  the  oxygen  income  can- 
not be  interfered  with,  or  the  experiment  comes  to  an  end.  It 
has  also  been  found  that  a  small  supply  of  water  to  drink  makes 
the  investigation  of  the  various  tissue  changes  more  reliable,  by 
facilitating  them  and  prolonging  life.  We  therefore  speak  of  a 
total  abstinence  from  solids  as  starvation. 

When  deprived  of  food,  those  tissues  upon  the  activity  of 
which  life  immediately  depends  must  feed  upon  materials  stored 
up  in  some  tissues  of  less  vital  importance  to  the  animal.  The 
first  questions  to  discuss  are  how  much  the  body  loses  daily  in 
weight  during  the  time  that  it  is  thus  feeding  on  itself,  and  how 
far  the  different  individual  tissues  contribute  to  this  loss. 

The  general  loss  of  weight  is  directly  estimated  by  weighing 
the  animal,  and  the  loss  of  the  individual  tissues  is  calculated  by 
a  careful  analysis  of  the  various  excreta,  by  which  the  exact 
amount  of  nitrogen,  carbonic  acid,  etc.,  is  ascertained  :  the  nitro- 
gen corresponds  to  the  loss  of  muscle ;  and  the  carbon  (after 
excluding  that  portion  which  is  the  outcome  of  muscle  change, 
which  may  be  calculated  from  the  nitrogen)  corresponds  to  the 
fats  oxidized. 

Loss  of  Weight. — It  has  been  found  that  a  starving  animal 


418  MANUAL    OF    PHYSIOLOGY. 

loses  weight  rapidly  at  first,  and  subsequently  more  slowly.  The 
cause  of  this  difference  is  that  the  food  last  eaten  continues  to 
have  influence  during  the  first  three  or  four  days,  and  the  mate- 
rials eliminated  are  proportionately  large  in  quantity.  When  the 
influence  of  the  food  taken  prior  to  starvation  has  ceased,  the 
daily  amount  of  materials  eliminated  is  much  reduced,  and 
remains  nearly  constant,  decreasing  slightly  in  proportion  as  the 
body  weight  diminishes  slowly  until  the  animal's  death. 

Adult  animals  generally  live  until  they  have  lost  about  half  of 
their  normal  body  weight.  Young  animals  die  when  they  have 
lost  about  20  per  cent,  of  their  weight. 

Relative  Loss  in  Various  Tissues. — Roughly  speaking,  we  may 
take  the  body  of  a  man  to  be  made  up  of  the  following  propor- 
tions of  the  more  important  textures : — 

Muscles 50  per  cent. 

Skin  and  fat 25       " 

Viscera 12       " 

Skeleton 13       u 

Seeing  that  the  muscle  tissue  contributes  such  a  large  propor- 
tion to  the  body  weight,  we  cannot  be  surprised  that  in  starva- 
tion the  greatest  absolute  loss  occurs  in  this  tissue,  except  in  the 
case  of  excessively  fat  animals.  Next  comes  adipose  tissue, 
which  almost  entirely  disappears,  so  that  the  relative  loss  is  here 
greatest,  but  the  absolute  loss  varies  in  proportion  to  the  fatness 
of  the  animal  at  the  beginning  of  the  investigation.  The  spleen 
and  liver  lose  more  than  half  their  weight,  and  the  amount  of 
blood  is  greatly  reduced.  The  smallness  of  the  loss  that  occurs 
in  the  great  nervous  centres  is  very  striking.  They  seem  to  feed 
on  the  other  tissues. 

The  following  table  gives  the  approximate  percentage  of  loss 
which  takes  place  in  each  individual  tissue  during  starvation  :  — 

£at"; 97.0  per  cent. 

Muscle 30.2       " 

Liyer 56.6       " 

Spleen gg.j       « 

Blood 176       « 

Nerve  centres 0          " 

With  regard  to  the  portals  by  which  the  various  materials  make 
their  escape,  it  has  been  found  that  practically  art  the  nitrogen 


INCOME    EQUAL   TO   OUTPUT    OF   STARVATION.  419 

passes  off  with  the  urine,  and  about  nine-tenths  of  the  carbon 
escapes  by  the  lungs  as  CO2,  the  remaining  one-tenth  passing  off 
by  the  intestine  and  kidneys.  Three-fourths  of  the  water  is  found 
in  the  urine,  and  one-fourth  goes  off  from  the  skin  and  lungs. 

The  following  table  shows  the  items  of  the  general  loss,  and 
the  amount  per  cent,  which  passes  out  by  the  chief  channels  of 
exit : — 


Total 
Elimination. 

Via 
Kidneys. 

Lungs 
and  Skin. 

Excrement. 

Water. 

995  34  gnu 

70  2  % 

26  1  % 

37% 

Gftrbon 

205  96     "' 

6  4  % 

92  6  % 

1  9  % 

j^itiweii  • 

30  81     " 

100  0  % 

Salts  

.     10.03     " 

97.0  % 

2.4  % 



As  the  loss  of  weight  of  an  animal's  body  during  starvation  is 
at  first  rapid  and  then  more  gradual,  so  also  the  amount  of  mate- 
rial eliminated  is  found  to  diminish  much  more  slowly  after  the 
first  few  days.  This  is  well  seen  from  the  nitrogenous  elimina- 
tion. For  the  first  four  days  the  fall  in  the  amount  of  urea 
excreted  is  very  rapid,  it  then  decreases  slowly  and  almost  con- 
stantly until  the  death  of  the  animal.  The  subsequent  fall  is  in 
proportion  to  the  slow  decrease  in  weight  of  the  animal.  This 
has  led  to  the  conclusion  that  the  nitrogenous  material  eliminated 
during  a  full  diet  comes  partly  from  used-up  nitrogenous  tissues, 
and  partly  from  nitrogenous  materials  which  have  never  really 
entered  into  the  composition  of  the  tissues,  but  are  the  surplus  of 
nitrogenous  food.  Hence,  two  kinds  of  proteid  are  supposed  to 
exist  in  the  body,  viz.,  (1)  that  forming  part  of  the  tissues,  and 
(2)  that  circulating  as  a  ready  supply  for  the  nutritive  demands 
of  the  tissues. 

AN  INCOME  EQUAL  TO  THE  OUTPUT  OF  STARVATION. 

In  the  second  case  mentioned,  namely,  where  an  amount  of 

food  is  supplied  which  is  just  equal  to  the  expenditure  which  was 

found  to  take  place  during  starvation,  one  might  suppose  that 

the  diet,  though  minimal,  would  yet  suffice  to  preserve  the  nor- 


420  MANUAL    OF    PHYSIOLOGY. 

mal  body  weight.     Practice,  however,  shows  this  to  be  far  from 
being  the  case. 

An  animal  fed  on  diet  equal  in  quantity  to  the  outgoings  dur- 
ing starvation  continues  to  lose  weight,  and  the  quantity  of  nitro- 
genous substance  eliminated  (urea)  is  in  excess  of  the  low 
standard  found  during  complete  abstinence  from  food.  From 
this  it  would  appear  that  even  when  supplied  with  an  amount  of 
nitrogenous  material  equal  to  that  used  by  the  tissues  during 
starvation,  an  animal  takes  a  further  supply  from  its  own  tex- 
tures, and  eliminates  some  of  the  nitrogenous  nutriment  without 
using  it.  The  body  subsists  on  the  scanty  allowance  of  nutriment 
it  borrows  from  the  tissues  during  starvation  only  so  long  as 
there  is  absolutely  no  food  income.  When  food  is  supplied,  an 
increased  expenditure  is  set  up,  the  income  is  exceeded,  and  a 
deficit  occurs  in  the  nitrogen  balance.  Or,  probably,  some  of  the 
nitrogenous  nutriment  is  rendered  useless  by  the  processes  it 
undergoes  in  the  intestine,  even  when  the  quantity  is  not  sufficient 
to  support  the  equilibrium  (compare  pp.  165,  166,  409,  410). 

It  follows,  then,  that  feeding  an  animal  on  an  amount  of  food 
stuffs  exactly  corresponding  to  the  quantity  of  nutriment  ab- 
stracted from  its  own  textures  during  total  abstinence  is  only  a 
slower  form  of  starvation. 

With  regard  to  nitrogenous  substances,  it  has  been  proved  that 
nearly  three  times  as  much  as  the  amount  eliminated  during 
starvation  is  required  to  establish  an  equilibrium  between  the 
income  and  expenditure  of  those  special  substances,  and  that  less 
than  this  leads  to  a  distinct  nitrogenous  deficit. 

NUTRITIVE  EQUILIBRIUM. 

The  third  case  mentioned,  viz.,  that  in  which  the  nutritive 
equilibrium  is  exactly  maintained,  so  that  the  body  weight 
remains  unaltered,  is  the  most  important  one  for  us  to  determine, 
since  its  final  settlement  would  enable  us  to  fix  the  most  bene- 
ficial standard  of  diet.  Unfortunately,  this  case  is  also  the  most 
difficult  upon  which  to  come  to  a  satisfactory  conclusion,  for  the 
following  reasons : — 

1.  The  elaborate  nature  of  the  conditions  imposed  during  the 


NUTRITIVE    EQUILIBRIUM.  421 

experiment  makes  it  difficult  to  carry  on  the  investigation  with 
scientific  accuracy. 

2.  Even  when  the  amounts  of  gain  and  loss  exactly  correspond 
we  cannot  say  that  we  have  the  best  dietary ;  because  some  of  the 
income  may   be  quite  useless,  and  pass  through  the   economy 
without  performing  any  function,  and  yet  appear  in  the  output 
so  as  to  give  an  accurate  balance. 

3.  We  have  just  seen  that  the  relative  amounts  of  outgoings 
and  of  material  laid  by  as  store  are  altered  and  regulated  by  the 
quantity  of  income.     And  we  find  that  the  quality  of  the  income, 
i.e.,  the  relative  proportions  of  the  various  food  stuffs,  has  a 
material  influence  on  the  quantities   of  material  laid  by  and 
eliminated  respectively.  We  must,  therefore,  consider  the  efficacy 
of  each  of  the  groups  of  the  food  stuffs  when  employed  alone 
and  mixed  in  different  proportions. 

4.  Different  animals  seem  to  have  different  powers  of  assimi- 
lation ;  and  under  various  circumstances  the  requirements  and 
assimilative  power  of  the  same  animal  may  vary. 

Nitrogenous  Diet. — An  animal  fed  upon  a  purely  meat  diet 
requires  a  great  amount  of  it  to  sustain  its  body  weight.  It  has  been 
found  that  from  -fa  to  -fa  of  the  body  weight  in  lean  meat  daily 
is  necessary  to  keep  an  animal  alive  without  either  losing  or  gain- 
ing weight.  If  more  than  this  amount  be  supplied  the  animal 
increases  in  weight,  and  as  its  weight  increases  a  greater  amount 
of  meat  is  required  to  keep  it  up  to  the  new  standard.  So 
that,  to  produce  a  progressive  increase  of  weight  with  a  purely 
meat  diet,  it  is  necessary  to  keep  on  increasing  the  quantity  of 
meat  given.  The  reason  of  this  is  found  in  the  fact  that  albu- 
minous diet  causes  an  increase  in  the  changes  occurring  in  the 
nitrogenous  tissues. 

If  an  animal  which  is  in  extremely  poor  condition  be  given  an 
ad  libitum  supply  of  lean  meat,  only  a  limited  portion  of  the 
albuminous  substance  is  retained  in  the  tissues.  By  far  the  larger 
proportion  of  the  nitrogenous  food  is  given  off  and  is  represented 
in  the  urine  by  urea,  and  a  comparatively  small  proportion  is 
stored  up.  If  this  large  supply  of  meat  diet  be  continued  for 
some  time,  less  and  less  of  the  albuminous  material  is  stored, 


422  MANUAL    OF    PHYSIOLOGY. 

more  and  more  being  eliminated  as  urea,  until  finally  the  urea 
excreted  just  corresponds  to  the  albuminous  materials  in  the 
ingesta.  When  only  meat  is  given,  it  must  be  supplied  in  large 
quantities  to  maintain  the  balance  of  nitrogenous  income  and 
expenditure,  which  is  spoken  of  as  nitrogenous  equilibrium. 
Upon  the  occurrence  of  a  change  in  the  amount  of  nitrogenous 
ingesta  this  nitrogenous  equilibrium  varies,  and  it  takes  some  time 
to  become  reestablished,  because  a  decrease  in  the  meat  diet  is 
accompanied  by  a  decrease  in  the  weight  of  the  animal,  and  an 
increase  causes  it  to  put  on  flesh.  For  each  new  body  weight 
there  is  a  new  nitrogenous  equilibrium,  which  is  only  attained 
after  the  disturbed  relation  between  the  nitrogenous  ingesta 
and  excreta  has  been  readjusted. 

The  increase  of  weight  which  follows  a  liberal  meat  diet 
depends  in  a  great  measure  on  fat  being  stored  up  in  the  body. 
Much  more  of  this  material  is  made  than  could  come  from  the 
fat  taken  with  the  meat ;  hence,  we  must  conclude  that  it  is  made 
from  the  albuminous  parts  of  the  meat. 

Non-nitrogenous  Diet. — The  effect  of  a  diet  without  any  albu- 
minous food  is  that  the  animal  dies  of  starvation  nearly  as  soon 
as  if  deprived  of  all  forms  of  food,  with  the  exception  that  the 
weight  of  the  body  is  much  less  reduced  at  the  time  of  death. 

Mixed  Diet. — The  addition  of  fat  or  sugar  to  meat  diet  allows 
of  a  considerable  reduction  in  the  supply  of  meat,  both  the  body 
weight  and  nitrogenous  tissue  change  preserving  their  equilibrium 
on  a  smaller  amount  of  food.  It  has  been  estimated  that  the 
nitrogenous  tissue  change  is  reduced  seven  per  cent,  by  the  addi- 
tion of  fat,  and  ten  per  cent,  by  the  addition  of  carbohydrate 
food  to  the  meat  diet ;  therefore  less  meat  is  wanted  to  make 
up  nitrogenous  tissues.  Further,  fats  and  sugars,  which  obvi- 
ously cannot  of  themselves  form  an  adequate  diet,  since  they 
contain  no  nitrogen,  seem  to  have  the  power  of  accomplishing 
some  end  in  the  economy  which,  in  their  absence,  requires  a  con- 
siderable expenditure  of  nitrogenous  materials  to  bring  about. 
Fats  and  sugars,  then,  supply  to  the  body  readily  oxidizable 
materials,  and  thus  shield  the  albuminous  tissues  from  oxida- 
tion, as  well  as  reduce  absolutely  the  nitrogenous  metabolism. 


NUTRITIVE    EQUILIBRIUM.  423 

It  would  further  appear  from  the  experience  gained  from  the 
stall  feeding  of  animals  that  a  good  supply  of  carbohydrates, 
together  with  a  limited  quantity  of  nitrogenous  food,  is  admirably 
adapted  to  produce  fat.  Since  much  more  fat  has  been  found 
to  be  produced  in  pigs  than  could  be  accounted  for  by  the  albu- 
minous and  fatty  constituents  of  their  diet,  we  must  suppose 
that  from  their  carbohydrate  food  fat  can  be  manufactured  in 
their  body. 

Much  of  the  difficulty  found  in  reconciling  the  opinions  of 
different  authors  concerning  the  sources  of  fat  in  the  body  can 
be  removed,  and  some  knowledge  of  the  manufacture  of  fats  from 
the  food  stuffs  can  be  gained  by  bearing  in  mind  the  properties 
of  the  protoplasm.  There  can  be  no  doubt  that  protoplasm,  if 
properly  nourished,  can  manufacture  fat.  As  examples,  we  may 
take  the  cells  of  the  mammary  gland  and  connective  tissue. 
This  fat  production  may  be  regarded  as  a  secretion  of  fat,  though 
only  in  one  of  the  examples  given  does  it  appear  externally  as 
a  definite  secretion — milk.  We  cannot  scrutinize  the  chemical 
methods  by  which  this  change  is  brought  about  in  protoplasm, 
any  more  than  those  which  give  rise  to  the  special  constituents 
of  other  secretions.  We  know  that  protoplasm  uses  as  pabulum, 
albumin,  fat,  and  carbohydrate,  and  we  have  no  reason  to  doubt 
that  the  proportion  of  these  materials  found  to  form  the  most 
nutritious  diet  for  the  body  generally,  is  also  the  proportion  in 
which  protoplasm  can  best  make  use  of  them.  Probably  cells 
which  secrete  a  material  containing  nitrogen,  such  as  mucin- 
yielding  gland  cells,  require  a  greater  proportion  of  albumin. 
Those  cells  which  produce  a  large  quantity  of  non-nitrogenous 
material  may  not  require  more  nitrogen  than  is  necessary  for 
their  perfect  re-integration  as  nitrogenous  bodies.  In  the  manu- 
facture of  their  secretion,  they  only  require  a  pabulum  which 
contains  the  same  chemical  elements  as  are  to  be  found  in  the 
output.  In  the  case  of  fat  formation,  a  supply  of  fat  or  carbo- 
hydrate ought  to  suffice  if  accompanied  by  a  small  amount  of 
albuminous  substance.  If  these  non-nitrogenous  substances  be 
withheld,  the  protoplasm  could  no  doubt  obtain  the  quantity  of 
carbon,  hydrogen,  and  oxygen  requisite  to  manufacture  fat  from 


424  MANUAL    OP   PHYSIOLOGY. 

albumin,  but  this  would  not  be  economical,  for  a  large  amount 
of  nitrogen  would  be  wasted. 

.  Fat  cannot  be  produced  by  the  tissue  cells  without  nitrogen 
in  the  diet,  because  the  fat-manufacturing  protoplasm  cannot 
live  without  nitrogen,  which  is  absolutely  necessary  for  its  own 
assimilative  re-integration.  A  good  supply  of  nitrogenous  food 
aids  in  fattening,  since  it  gives  vigor  to  all  the  protoplasmic  meta- 
bolism, and  among  them  fat  formation. 

The  albuminoid  substance  gelatine,  which  is  an  important  item 
in  the  food  we  ordinarily  make  use  of,  is  able  to  effect  a  saving 
in  the  albuminous  food  stuffs.  Although  it  contains  a  sufficiently 
large  proportion  of  nitrogen,  it  cannot  satisfactorily  replace 
albumin  in  the  food.  Indeed,  in  spite  of  the  great  similarity  in 
its  chemical  composition  to  albuminous  bodies,  it  can  no  better 
replace  the  proteids  in  a  dietary  than  fat  or  carbohydrate  ;  and, 
although  an  animal  uses  up  less  of  its  tissue  nitrogen  on  a  diet 
containing  gelatine  and  fat  than  when  it  is  fed  on  fat  alone, 
it  dies  of  starvation  almost  as  soon  as  if  its  diet  contained  no 
nitrogenous  substance. 

EXCESSIVE  CONSUMPTION. 

The  last  case  we  have  to  consider  is  that  in  which  the  supply 
of  food  material  is  in  excess  of  the  requirements  of  the  economy. 
This  is  certainly  the  commonest  case  in  man. 

Much  of  the  surplus  food  never  really  enters  the  system,  but  is 
conveyed  away  with  the  fseces. 

In  speaking  of  pancreatic  digestion,  reference  has  been  made 
to  the  possible  destiny  of  excess  of  nitrogenous  food.  In  the 
intestine,  some  of  it  is  decomposed  into  leucin  and  tyrosin,  which 
are  absorbed  into  the  intestinal  blood  vessels.  In  the  body  these 
substances  undergo  further  changes,  which  probably  take  place 
in  the  liver.  As  a  result  of  the  absorption  of  leucin,  a  larger 
quantity  of  urea  appears  in  the  urine,  and  hence  the  leucin 
formed  in  the  intestine  by  prolonged  pancreatic  digestion  is  an 
important  source  of  urea.  This  view  is  supported  by  the  almost 
immediate  increase  in  the  quantity  of  urea  eliminated  when 
albuminous  food  is  taken  in  large  quantity. 


EXCESSIVE    CONSUMPTION.  425 

From  the  fact  that  a  considerable  amount  of  fat  may  be  stored 
up  by  an  animal  supplied  with  a  liberal  diet  of  lean  meat,  we 
must  conclude  that  part  at  least  of  the  surplus  albumin  goes  to 
form  fat.  It  has  been  suggested  that,  after  sufficient  albumin  has 
been  absorbed  for  the  nutritive  requirements  of  the  nitrogenous 
tissues,  the  rest  is  split  up  into  two  parts,  one  of  which  is  imme- 
diately prepared  for  elimination  as  urea  by  the  liver,  and  the 
other  undergoes  changes,  probably  in  the  same  organ,  which 
result  in  its  being  converted  into  fat. 

It  would  further  seem  probable,  from  the  manner  in  which  the 
urea  excretion  changes  during  starvation,  that,  as  before  men- 
tioned, the  absorbed  albumin  exists  in  the  economy  in  two  forms  : 
one  in  which  it  has  been  actually  assimilated  by  the  nitrogenous 
tissues  and  forms  part  of  them,  and  hence  is  called  organ 
albumin  ;  the  other,  which  is  merely  in  solution  in  the  fluids 
of  the  body,  being  in  stock,  but  not  yet  ultimately  assimilated, 
and  hence  called  circulating  albumin.  The  latter  passes  away 
during  the  first  few  days  of  starvation,  being  probably  broken 
up  to  form  urea,  and  a  material  which  serves  the  turn  of  nou- 
nitrogenous  food.  The  organ  albumin  appears  to  supply  the  urea 
after  the  circulating  albumin  has  completely  disappeared. 

From  the  foregoing  it  will  be  gathered  that  we  cannot  say 
what  are  the  exact  destinies  of  the  various  food  stuffs  in  the 
body.  Proteids  are  not  exclusively  utilized  in  the  re-integration 
of  proteid  tissues,  as  an  excess  gives  rise  to  a  deposit  of  fat. 
Carbohydrates  are  not  employed  simply  to  replace  the  carbohy- 
drates constituting  part  of  the  tissues,  but,  as  will  be  shown  when 
speaking  of  muscle  metabolism,  they  are  intimately  related  to 
the  chemical  changes  which  take  place  during  the  activity  of 
that  tissue.  If  fats  are  chiefly  devoted  to  the  restitution  of  the 
fat  of  the  body,  they  certainly  are  not  the  only  kind  of  food 
from  which  fat  can  be  made. 

We  may  say,  then,  that  all  food  stuffs  are  destined  to  feed  the 
living  protoplasm,  whether  it  be  in  the  form  of  gland  cells,  the 
cells  of  the  connective  tissues,  or  muscle  plasma,  so  that  all  the 
food  stuffs  that  are  really  assimilated  contribute  to  the  main- 
tenance of  protoplasm  and  subserve  its  various  functions. 
36 


426  MANUAL    OF    PHYSIOLOGY. 

Besides  nourishing  itself  and  keeping  itself  up  to  a  certain 
standard  composition,  protoplasm,  or  rather  the  various  proto- 
plasmata,  can  make  the  different  chemical  materials  we  find  in 
the  body.  Some  produce  fat,  some  animal  starch  (glycogen), 
and  others  manufacture  the  various  substances  we  find  in  the 
secretions ;  while  yet  another  group  is  devoted  to  setting  free 
and  utilizing  the  energy  of  the  various  chemical  associations. 

But  all  the  food  we  eat  is  not  assimilated  ;  indeed,  the  destiny 
of  the  numerous  ingredients  of  our  complex  dietaries  is  not  easily 
traced.  Of  food  stuffs  proper,  the  following  classification  may  be 
made,  showing  that  even  the  same  stuff  may  meet  with  a  different 
fate  under  different  circumstances: — 

1.  Stuffs  which  never  enter  the  economy  (faeces). 

2.  Materials   absorbed   and   arriving    at    the   blood    are    at 

once  carried  to  certain  portals  of  excretion  (excess  of 
salts). 

3.  Substances  which  are  broken  up  in  the  intestine  to  facilitate 

their  elimination  (excess  of  proteid). 

4.  Substances  absorbed  and  carried  along  by  the  fluids,  but 

not  really  united  to  the  tissues  (circulating  albumin). 

5.  Materials  which  after  their  absorption  are  really  assimilated 

by  the  protoplasm  of  the  tissues  (a  certain  amount  of  all 
food  stuffs). 

6.  Substances  which,  after   their   assimilation  by  the  proto- 

plasm, reappear  in  their  original  form  and  are  stored  up 
(fats). 

The  question  of  the  exact  amounts  and  materials  required  to 
form  the  most  economic  and  wholesome  dietary  is  one  of  too  great 
practical  importance  to  receive  adequate  attention  in  this  manual. 
As  a  rule,  men,  like  other  animals,  partake  of  food  largely  in 
excess  of  their  physiological  requirements  when  they  can  get  it. 
This  may  be  seen  by  contrasting  one's  own  daily  food  with  the 
amount  which  has  been  found  to  be  adequate  in  the  case  of  indi- 
viduals who  have  not  the  opportunity  of  regulating  their  own 
supplies  of  comestibles. 

An  adult  man  should  be  well  nourished  if  he  be  supplied  with 
the  following  daily  diet : — 


ULTIMATE   USES   OF    FOOD   STUFFS.  427 

Albuminous  foods 100  grms.  or    3.5  ozs. 

Fats 90     "       "     3.1    " 

Starch 300     "       "10.7    " 

Salts 30      "       u     1.0    " 

Water 2000     "       "     3  pints. 

As  a  matter  of  fact,  many  persons  do  thrive  on  a  much  less 
quantity  of  proteid  than  that  given  in  this  table,  but  in  their 
cases  the  fats  and  starches  should  be  proportionately  increased. 

Such  a  dietary  could  be  obtained  from  many  comestibles 
alone,  and  hence  the  taste  of  the  individual  may  be  exercised  in 
selecting  his  food  without  much  departing  from  such  a  standard. 
Individual  taste  commonly  selects  foods  with  too  much  proteid — 
i.  e.,  an  excess  of  nitrogen — while  the  cheapness  of  vegetable 
products  dictates  their  use  in  greater  abundance  as  food. 

Compare  Chap,  v,  p.  102,  where  the  quantity  of  the  different 
food  stuffs  in  some  of  our  common  articles  of  diet  is  given. 


428  MANUAL   OF   PHYSIOLOGY. 


CHAPTER  XXIV. 

ANIMAL  HEAT. 

The  bodies  of  most  animals  are  considerably  warmer  than 
their  surroundings.  Part  of  the  energy  set  free  by  the  chemical 
changes  in  the  animal  tissues  appears  as  heat  which  is  devoted  to 
this  purpose.  Warm-blooded  animals  are  those  which  habitually 
preserve  an  even  temperature,  independent  of  the  changes  which 
take  place  in  that  of  the  medium  in  which  they  live ;  and,  as  the 
term  warm  blooded  implies,  their  temperature  is,  as  a  rule,  higher 
than  the  surrounding  air  or  water.  Cold-blooded  animals,  on  the 
other  hand,  are  those  whose  temperature  is  considerably  affected 
by,  or  more  or  less  closely  follows,  that  of  the  medium  surround- 
ing them. 

The  blood  of  all  mammalia  has  pretty  much  the  same  tempera- 
ture as  that  of  man,  about  37.5°  C.,  and  probably  varies  under 
similar  circumstances.  But  birds,  the  other  class  of  warm-blooded 
animals,  have  a  temperature  about  4°  or  6°  C.  higher  than  that 
of  mammals. 

The  blood  of  those  animals  whose  temperature  follows  the 
changes  that  occur  around  them  is  generally  from  1°  to  5°  C. 
higher  than  the  medium  in  which  they  live.  They  produce  some 
heat,  though  it  be  small  in  quantity,  and,  since  they  have  no  spe- 
cial plan  for  its  regulation,  it  does  not  remain  at  a  fixed  standard. 
In  every  part  where  active  oxidation  takes  place,  heat  is  pro- 
duced ;  so  even  in  invertebrate  animals  an  elevation  of  tempera- 
ture occurs ;  this  can  be  ascertained  where  they  exist  in  masses, 
as  in  bee  hives,  an  active  hive  sometimes  reaching  a  temperature 
of  35°  C. 

Instead  of  the  term  "  warm  blooded,"  it  is  more  accurate  to 
apply  to  animals  whose  temperature  remains  uniformly  even, 
and  independent  of  their  surroundings,  the  term  "  Homceother- 
mic"  (of  constant  temperature),  and  to  animals  with  temperatures 
varying  with  their  surroundings  "  Poikilothermie"  (of  changing 
temperature),  instead  of  the  words  warm  and  cold  blooded. 


NORMAL    TEMPERATURE.  429 

MEASUREMENT  OF  TEMPERATURE. 

On  account  of  the  slight  amount  of  variation  that  occurs  in 
the  temperature  of  man,  all  the  changes  can  be  measured  with  a 
thermometer  having  a  short  scale  of  some  twenty  degrees,  each 
degree  of  which  occupies  considerable  length  on  the  instrument, 
so  that  very  slight  variations  may  be  easily  appreciated.  Such 
thermometers,  with  an  arrangement  for  self-registering  the  maxi- 
mum height  attained  by  the  column  of  mercury,  are  in  daily  use 
for  clinical  observation,  for  the  temperature  of  the  body  is  now  a 
most  important  aid  to  diagnosis  and  prognosis  in  a  large  class  of 
diseases. 

As  heat  is  constantly  being  lost  at  the  surface  of  the  body,  the 
skin  is  colder  than  the  deeper  parts,  and  in  order  to  avoid  varia- 
tions caused  by  this  surface  loss — which  depends  in  a  measure 
on  the  temperature  of  the  air — special  arrangements  are  neces- 
sary to  prevent  the  thermometer  being  too  much  influenced  by 
it.  The  instrument  may  be  brought  into  close  proximity  to  the 
deeper  parts  by  being  introduced  into  one  of  the  raucous  passages, 
where  it  is  closely  surrounded  by  vascular  tissue.  In  animals, 
the  rectum  is  the  most  convenient  part  for  the  application  of  the 
thermometer,  but  in  clinical  practice  it  is  usually  placed  under 
the  tongue,  or  in  the  armpit,  the  bulb  being  held  so  that  on  all 
sides  it  is  in  contact  with  the  skin  and  protected  from  the  cool  air. 

The  variations  at  different  parts  of  the  body  are  but  slight,  and 
the  average  normal  surface  temperature  in  man  is  found  to  be 
about  37°  C. 

NORMAL  VARIATIONS  IN  TEMPERATURE. 

I.  The  temperature  of  the  ivhole  body  normally  undergoes  cer- 
tain variations,  some  of  which  are :  1.  Regular  and  periodical, 
depending  upon  the  time  of  day,  the  ingestion  of  food,  and  the 
age  of  the  individual.  2.  Accidental,  such  as  those  caused  by 
mental  or  bodily  exertion. 

(a)  The  temperature  is  highest  between  4  and  5  P.  M.  and  lowest 
between  2  and  4  A.  M.,  the  transition  being  gradual.  This  diurnal 
variation,  which  normally  does  not  much  exceed  1°  C.,  is  much 
exaggerated  in  certain  fevers. 


430  MANUAL   OF   PHYSIOLOGY. 

(6)  The  temperature  rises  after  a  hearty  meal  and  falls  during 
fasting.  During  starvation  the  temperature  sinks  gradually  until 
the  death  of  the  individual. 

(c)  The  temperature  is  highest  at  birth,  and  falls  about  1°  C. 
between  that  and  the  age  of  50  years  ;  in  extreme  old  age  it  is 
said  that  it  again  rises. 

(d)  Muscular  exertion,  which  gives  the  individual  the  sensation 
of  great  warmth,  only  changes  the  temperature  of   the  blood 
about  .5°  C.     The  very  high  temperature   which  accompanies 
the  disease  tetanus,  where  all  the  muscles  are  thrown  into  a  state 
of  spasm,  probably  depends  more  on  pathological  changes  than 
on  muscular  action. 

(e)  Mental  exertion  is  also  said  to  cause  a  rise  of  temperature. 
(/)  Slight  differences  in  the  heat  of  the  blood  may  be  brought 

about  by  variations  in  the  surrounding  temperature.  The  abnor- 
mally high  temperature  of  fever  is  much  more  easily  affected 
by  changes  in  the  rate  of  removal  of  the  heat  from  the  body 
than  is  the  normal  temperature,  and  hence  the  therapeutic  value 
of  cold  applications  in  this  class  of  disease. 

II.  The  temperature  of  different  parts  of  the  body  varies  in  a 
slight  degree,  and  depends  upon  the  following  circumstances : 
1.  The  amount  of  blood  flowing  through  them  ;  the  blood  being 
the  greater  carrier  of  warmth  from  one  part  to  another,  supply- 
ing heat  wliere  it  is  lost,  and  conveying  material  to  those  parts 
where  the  heat  is  generated.  2.  The  amount  of  heat  produced, 
i.  e.,  the  activity  of  its  tissue  change.  3.  The  amount  of  heat 
lost,  which  depends  on  (a)  the  extent  of  surface ;  (6)  the 
external  temperature ;  (c)  the  power  of  conduction  of,  and  the 
capacity  for  heat  of,  the  surrounding  medium. 

From  this  it  is  obvious  that  the  deeper  parts  of  the  body, 
where  active  chemical  change  takes  place,  and  which  are  pro- 
tected from  exposure,  must  be  warmer  than  the  exterior,  which 
is  constantly  giving  out  its  heat.  The  blood  which  flows  through 
the  surface  vessels  is  cooled,  and  that  which  flows  through  the 
deeper  vascular  viscera  is  warmed.  Thus  the  skin  is  usually 
about  37°  C.,  while  the  mouth  beneath  the  tongue  is  about  37.5° 
C.,  and  the  rectum  aboutj38°  C.  The  temperature  of  the  blood 


PRODUCTION   OF   ANIMAL    HEAT.  431 

therefore  varies  within  narrow  limits  according  to  the  part  of 
the  body  through  which  it  has  recently  passed.  The  mean  tem- 
perature of  the  blood  is  higher  than  that  of  any  tissue.  The 
blood  in  the  hepatic  capillaries  is  the  warmest  in  the  body. 
This  reaches  40.73°  in  the  dog,  or  nearly  two  degrees  higher 
than  that  in  the  aorta  of  that  animal.  The  cool  blood  from  the 
extremities  and  head  mingling  in  the  right  side  of  the  heart 
with  the  unusually  warm  blood  from  the  liver  keeps  the  blood 
going  to  the  lungs  at  the  standard  temperature.  The  blood  in 
the  left  side  of  the  heart  is  a  little  cooler  than  that  in  the  right, 
probably  because  the  latter  lies  on  the  warm  liver,  as  is  proved 
by  the  substitution  of  a  cold  object  for  this  organ,  when  the 
blood  on  the  right  side  becomes  colder  than  the  left.  It  is  not 
because  the  blood  is  cooled  going  through  the  lungs,  for  the 
heat  used  in  warming  the  respired  air  is  given  off  by  the  nose 
and  other  air  passages,  and  not  by  the  alveoli  of  the  lungs. 

III.  The  temperature  of  an  organ  varies  with  the  state  of  its 
activity.  During  the  active  state  the  glands,  etc.,  receive  more 
blood  and  undergo  more  active  chemical  change,  so  that  they 
become  warmer. 

MODE  OF  PRODUCTION  OF  ANIMAL  HEAT. 

It  has  already  been  indicated  that  the  general  effect  of  the 
tissue  change  of  the  body  is  a  kind  of  combustion  in  the  tissues 
of  certain  substances  obtained  from  the  vegetable  kingdom,  viz., 
proteid,  fat,  carbohydrate,  etc.  The  combustible  substances  are 
capable  of  being  burned  in  the  open  air,  or  made  to  unite  with 
oxygen  so  as  to  produce  a  certain  amount  of  heat,  being  thus 
converted  into  CO.2  and  H.2O.  In  the  body  the  oxidation  goes 
on  in  a  gradual  or  modified  way,  and  the  end  products  of  the 
process  can  be  recognized  as  CO2  eliminated  from  the  lungs, 
and  as  water  and  urea  got  rid  of  by  the  kidneys.  The  general 
tendency  of  the  chemical  changes  in  the  tissues  is  such  as  will 
set  free  energy  in  the  form  of  heat. 

The  amount  of  heat  that  any  substance  is  capable  of  giving 
off  corresponds  to  the  amount  of  energy  required  for  the  forma- 
tion from  CO2  and  H2O,  etc.,  of  the  compounds  contained  in  it, 


432  MANUAL   OF    PHYSIOLOGY. 

and  this  correspondence  remains  whether  the  dissociation  take 
place  rapidly  or  slowly.  The  substances  we  make  use  of  as  food 
have  thus  a  certain  heat  value  which  depends  upon  their  chemi- 
cal composition. 

The  high  temperature  which  homoeothermic  animals  can  keep 
up  in  spite  of  the  cold  of  the  atmosphere  in  which  they  live  is 
readily  accounted  for  by  the  chemical  change  which  is  con- 
stantly occurring  in  the  tissue  of  their  bodies. 

The  amount  of  heat  produced  in  any  part  depends  upon  the 
activity  of  its  tissue  change,  for  we  find  that  the  temperature 
varies  with  the  elimination  of  CO2  and  urea,  which  gives  a  fair 
estimate  of  the  normal  chemical  changes  of  the  tissues. 

1.  The  diurnal  changes  in  temperature  are  accompanied  by  an 
afternoon  increase  and  a  morning  decrease  of  CO2  and  urea. 

2.  The  tissue  change  giving  rise  to  CO2  decreases  in  a  fasting 
animal,  as  does  also  the  production  of  heat. 

3.  More  CO2  is  eliminated  after  meals,  when  the  temperature 
also  rises. 

4.  The  activity  of  various  organs,  such  as  the  muscles  and 
glands,  is  associated  with  a  local  increase  of  temperature. 

INCOME  AND  EXPENDITURE  OF  HEAT. 

Income. — The  chemical  changes  which  give  rise  to  heat  cause 
a  certain  waste  of  the  tissues,  which  have  again  to  be  renewed 
by  the  assimilation  of  various  nutrient  materials.  Food  is  thus 
the  fuel  of  the  animal  body,  and  the  peculiarity  of  the  combus- 
tion is  that  the  tissues  assimilate  or  convert  into  their  own  sub- 
stance the  fuel,  and  then  themselves  undergo  a  kind  of  partial 
combustion,  by  means  of  which  they  perform  their  several  func- 
tions, among  others  heat  production. 

As  already  mentioned,  heat  is  produced  most  abundantly  in 
those  tissues  which  undergo  most  active  chemical  changes,  hence 
the  protoplasmic  cells  of  glands,  and  the  contractile  substance 
of  muscle,  must  be  looked  upon  as  the  chief  agents  in  setting 
heat  free. 

The  possible  heat  income  depends  on  the  amount  of  nutrient 
matter  assimilated.  As  each  kind  of  food  has  a  certain  heat 


INCOME   AND    EXPENDITURE   OF   HEAT.  433 

value,  L  e.,  the  number  of  heat  units  its  combustion  will  produce, 
we  ought  to  be  able  to  estimate  the  amount  of  heat  produced 
by  ascertaining  this  value  and  subtracting  the  calorific  value 
of  the  various  excreta,  and  the  energy  used  in  producing 
the  muscular  movements  of  the  body.  Since,  practically,  the 
temperature  of  the  body  remains  the  same,  the  amount  of  heat 
lost  during  a  given  time  should  correspond  to  the  income  esti- 
mated from  the  number  of  heat  units  of  the  food.  So  far, 
however,  attempts  to  make  the  calculated  heat  income  corre- 
spond with  the  expenditure  have  not  been  productive  of 
satisfactory  results,  the  calorific  value  of  the  food  being  hardly 
sufficient  to  produce  the  heat  calculated  to  be  given  off,  and 
the  other  work  done  by  the  body  in  the  form  of  muscular  move- 
ment, etc. 

Since  the  activity  of  muscle  and  gland  tissue  is  constantly 
undergoing  variations  in  intensity,  the  amount  of  chemical 
change  differs  at  different  times,  so  that  the  amount  of  heat  pro- 
duced must  also  vary.  We  know  that  the  heat  set  free  by  any 
organ,  such  as  a  gland  or  a  muscle,  increases  in  proportion  to 
the  increase  of  its  functional  activity,  but  we  cannot  say  that  the 
calorific  activity  can  vary  independently  of  other  circumstances. 
Without  such  a  special  calorific  function  of  some  tissues,  such 
as  muscle,  the  actual  net  heat  income  must  vary  with  circum- 
stances which  are  accidental,  and  therefore  irregular. 

Since  we  know  that  the  nervous  system  controls  the  tissue 
activities  which  are  accompanied  by  the  setting  free  of  heat,  we 
can  see  how  the  nerve  centres  can  materially  influence  the  heat 
production  of  the  body.  The  more  active  the  muscles,  glands, 
etc.,  which  are  under  the  control  of  nerves,  the  greater  is  the 
amount  of  heat  produced  in  a  given  time.  That  the  nervous 
system  can  cause  in  any  tissue  a  chemical  change,  giving  rise  to 
a  greater  production  of  heat,  without  any  other  display  of 
functional  activity,  we  do  not  know,  but  many  facts  seem  to  point 
to  such  a  possibility. 

The  effect  of  nerve  influence  on  the  production  of  heat  is  greatly 
complicated  by  the  power  exercised  by  the  vasomotor  nerves 
over  the  blood  supply  to  the  great  viscera,  etc.,  for  the  tempera- 
37 


434  MANUAL   OF   PHYSIOLOGY. 

ture  of  any  given  part  is  so  intimately  related  to  the  amount  "of 
blood  flowing  through  it  that  the  former  has  been  accepted  as  an 
adequate  measure  of  the  latter. 

For  the  present,  therefore,  we  are  not  in  a  position  to  speak 
with  decision  of  nerves  with  a  purely  thermic  action. 

The  Expenditure  of  the  heat  may  be  classed  under  the  follow- 
ing headings : — 

1.  In  warming  ingesta :    As   a  rule,  the  food  and  drink  we 
use,  as  well  as  the  oxygen  we  breathe,  are  colder  than  the  body, 
and  before  they  pass   out    they  are  raised    to    the    body  tem- 
perature. 

2.  Radiation  and  Conduction :  From  the  surface  of  the  body 
a  quantity  of  heat  is  being  expended  in  warming  the  surround- 
ing medium,  which  is  habitually  colder  than  our  bodies.     The 
colder  the  medium,  the  greater  its  capacity  for  heat,  and  the  more 
quickly  it  comes  in  contact  with  new  portions  of  the  surface,  the 
more  warmth  it  robs  us  of.     Water  or  damp  air  takes  up  much 
more  heat  from  our  surface  than  dry  air  of  the  same  tempera- 
ture, and  the  quantity  of  heat  lost  is  still  further  increased  if 
the  medium  be  in  motion,  so  that  the  relatively  colder  fluid  is 
constantly  renewed. 

3.  Evaporation :  (a)  From  the  larger  air  passages  :  a  quantity 
of  water  passes  into  the  vaporous  state  and  saturates  the  tidal 
air,  and  this  change  of  condition  from  liquid  to   that  of  vapor 
absorbs  much  heat ;  (6)    From  the  skin  :    surface   evaporation 
is  always  going  on,  even  when  no  moisture  is   perceptible  on 
the  skin,  and  much  fluid  of  which  we  are  not  sensible,  is  lost 
in  this  way.      The  quantity  of  heat  lost  by  evaporation  from 
the  skin  will  depend    on   the   temperature   and   the  degree  of 
moisture  of  the  air  in  proportion  to  that  of  the  surface  of  the 
body. 

Balance. — As  has  been  said,  the  exact  income  of  heat  is 
uncertain  and  variable,  because  the  data  upon  which  the  abso- 
lute amount  can  be  calculated  are  not  scientifically  free  from  error. 
According  to  the  most  careful  estimates,  an  adult  weighing  82 
kilo,  produces  2,700,000  units  of  heat  in  the  twenty- four  hours, 
which  are  expended  in  the  following  way  : — 


MAINTENANCE   OF   UNIFORM    TEMPERATURE.  435 

In  warming  ingesta 70,157  units  of  heat. 

In  warming  tidal  air 140,064       " 

By  the  evaporation  of  656  grin,   of 

water  from  the  air  passages 397, 536       ' ' 

By  surface  loss 2,092,243 

From  this  it  appears  that  more  than  three-quarters  of  our 
heat  is  lost  by  the  skin  (77.5  per  cent.) ;  by  pulmonary  evapora- 
tion, 14.7  per  cent.;  in  heating  the  air  breathed,  5.2  per  cent.; 
in  heating  ingesta,  2.6  per  cent. 

MAINTENANCE  OF  UNIFORM  TEMPERATURE. 

In  order  that  the  vital  processes  of  man  and  the  other  homoeo- 
thermic  animals  should  go  on  in  a  normal  manner,  it  is  neces- 
sary that  their  mean  temperature  remain  nearly  the  same,  and 
we  have  seen  that  under  ordinary  circumstances  it  varies  only 
about  one  degree  below  or  above  the  standard  37°  C.,  notwith- 
standing the  changes  taking  place  in  the  temperature  around  us. 
Thus  we  can  live  in  any  climate,  however  cold  or  warm,  and  if 
our  body  temperature  remains  unaltered,  we  suffer  no  imme- 
diate injury. 

There  is  a  limit,  however,  to  this  power  of  maintaining  a  uni- 
form standard  temperature.  If  a  mammal  be  kept  for  some 
time  in  a  moist  medium,  where  evaporation  cannot  take  place, 
at  a  temperature  but  little  higher  than  its  body,  say  over  45°  C., 
its  temperature  soon  begins  to  rise,  and  it  dies  with  the  signs  of 
dyspnoaa  and  convulsions  (probably  from  the  nervous  centres 
being  affected)  when  its  temperature  arrives  at  43°-45°.  If 
placed  in  water  at  freezing  point  an  animal  loses  its  heat  quickly, 
and  when  its  body  temperature  has  fallen  to  about  20°  C.  it 
dies  in  a  condition  resembling  somnolence,  the  circulation  and 
respiration  gradually  failing. 

Since  a  variation  of  more  than  one  or  two  degrees  in  the  tem- 
perature of  our  bodies  interferes  with  the  vital  activities  of  the 
controlling  tissue  in  the  nervous  centres,  it  is,  of  course,  of  the 
utmost  importance  that  adequate  means  for  the  regulation  of  the 
mean  temperature  of  our  bodies  should  exist. 

The  temperature  of  an  animal's  body  must  depend  on  the 
relations  existing  between  the  amount  of  heat  generated  in  the 


436  MANUAL   OF   PHYSIOLOGY. 

tissues  and  organs  and  the  amount  allowed  to  escape  at  the  sur- 
face, and  these  must  closely  correspond  in  order  that  the  heat  of 
the  body  may  remain  uniform.  Both  these  factors  are  found  to 
be  very  variable.  Every  increase  in  the  activity  of  the  muscles, 
liver,  etc.,  causes  a  greater  production  of  heat,  while  a  fall  in 
external  temperature  or  increase  in  the  moisture  of  cool  air 
causes  a  greater  escape  of  heat  from  the  surface. 

The  maintenance  of  uniform  temperature  may  be  accom- 
plished by  (1)  variations  in  the  heat  income,  so  arranged  as 
to  make  up  for  the  irregularities  of  expenditure,  or  (2)  varia- 
tions in  the  loss  to  compensate  for  the  differences  of  heat 
generated. 

Since  the  temperature  and  moisture  of  our  surroundings  are 
constantly  varying  between  tolerably  wide  limits,  the  amount  of 
heat  given  off  by  our  bodies  must  also  vary.  In  cold,  damp 
weather  a  great  quantity  of  heat  is  lost  in  comparison  with 
that  which  escapes  from  the  body  when  the  air  is  dry  or  warm. 
If  the  heat  generated  had  to  make  up  for  the  changes  in  the 
heat  lost,  we  should  expect  to  find  a  correspondingly  great  differ- 
ence in  the  amount  of  heat  generated  at  different  times  of  the 
year.  No  doubt  we  have  some  evidence  in  the  keener  appetite 
or  use  of  more  fuel,  and  the  natural  tendency  to  active  muscular 
exertion  during  cold  weather,  to  show  that  a  greater  amount  of 
combustion  takes  place  in  winter  than  in  summer.  Further,  if 
the  preservation  of  a  uniform  body  temperature  depended  solely 
upon  the  variations  in  the  amount  of  incorrik  keeping  pace  with 
the  variation  in  the  expenditure,  we  should  find  it  inconvenient 
to  set  our  muscular  or  glandular  tissues  in  action  except  when 
the  external  temperature  was  such  as  would  enable  us  easily  to 
get  rid  of  the  increased  heat  following  their  activity.  It  no 
doubt  appears  that  the  general  tissue  combustion,  as  measured 
by  the  amount  of  CO2  given  off,  increases  when  we  are  placed 
in  colder  surroundings — such  as  a  cold  bath  ;  still,  it  is  probable 
that  the  variations  in  heat  income  have  but  secondary  regulating 
influence  on  the  body  temperature.  If  the  rate  of  income  have 
any  regulating  influence,  we  are  ignorant  of  the  manner  in  which 
such  influence  is  exerted,  for  it  must  act  more  slowly  and  cannot 


HEAT   REGULATION.  437 

follow  so  closely,  as  the  variations  in  expenditure  do,  extrinsic 
changes  of  temperature. 

On  the  other  hand,  we  know  that  the  amount  of  heat  expendi- 
ture may  be  varied  by  mechanisms  which  are  almost  self  regu- 
lating. It  has  already  been  stated  that  the  great  majority  of 
the  heat  is  lost  by  the  parts  in  contact  with  the  air,  namely,  the 
skin  and  air  passages.  In  these  places  the  warm  blood  is  exposed 
to  the  cool  air,  and  loses  much  of  its  heat  by  radiation,  conduc- 
tion and  evaporation.  It  is  obvious  that  the  greater  the  quantity 
of  blood  thus  exposed  for  cooling,  the  greater  will  be  the  amount 
of  heat  lost  in  a  given  time  by  the  body  as  a  whole. 

If  we  review  the  circumstances  which  interfere  with  the  uni- 
formity of  the  temperature  of  the  body,  we  shall  see  that  each 
one  is  accompanied  by  certain  physiological  actions  which  tend 
to  compensate  for  the  disturbing  influences. 

The  chief  common  events  tending  to  make  our  temperature 
exceed  or  fall  short  of  its  normal  standard  may  be  enumerated 
as  follows,  and  the  explanation  of  their  modes  of  compensation 
will  at  the  same  time  be  given  : — 

COMPENSATION  FOR  INTERNAL  VARIATIONS. 

A  casual  increase  in  the  .heat  income  may  be  induced  by  any 
increased  chemical  activity  in  the  tissues,  notably  the  action  of 
the  muscfes  and  large  glands.  When  this  increased  heat  is  com- 
municated to  it,  the  warm  blood,  by  the  help  of  certain  nerve 
centres,  brings  aboutxthe  following  results :  (a)  An  acceleration 
of  respiratory  movement,  increasing  the  amount  of  cold  air  to 
be  warmed  and  saturated  with  moisture  by  the  air  passages,  and 
facilitating  the  escape  of  the  surplus  caloric.  (6)  Relaxation 
of  the  cutaneous  arterioles,  exposing  a  greater  quantity  of  blood 
to  the  cooling  influence  of  the  air.  (c)  Greater  rapidity  of 
the  heart  beat,  supplying  a  greater  quantity  of  blood  to  the 
air  passages  and  to  the  surface  vessels,  (d)  An  increase  in  the 
amount  of  sweat  secreted,  affording  opportunity  for  greater  sur- 
face evaporation. 

As  examples  of  these  points  may  be  mentioned  active  muscular 
exercise,  which  daily  experience  shows  us  is  always  accompanied 


438  MANUAL   OF   PHYSIOLOGY. 

by  quick  breathing,  rapid  heart's  action,  and  a  moist  skin.  The 
increased  production  of  heat  in  fever  gives  rise  to  the  same  results, 
with  the  exception  of  the  secretion  of  the  sweat,  which  want 
(probably  owing  to  the  toxic  inhibition  of  the  special  nerve  mech- 
anisms of  the  glands)  is  an  important  deficiency  in  the  heat-regu- 
lating arrangements,  and  has  much  to  do  with  the  abnormally 
high  temperature  of  the  disease. 

When  a  lesser  quantity  of  heat  is  produced,  owing  to  inactivity 
of  the  heat-producing  tissues,  the  reverse  takes  place,  namely, 
the  respiration  and  heart's  action  are  slow,  the  skin  is  pale  and 
dry,  so  that  little  heat  can  escape. 

COMPENSATION  FOR  EXTERNAL  VARIATIONS  OF  TEMPERATURE. 

When  the  temperature  of  the  air  rises  much  above  the  aver- 
age, the  escape  of  heat  is  correspondingly  hindered ;  and  when 
the  general  body  temperature  begins  to  rise  by  this  retention  of 
caloric,  we  have  the  sequence  of  events  detailed  in  the  last  para- 
graph. But  before  the  blood  can  become  warmer  by  the  influ- 
ence of  the  increased  external  temperature,  the  warm  air,  by 
stimulating  the  skin,  brings  about  certain  changes,  independent  of 
the  body  temperature,  which  satisfactorily  check  the  tendency  to 
an  abnormal  rise.  This  can  be  shown  by  the  local  application  of 
external  heat,  by  means  of  which  (a)  a  rush  of  blood  to  the  skin, 
and  (6)  copious  sweat  secretion  may  be  induced  in  a  pdrt.  This 
is  brought  about  by  impulses  sent  directly  from  the  skin  to  the 
centres  regulating  the  vasomotor  and  secretory  mechanisms,  and 
thus  causing  vascular  dilatation  and  secretive  activity.  If  a 
part  only  be  warmed,  a  local  effort  is  made  to  cool  that  part, 
and  this  has  but  little  influence  on  the  general  body  tem- 
perature. 

When,  however,  the  atmosphere  becomes  very  warm,  all  the 
cutaneous  vessels  dilate  simultaneously,  and  the  escape  of  heat  is 
greatly  increased ;  while,  at  the  same  time,  so  much  blood  being 
occupied  in  circulating  through  the  skin,  the  deeper — heat  pro- 
ducing— tissues  are  supplied  with  less  blood,  and  therefore  gener- 
ate a  lesser  quantity  of  heat.  Thus  a  marked  rise  in  the  external 
temperature,  which  at  first  sight  would  seem  to  impede  the  escape 


HEAT    REGULATION.  439 

of  heat  from  the  body,  really  facilitates  it,  by  causing,  through 
the  vascular  and  glandular  nerve  mechanisms  of  the  skin,  a 
greater  exposure  of  the  blood  to  the  cooler  air,  and  a  greater 
quantity  of  moisture  to  be  evaporated  from  the  warm  skin. 
When  the  temperature  of  the  air  reaches  that  of  the  body,  the 
only  way  of  disposing  of  the  heat  generated  in  the  body  is  by 
evaporation,  for  radiation  and  conduction  become  impossible. 
In  animals  like  man,  whose  cutaneous  moisture  is  great,  external 
heat  seldom  causes  marked  change  in  the  rate  of  breathing,  but 
in  animals  whose  cutaneous  secretion  is  limited,  external  heat 
distinctly  affects  their  respiratory  movements,  as  may  be  seen  by 
the  panting  of  a  dog  on  a  very  warm  day,  even  when  the  animal 
is  at  rest. 

Almost  more  important  than  facilitating  the  escape  of  heat  in 
very  warm  weather,  are  the  arrangements  for  preventing  its  loss 
when  the  surroundings  are  unusually  cold.  In  this  case,  the  cold, 
acting  as  a  stimulus  to  the  vaso-constrictor  nerve  agencies  of 
the  skin,  causes  the  blood  to  retire  from  the  surface  and  fill  the 
deeper  organs,  where  more  heat  is  produced.  This  bloodless  skin 
and  the  underlying  fat  then  act  as  a  non-conducting  layer  or 
boundary  protecting  the  warm  blood  from  the  cooling  exposure. 
At  the  same  time  the  secretion  of  the  sweat  is  controlled  by  a 
special  nerve  mechanism,  which  lessens  evaporation  and  soon 
checks  the  secretion,  thereby  enabling  the  body  to  remain  at  the 
normal  standard  temperature. 

It  would  then  appear  that  the  chief  factors  regulating  the  body 
temperature  belong  to  the  expenditure  department,  and  may  be 
said  to  be — (a)  variation  in  the  quantity  of  blood  exposed  to  be 
cooled,  and  (b)  variation  in  the  quantity  of  moisture  produced 
for  evaporation. 

These  regulators  have  to  compensate  not  only  for  differences 
of  external  temperature,  but  also  for  great  fluctuations  in  the 
amount  of  heat  produced  in  the  tissues. 

The  regulating  power  of  the  skin,  etc.,  appears  to  be  adequate 
for  the  perfect  maintenance  of  uniform  temperature  only  within 
certain  limits.  When  these  limits  are  passed  by  the  rise  or  fall 
in  the  surrounding  medium,  the  preservation  of  a  uniform  tern- 


440  MANUAL   OF   PHYSIOLOGY. 

perature  soon  becomes  impossible.  These  limits  vary  much  in 
different  animals,  many  of  which  have  special  coverings  protect- 
ing them  from  external  influences,  and  retain  their  warmth  in  a- 
temperature  seldom  above  0°  C.  In  man  the  limits  vary  accord- 
ing to  many  circumstances,  e.  g.,  both  extremes  of  age  are  more 
sensitive  to  changes  of  temperature.  It  would  appear  that  for 
about  10°  C.  above  and  below  the  body  temperature  our  skin- 
regulating  mechanisms  are  adequate,  but  beyond  these  limits 
external  changes  affect  our  general  temperature,  and  if  continued 
become  injurious.  Of  course,  by  imitating  with  clothing  the 
natural  protection  with  which  some  animals  are  endowed,  we  can 
aid  the  normal  regulating  factors,  and  bear  much  greater  extremes 
of  temperature  with  safety  or  even  comfort. 

It  is  somewhat  surprising  that  our  bodies  are  always  at  the 
same  temperature,  no  matter  how  hot  or  cold  we  feel.  This  is 
quite  true,  and  our  sensations  of  being  hot  or  cold  are  explained 
as  follows :  When  we  feel  hot  our  cutaneous  vessels  are  full  of 
warm  blood,  and  this  communicates  to  the  cutaneous  nerve  ter- 
minals— the  sensory  nerves — the  sensation  of  general  warmth. 
On  the  other  hand,  when  the  cutaneous  vessels  are  empty,  the 
sensory  nerves  are  directly  affected  by  the  cold  of  the  external 
air.  Since  the  full  or  empty  state  of  the  vessels  of  the  skin 
depends  generally  on  the  heat  or  cold  of  the  air,  we  use  the 
expressions  "  it  is  hot  or  cold "  and  "  we  are  hot  or  cold,"  as 
synonymous,  because  both  ideas  arise  from  the  state  of  the  skin. 
But  we  can  make  ourselves  feel  warm  by  violent  exercise  even 
on  a  frosty  day,  because  we  generate  so  much  heat  by  muscular 
action  that  the  cutaneous  vessels  have  to  be  dilated  in  order  to 
get  rid  of  the  surplus,  and  our  skin  vessels  being  full  we  feel 
warm.  Our  feelings,  when  we  say  we  are  warm  or  cold,  simply 
depend  upon  our  cutaneous  vessels  being  full  or  empty  of  warm 
blood. 

The  local  appreciation  of  differences  of  temperature  will  be 
discussed  in  the  chapter  dealing  with  the  sense  of  Touch. 


CONTRACTILE   TISSUES.  441 


CHAPTER  XXV. 

CONTRACTILE  TISSUES. 

In  the  lower  forms  of  organisms  the  motions  executed  by  pro- 
toplasm suffice  for  all  their  requirements.  Thus  the  amoeba 
manages  to  pass  through  its  lifetime  with  no  other  kind  of  motion 
at  its  disposal  than  the  flowing  circulation  and  the  budding  out 
of  its  soft  protoplasm.  A  vast  number  of  minute  organisms 
depend  wholly  upon  the  protoplasmic  stream  and  the  twitching 
of  cilia  for  their  digestive  and  progressive  movements.  Before 
we  leave  the  class  of  animals  which  never  pass  beyond  the  uni- 
cellular stage,  we  find,  however,  examples  in  which  a  portion  of 
their  protoplasm  is  specially  adapted  to  the  performance  of  sud- 
den and  rapid  motions.  The  protoplasm  so  modified  in  function 
deserves  the  name  of  contractile  material.  Thus,  though  the 
protoplasm  which  lies  within  the  stalk  of  the  bell  animalcule 
is  morphologically  undifferentiated,  it  can  contract  with  such 
rapidity  that  the  eye  cannot  follow  the  motion. 

As  we  ascend  in  the  scale  of  animal  life,  the  necessity  for 
motions  of  various  rapidity  and  duration  at  the  command  of  the 
animal  becomes  more  and  more  urgent,  and  so  we  find  not  only 
one,  but  several  kinds  of  tissue  specially  adapted  for  carrying  out 
motions  of  different  rate  and  duration. 

As  a  general  rule,  the  more  rapid  the  contraction  it  performs 
the  more  the  tissue  differs  from  the  original  type  of  protoplasm ; 
and  the  slower  and  more  persistent  the  contraction,  the  more  the 
tissue  elements  resemble  protoplasmic  cells.  Thus,  in  the  minute 
blood  vessels,  as  we  have  seen,  a  very  prolonged  form  of  contrac- 
tion, only  varied  by  partial  relaxations,  is  the  rule,  and  gives  rise 
to  the  tone  of  the  arterioles,  and  the  contractile  elements  differ 
but  little  from  ordinary  protoplasmic  cells.  The  intestinal  move- 
ments are  rapid  compared  with  those  of  the  arterial  muscles,  and 
in  them  we  find  a  thin,  elongated  form  of  muscle  cell.  In  the 
heart  a  forcible  and  quick  contraction  takes  place,  which,  how- 


442 


MANUAL   OF    PHYSIOLOGY. 


Fin.  177. 


ever,  is  slow  when  compared  with  the  sudden  jerk  of  a  single 
spasm  of  a  skeletal  muscle,  and  its  texture  is  different,  being  a 
form  intermediate  between  the  slow-contract- 
ing smooth  muscle  and  the  quick-contracting 
striated  skeletal  muscle. 

By  borrowing  examples  from  the  lower 
animals,  this  parallelism  of  structural  differen- 
tiation and  increase  -of  functional  energy  can 
be  more  perfectly  demonstrated,  and  we  can 
make  out  a  gradual  scale  of  increasingly  rapid 
motion  corresponding  with  greater  complexity 
of  structure. 


HISTOLOGY  OF  MUSCLE. 

The  term  muscle  includes  the  textures  in 
which  the  protoplasm  is  specially  differentiated 
for  purposes  of  contraction. 

The  muscle  tissues  of  the  higher  animals 
may  be  divided  into  two  classes :  (1)  non-striated 
or  smooth,  and  (2)  striated,  in  which  again  there 
are  some  slight  variations. 

The  non-striated  muscle  tissue  is  that  in  which 
the  elements  are  most  like  contractile  proto- 
plasmic cells,  and  have  so  far  retained  the  typi- 
cal form  as  to  be  easily  recognizable  as  cells 
when  separated  one  from  the  other.  These  cells 
are  more  or  less  elongated,  flattened,  homo- 
geneous elements  with  a  single,  long,  rod-shaped 
nucleus  and  no  cell  wall.  They  are  tightly 
cemented  together  by  a  tough  elastic  substance, 
so  that  their  tapering  extremities  fit  closely 
together  and  form  commonly  a  dense  mass  or 
Muscle  ceils,  showing  sheet.  Sometimes  thev  branch  more  or  less 

differentcondition  of  ,      ,  .      . 

the  protoplasm  of  the  regularly,   and    then   are    arranged    in    net- 

cell  and  nucleus.  , 

works. 

These  cells  vary  greatly  in  size  as  jvell  as  in  the  relation 
of  their  length  to  their  width,  in    some  places  deserving  the 


HISTOLOGY   OF    MUSCLE. 


443 


FIG.  178. 


name  fibres,  or  fibre  cells,  and  in  others  being  only  elongated 
cells. 

The  striated  muscle  tissue  is  that  of  which  the  skeletal  muscles 
and  the  heart  are  composed.  It  therefore  forms  the  larger  pro- 
portion of  the  animal,  known  as  flesh.  The  flesh  can,  by  judi- 
cious dissection,  easily  be  divided  into  single  parts  called  muscles, 
each  of  which  contains  many  other  tissues,  and  is  so  attached  as 
to  carry  on  certain  movements,  and  may,  therefore,  be  regarded 
as  an  organ. 

Such  a  muscle  is  enclosed  in  a  sheath  of  connective  tissue, 
from  which  sheet-like  partitions  or  septa  pass  into  the  mass  of 
the   muscle  and  divide  it  into  bundles  of  fibres,  which   they 
enclose.     These  septa  also  act  as  the 
bed   in  which  the  vessels  and  nerves 
lie. 

The  tissue  of  the  heart  differs  from 
the  striated  muscle  in  being  made 
up  of  truncated,  oblong  branching 
cells  with  a  central  nucleus  and  no 
sarcolemma  (see  page  262.) 

The  bundles  of  fibres  of  skeletal 
muscle  vary  much  in  size,  giving  a 
coarse  or  fine  grain  in  different  mus- 
cles ;  they  are  composed  of  a  greater 
or  less*  number  of  fibres,  which,  lying 
side  by  side,  run  parallel  one  to  the 
other.  The  single  fibres  of  striated 

,  •         i          .\  .'  Short  striated  cells  of  the   heart 

muscle     Vary      111      length,      Sometimes       muscles,  separated  one  showing 


reaching   4-5   cm.    (2    inches),    but 
being   on  an  average  much    shorter, 

they  only  extend  the  entire  length  of  a  muscle  in  the  case  of 
very  short  muscles.  In  long  muscles  their  tapering  points  are 
made  to  correspond  with  those  of  other  fibres  to  which  they  are 
firmly  attached.  The  soft  fibres  are  pressed  by  juxtaposition 
into  prismatic  forms,  so  that  in  a  fresh  condition  they  appear 
polygonal  in  transverse  section.  When  freed  from  all  pressure 
or  traction  they  become  cylindrical,  and  the  transverse  striation 


444 


MANUAL   OF   PHYSIOLOGY. 


of  the   contractile  substance    appears    regular,   and   is   easily 
recognized.     Each  fibre  consists  of  a  deli- 
FIO.  170.  Cate   case  of   thin,  elastic,  homogeneous 

membrane,  forming  a  sheath  called 
sarcolemma,  within  which  the  essential 
contractile  substance  is  enclosed.  The 
soft  contractile  substance  completely  fills 
and  distends  the  elastic  sarcolemma,  so 
that  when  the  latter  is  broken  its  con- 
tents bulge  out  or  escape.  After  death, 
particularly  if  preserved  in  weak  acid 
(HC1),  the  striation  becomes  more 
marked,  and  the  dead  and  now  rigid 
contractile  substance  can  easily  be 
broken  up  into  transverse  plates  or 
discs. 

Besides  the  transverse  striation,  a  longi- 
tudinal marking  can  be  seen  in  the 
muscle  fibre,  which  indicates  the  sub- 
division of  the  contractile  substance  into 
thin  threads  called  primitive  fibrillse. 
Each  primitive  fibril  shows  a  transverse 
marking,  corresponding  with  the  trans- 
verse striation,  which  divides  the  fibrils 
into  short  blocks  called  sarcous,  or  muscle 
elements.  These  markings,  and  the 
transverse  striations  of  the  muscle  fibre 
in  general,  depend  on  different  parts  of 
the  contractile  substance  having  different 
powers  of  refraction,  and  giving  the 
appearance  of  dark  and  light  hands. 

Twofibresof  striated  muscle,       In   the   muscle   fibre   are  found  loner 

in    which    the   contractile 

substance  (m)  has  been  rup-  granular  masses  like  protoplasm;  these 

tured    and  separated  from  .  ,  r  ., 

the  sarcolemma  (a)  and  (5) -,   are   the   nuclei   or    the  contractile   sub- 

(B)  shows   a  thin  strip  of  ™, 

torn  contractile  substance  stance,     ihev   must  not   be  contounded 

in    which     the    transverse        .,,     ,,  ,    .      „     .  ,  .   , 

markings  are  clearer;    (n)    With  the  nuclei  of  the  Sarcolemma,  which 
nuclei,  (p)  space  under  sar-  ,  ,11 

coiemma.  (Ranvier.)  are  much  more  numerous  along  the  edge 


PROPERTIES    OF    MUSCLE.  445 

of  the  fibre,  or  with  the  other  short  nuclei  seen  in  such  numbers 
between  the  fibres,  which  indicate  the  position  of  the  capillary 
vessels. 

It  is  stated  that  each  striated  muscle  fibre  has  a  nerve  fibre 
passing  directly  into  it,  but  the  exact  details  of  the  mode  of 
union  in  mammalia  are  not  yet  satisfactorily  made  out. 

PROPERTIES  OF  MUSCLE  IN  THE  PASSIVE  STATE. 

Consistence. — The  contractile  substance  of  muscle  is  so  soft  as 
to  deserve  rather  the  name  fluid  than  solid  ;  it  will  not  drop  as 
a  liquid,  but  its  separate  parts  will  flow  together  again  like 
half-melted  jelly.  In  this  respect  it  resembles  the  protoplasm  of 
some  elementary  organisms,  the  buds  from  which  are  so  soft  that 
they  can  unite  around  foreign  bodies  and  yet  have  sufficient 
consistence  to  distinguish  them  from  fluid. 

Chemical  Composition. — The  chemical  constitution  of  the  con- 
tractile substance  of  muscle  in  the  living  state  is  not  accurately 
known.  The  death  of  the  tissue  is  accompanied  by  certain 
changes  of  a  chemical  nature  which  give  rise  to  a  kind  of 
coagulation,  resulting  in  the  formation  of  two  substances,  viz., 
muscle  serum  and  muscle  dot  or  myosin.  This  coagulation  can  be 
postponed  almost  indefinitely  in  the  contractile  substance  of  the 
muscles  of  cold-blooded  animals,  by  keeping  the  muscle  after 
its  removal  at  about  5°  C.  In  this  way  a  pale  yellow,  opales- 
cent, alkaline  juice  may  be  pressed  out  of  the  muscle,  and 
separated  on  a  cold  filter.  This  substance  turns  to  a  jelly  at 
freezing  point,  and  if  brought  to  the  ordinary  temperature  of 
the  room  it  passes  through  the  stages  of  coagulation  seen  in  the 
contractile  substance  of  dead  muscle,  and  gives  the  same  fluid 
serum  and  clot  of  myosin.  Since  a  frog's  muscle  can  be  frozen 
and  thawed  without  the  tissue  being  killed,  it  is  supposed  that 
the  thick  juice  is  really  the  contractile  substance,  which  has 
been  called  muscle  plasma. 

The  coagulation  of  muscle  plasma  reminds  us  in  many  ways 
of  the  clotting  of  the  blood  plasma,  but  the  muscle  clot,  or 
myosin,  is  gelatinous  and  not  in  threads  like  fibrin.  It  is  a 
globulin,  and  is  soluble  in  10  per  cent,  solution  of  salt.  It  is 


446  MANUAL   OF    PHYSIOLOGY. 

readily  changed  into  syntonin  or  acid  albumin,  and  forms  the 
preponderant  albuminous  substance  of  muscle. 

The  serum  of  dead  muscle  has  an  acid  reaction,  and  contains 
three  distinct  albuminous  bodies  coagulating  at  different 
temperatures,  one  of  which  is  serum-albumin,  and  another  a 
derived  albumin,  potassium-albumin.  The  serum  of  muscle 
also  contains:  (1)  Kreatin,  kreatinin,  xanthin,  etc.  (2) 
Haemoglobin.  (3)  Grape  sugar,  muscle  sugar,  of  inosit,  and 
glycogen.  (4)  Sarcolactic  acid.  (5)  Carbonic  acid.  (6) 
Potassium  salts ;  and  (7)  75  per  cent,  of  water.  Traces  of 
pepsin  and  other  ferments  have  also  been  found. 

FIG.  180. 


1.  Shows  graphically  the  amount  of  extension  caused  by  equal  weight  increments 

applied  to  a  steel  spring. 

2.  Shows  graphically  the  amount  of  extension  caused  by  equal  weight  increments 
r  applied  to  an  india-rubber  band. 

3.  The  same  applied  to  a  frog's  muscle.    Showing  the  decreasing  increments  of  exten- 

sion ;  the  gradual  continuing  stretching,  and  the  failure  to  return  to  the  abscissa 
when  the  weight  is  removed. 

Chemical  Change.— In  the  state  of  rest  a  certain  amount  of 
chemical  change  constantly  goes  on,  by  which  oxygen  is  taken 
from  the  haemoglobin  of  the  -blood  in  the  capillaries,  and  car- 
bonic acid  is  given  up  to  the  blood.  These  changes  seem 
necessary  for  the  nutrition,  and  therefore  the  preservation  of  the 
life  and  active  powers, of  the  tissue,  because  if  a  muscle  after 
removal  be  placed  in  an  atmosphere  free  from  oxygen,  it  more 
quickly  loses  its  chief  vital  character,  viz.,  its  irritability. 


ELASTICITY   OP   MUSCLE.  447 

Elasticity. — Striated  muscle  is  easily  stretched,  and,  if  the 
extension  be  not  carried  too  far,  recovers  very  completely  its 
original  length.  That  is  to  say,  the  elasticity  of  muscle  is  small 
or  weak,  but  very  perfect.  When  a  muscle  is  stretched  to  a 
given  extent  by  a  weight — say  of  1  gramme — if  another  gramme 
be  then  added,  it  will  not  stretch  the  muscle  so  much  as  the  first 
did ;  and  so  on  if  repeated  gramme  weights  be  added  one  after 
the  other,  each  succeeding  gramme  will  cause  less  extension  of 
the  muscle  than  the  previous  one ;  so  that  the  more  a  muscle  is 
stretched  the  more  force  is  required  to  stretch  it  to  the  given 
extent,  or,  in  other  words,  the  elastic  force  of  muscle  increases 
with  its  extension. 

If  a  tracing  be  drawn,  showing  the  extending  effect  of  a  series 
of  equal  weights  attached  to  a  fresh  muscle,  it  will  be  found 
that  a  great  difference  exists  between  it  and  a  similar  record 
drawn  by  inorganic  bodies  or  an  elastic  band  of  rubber. 

When  a  weight  is  applied  to  a  muscle,  it  does  not  immediately 
stretch  to  the  full  extent  the  weight  is  capable  of  effecting,  but  a 
certain'  time,  which  varies  with  circumstances,  is  required  for  its 
complete  extension.  The  rate  of  extension  is  at  first  rapid,  then 
slower,  until  it  ceases.  As  a  muscle  loses  its  powers  of  contrac- 
tion from  fatigue,  it  becomes  more  easily  extended.  Dead 
muscle  has  a  greater  but  less  perfect  elasticity  than  living,  i.  e., 
it  requires  greater  force  to  stretch  it,  but  does  not  return  so 
perfectly  to  its  former  shape.  The  importance  of  the  elastic 
property  of  muscle  in  the  movements  of  the  body  is  noteworthy. 
The  muscles  are  always  in  some  degree  on  the  stretch  (as  can  be 
seen  in  a  fractured  patella,  the  fragments  of  which  remain  far 
apart  and  cause  the  surgeon  much  anxiety),  and  brace  the  bones 
together  like  a  series  of  springs,  the  various  skeletal  muscles 
being  so  arranged  as  to  stretch  others  by  their  contraction. 
When  one  muscle — for  example,  the  biceps — contracts,  it  finds 
an  elastic  antagonist  already  tense,  and  has  to  shorten  this 
antagonist  as  it  contracts  itself.  The  triceps  in  this  case  acts  as 
a  weak  spring,  opposing  the  biceps,  and  it  gently  returns  to  its 
natural  length  when  the  contraction  of  the  biceps  ceases.  The 
muscles  are  kept  tense  and  ready  for  action  by  their  mere 


448  MANUAL   OF   PHYSIOLOGY. 

elasticity,  and  have  to  act  against  a  gentle  spring-like  resistance, 
so  that  the  motions  occur  evenly,  and  there  is  no  jarring  or  jerk- 
ing, as  might  take  place  if  the  attachments  of  the  inactive 
muscles  were  allowed  to  become  slack. 

Electric  Phenomena.  —  In  a  living  muscle  electric  currents  may 
be  detected,  having  a  definite  direction,  and  certain  relations  to 
the  vitality  of  the  tissue.  As  they  seem  to  be  invariably  present 
in  a  passive  muscle,  they  have  been  called  natural  muscle  currents. 

They  are  generally  studied  in  the  muscles  of  cold-blooded 
animals  after  removal  from  the  body.  The  muscle  is  spoken  of 
as  if  it  were  a  cylinder,  with  longitudinal  and  transverse  sur- 


FlG.  181. 


Non-polarizable  Electrodes.  The  glass  tubes  (a  a)  contain  sulphate  of  zinc  solution 
(z.  *.),  into  which  well  amalgamated  zinc  rods  dip.  The  lower  extremity  is  plugged 
with  china  clay  (ch.c),  which  protrudes  at  (c')  the  point.  The  tubes  can  be  moved 
in  the  holders  (h  h),  so  as  to  be  brought  accurately  into  contact  with  the  muscle. 
(Foster.) 

faces  corresponding  to  its  natural  surface  and  its  cut  extremities. 
In  such  a  block  of  frog's  muscle  the  measurement  of  the  electric 
currents  requires  considerable  care,  because  they  are  so  difficult 
to  detect  that  a  most  sensitive  galvanometer  must  be  used ;  and 
such  an  instrument  can  easily  be  disturbed  by  currents  due  to 
bringing  metal  electrodes  into  contact  with  the  moist  saline 
tissues.  Specially  constructed  electrodes  must  be  used  to  avoid 
these  currents  of  polarization  taking  place  in  the  terminals 
touching  the  muscle.  These  are  called  non-polar izable  electrodes, 
and  may  be  made  on  the  following  plan :  Some  innocuous 


ELECTRIC   PHENOMENA   OF   MUSCLE.  449 

material  moistened  in  saline  solution  (.65  per  cent.)  is  brought 
into  direct  contact  with  the  muscle,  and,  by  means  of  saturated 
solution  of  zinc  sulphate,  into  electrical  connection  with  amalga- 
mated zinc  terminals  from  the  galvanometer.  Thus  the  muscle 
is  not  injured,  and  the  zinc  solution  prevents  the  metal  termi- 
nals from  producing  adventitious  currents. 

Small  glass  tubes  drawn  to  a  point,  the  opening  of  which  is 
plugged  with  china  clay  moistened,  with  salt  solution,  make  a 
suitable  receptacle  for  the  zinc  solution.  If  a  pair  of  such 
electrodes  be  applied  to  the  middle  of  the  longitudinal  surface  at 
(e)  (Fig.  182),  and  of  the  transverse  surface  at  (p),  respectively, 
and  then  be  brought  into  connection  with  a  delicate  galvano- 
meter, it  is  found  that  a  current  passes  through  the  galvano- 
meter from  the  longitudinal  to  the  transverse  surface.  A  current 
in  this  direction  can  be  detected  in  any  piece  of  muscle,  no 
matter  how  much  it  be  divided  longitudinally,  and  probably 
would  be  found  in  a  single  fibre,  had  we  the  means  of  examining 
it.  The  nearer  to  the  centre  of  the  longitudinal  and  transverse 
sections  the  electrodes  are  placed,  the  stronger  will  be  the  cur- 
rent received  by  them.  If  both  the  electrodes  be  placed  on  the 
longitudinal  section  or  on  the  transverse  surfaces,  a  current  will 
pass  through  the  galvanometer  from  that  electrode  nearer  the  mid- 
dle of  the  longitudinal  section  (called  the  equator  of  the  muscle 
cylinder)  to  the  electrode  nearer  the  centre  of  the  transverse 
section  (pole  of  muscle  cylinder).  If  the  electrodes  be  placed 
equidistant  from  the  poles  or  from  the  equator  no  current  can  be 
detected. 

The  central  part  of  the  longitudinal  surface  of  a  piece  of 
muscle  is  then  positive,  compared  with  the  central  part  of  the 
extremities  or  transverse  sections.  And  between  these  parts — 
the  equator  and  poles  of  the  muscle  cylinder,  where  the  differ- 
ence is  most  marked — are  various  gradations,  so  that  any  point 
near  the  equator  is  positive  when  compared  with  one  near  the 
poles. 

There  is,  then,  a  current  passing  through  the  substance  of  the 
piece  of  muscle  from  both  the  transverse  sections  or  extremities 
of  the  muscle  block  to  the  middle  of  the  longitudinal  surface, 
38 


450  MANUAL   OF   PHYSIOLOGY. 

whether  it  be  a  cut  surface  (longitudinal  section)  or  the  natural 
surface  of  the  muscle.  This  is  called  the  muscle  current,  or  some- 
times natural  muscle  current. 

If  the  cylinder  in  the  accompanying  figure  be  taken  to  repre- 
sent a  block  of  muscle,  e  would  correspond  to  the  equator,  and 
p  to  the  poles,  and  the  arrow  heads  show  the  direction  of  the 
» currents  passing  through  the  galvanometer,  the  thickness  of  the 
lines  indicating  their  force..   The  dotted  lines  o  are  connected 


FIG.  182. 


Diagram  to  illustrate  the  curreuts  in  muscle. 
(e)  Equator,  corresponds  to  the  centre  of  the  muscle  cylinder. 
(p)  Polar  regions  of  cylinder  representing  the  extremities  of  the  muscle. 
The  arrow  heads  show  the  direction  of  the  surface  currents,  and  the  thickness  of 
lines  indicates  the  strength  of  the  currents.    (After  Fick.) 

with  points  where  the  electro-motive  force  is  equal,  and,  therefore, 
no  current  exists. 

The  electro-motive  force  of  the  muscle  current  in  a  frog's 
gracilis  has  been  estimated  to  be  about  .05-.08  of  a  Daniell  cell. 
It  gradually  diminishes  as  the  muscle  loses  its  vital  properties, 
and  is  also  reduced  by  fatigue.  The  electro-motive  force  rises 
with  the  temperature  from  5°  C.  until  a  maximum  is  reached  at 
about  the  body  temperature  of  mammals. 

These  muscle  currents  are  very  weak  if  the  uninjured  muscle 


LCTIVE   STATE   OF   MUSCLE.  451 

be  examined  in  situ,  the  tendon  being  used  as  the  transverse 
section  ;  they  soon  become  more  marked  after  the  exposure  of 
the  muscle,  and  if  the  tendon  be  injured  they  appear  at  once  in 
almost  full  force.  In  animals  quite  inactive  from  cold  the 
muscles  naturally  are  but  slowly  altered  by  exposure,  etc.,  and 
the  muscle  currents  do  not  appear  for  a  considerable  time,  which 
is  shortened  on  elevating  the  temperature.  It  has,  therefore, 
b3en  supposed  that  in  the  perfectly  normal  state  of  a  living 
animal  there  are  no  muscle  currents  so  long  as  the  muscle  remains 
in  the  passive  state. 

ACTIVE  STATE  OF  MUSCLE. 

A  muscle  is  capable  of  changing  from  the  passive  elongated 
condition,  the  properties  of  which  have  just  been  described,  into 
a  state  of  contraction  or  activity.  Besides  the  change  in  form, 
obvious  in  the  contracted  state  of  the  muscle,  its  chemical,  elastic, 
electric,  and  thermic  properties  are  altered.  The  capability  of 
passing  into  this  active  condition  is  spoken  of  as  the  irritability 
of  muscle.  This  is  directly  dependent  upon  its  chemical  condi- 
tion, and  therefore  related  to  its  nutrition  and  to  the  amount  of 
activity  recently  exerted,  which,  as  will  hereafter  appear,  changes 
its  chemical  state. 

Under  ordinary  circumstances,  during  life,  the  muscles  change 
from  the  passive  state  into  that  of  contraction  in  response  to  cer- 
tain impulses  communicated  to  them  by  nerves,  which  carry 
impressions  from  the  brain  or  spinal  cord  to  the  skeletal  muscles. 
The  influence  of  the  will  generally  excites  most  skeletal  muscles 
to  action.  Nearly  all  muscular  contraction  depends  on  nervous 
impulses  of  one  kind  or  another.  But  there  are  many  other 
influences  which,  when  applied  to  a  muscle,  can  bring  about  the 
same  change.  These  influences  are  called  stimuli. 

We  utilize  the  nerve  belonging  to  a  muscle  in  order  to  throw 
it  into  the  contracted  state,  but  the  great  majority  of  stimuli 
can  bring  about  the  change  when  applied  to  the  muscle  directly. 
Since  the  nerves  branch  in  the  substance  of  the  muscle,  and  are 
distributed  to  the  individual  fibres,  it  might,  as  has  been  argued, 
be  the  stimulation  of  the  terminal  nerve  ramifications  that 


452  MANUAL   OF   PHYSIOLOGY. 

brings  about  the  contraction,  even  when  the  stimulus  is  applied 
to  the  muscle  directly,  for  the  terminal  branches  of  the  nerves 
are  affected  by  the  stimulus  applied  to  the  muscle. 

That  muscles  can  be  stimulated  without  the  intervention  of 
nerves  is  satisfactorily  proved  by  the  following  facts  : — 

1.  Some  parts  of  muscles,  such  as  the  lower  end  of  the  sarto- 
rius,  and  many  muscular  structures  which  have  no   nerve  ter- 
minals in  them,  respond  energetically  to  all  kinds  of  muscle 
stimuli. 

2.  There  are  some  substances  which  act  as  stimuli  when  applied 
directly  to  the  muscle,  but  have  no  such  effect  upon  nerves,  viz., 
ammonia. 

3.  For  some   time   after  the  nerve   has   ceased  to   react,  on 
account  of  its  dying  after  removal  from  the  body,  the  attached 
muscle  will  be  found  quite  irritable  if  directly  stimulated. 

4.  The  arrow  poison,  Ourara,  has  the  extraordinary  effect  of 
paralyzing  the  nerve  terminals,  so  that  the  strongest  stimulation 
of  the  nerve  calls  forth  no  muscle  contraction.     If  the  muscles 
in  an  animal  under  the  influence  of  this  poison  be  directly  stimu- 
lated, they  respond  with  a  contraction. 

MUSCLE  STIMULI. 

The  circumstances  which  call  forth  muscle  contraction  may  be 
enumerated  thus:— 

1.  Mechanical  Stimulation. — Any  sudden  blow,  pinch,  etc.,  of 
a  living  muscle  causes  a  momentary  contraction,  which  rapidly 
passes  off  when  the  irritation  is  removed. 

2.  Thermic  Stimulation. — If  a   frog's   muscle  be  warmed  to 
over  30°  C.  it  will  begin  to  contract,  and  before  it  reaches  40°  C. 
it  will  pass  into  a  condition  known  as  heat  rigor,  which  will  be 
mentioned  presently.     If  the  temperature  of  a  muscle  be  reduced 
below  0°  C.  it  shortens  before  it  becomes  frozen. 

3.  Chemical  Stimulation. — A  number  of  chemical  compounds 
act  as  stimuli  when  they  are  applied  to  the  transverse  section  of 
a  divided  muscle.     Among  these  may  be  named  :  (1)  the  mineral 
acids  (HC1,  .1  per  cent.)  and  many  organic  acids ;  (2)  salts  of 
iron,  zinc,  silver,  copper  and  lead ;  (3)  the  neutral  salts  of  the 


MUSCLE   STIMULI. 


453 


alkalies  of  a  certain  strength  ;  (4)  weak  glycerine  and  weak 
lactic  acid ;  these  substances  only  excite  nerves  when  concen- 
trated ;  (5)  bile  is  also  said  to  stimulate  muscle  in  much  weaker 
solutions  than  it  will  nerve  fibres. 

4.  Electric  Stimulation. — Electricity  is  the  most  convenient 
form  of  stimulation,  because  we  can  accurately  regulate  the 
force  of  the  stimulus.  The  occurrence  of  variation  in  the 

FIG.  183. 


Du  Bois-Reymond's  Inductorium  with  Magnetic  Interrupter. 
c.  Primary  coil  through  which  the  primary,  inducing,  current  passes,  on  its  way  to  the 

electro-magnet  (&). 
i.  Secondary  coil,  which  can  be  moved  nearer  to  or  further  from  the  primary  coil  (c), 

thereby  allowing  a  stronger  or  weaker  current  to  be  induced  in  it.    This  induced 

current  is  the  stimulus. 
6.  Electro-magnet,  which  on  receiving  the  current  breaks  the  contact  in  the  circuit  of 

the  primary  coil  by  pulling  down  the  iron  hammer  (A),  and  separating  the  spring 

from  the  screw  of  e.    When  using  Helmholtz's  modification  (#'),  «  is  screwed  up,  and 

the  current  brings  the  spring  in  contact  with  the  point  of  the  pillar  (a),  and  so 

demagnetizes  (6)  by  "short  circuiting"  the  battery. 

When  tetanus  is  to  be  produced,  the  wires  from  the  battery  are  to  be  connected  with 
g  and  a. 

When  a  single  contraction  is  required,  the  magnetic  interrupter  is  cut  out  by  shift- 
ing the  wire  from  a  to  the  binding  screw  to  the  right  of/. 

intensity  of  an  electric  current  passing  through  a  muscle  causes 
it  to  contract.  The  sudden  increase  or  decrease  in  the  strength 
of  a  current  acts  as  a  stimulus,  but  a  current  of  exactly  even 
intensity  may  pass  through  a  muscle  without  further  exciting  it, 
after  the  initial  contraction  has  ceased.  A  common  method  of 
producing  such  a  variation  is  that  of  opening  or  closing  an 


454  MANUAL   OF    PHYSIOLOGY. 

electric  circuit  of  which  the  muscle  forms  a  part,  so  as  to  make 
or  break  the  current ;  and  thus  a  variation  of  intensity  equal  to 
the  entire  strength  of  the  current  takes  place  in  the  muscle,  and 
acts  as  a  stimulus. 

The  direct  current  from  a  battery  (continuous  current)  is  used 
to  stimulate  a  muscle  in  certain  cases,  but  a  current  induced  in 
a  secondary  coil  by  the  entrance  or  cessation  of  a  current  in 
a  primary  coil  of  wire  (induced  current)  is  more  commonly 
employed  on  account  of  the  greater  efficacy  of  its  action.  The 
instrument  used  for  this  purpose  in  physiological  laboratories  is 
Du  Bois-Reymond's  inductoriurn,  in  which  the  strength  of  the 
stimulus  can  be  reduced  by  removal  of  the  secondary  coil  from 
the  primary.  It  is  supplied  with  a  magnetic  interrupter,  by  means 
of  which  repeated  stimuli  may  be  given  by  rapidly  making  and 
breaking  the  primary  current  (interrupted  current}  (Fig.  183). 

The  irritability  of  muscle  substance  is  not  so  great  as  that  of 
the  motor  nerves ;  that  is  to  say,  a  less  stimulus  applied  to  the 
nerve  of  a  nerv7e-muscle  preparation*  will  cause  contraction  than 
if  applied  to  the  muscle  directly.  In  experimenting  on  the  con- 
traction of  muscle,  as  already  stated,  the  nerve  is  commonly 
used  to  convey  the  stimulus,  because,  when  an  electric  current  is 
applied  to  the  nerve,  the  stimulus  is  the  more  safely  and  com- 
pletely distributed  throughout  the  muscle  fibres  than  when  it  is 
applied  directly. 

( 'H  .\\UES  OCCURRING  IN  MUSCLE  ON  ITS  ENTERING  THE  ACTIVE 

STATE. 

Changes  in  Structure. — The  examination  of  muscle  with  the 
microscope  during  its  contraction  is  attended  with  considerable 
difficulty,  and  in  the  higher  animals  has  not  led  to  satisfactory 
results.  In  the  muscles  of  insects,  where  the  differentiation  of 
the  contractile  substance  is  more  marked,  certain  changes  can  be 
observed.  The  fibres,  and  even  the  fibrillse  within  them,  can  easily 
enough  be  seen  to  undergo  changes  in  form  corresponding  to  those 

*By  a  nerve-muscle  preparation  is  meant  a  muscle  of  a  frog  (commonly  the  gastro- 
cnemius  and  the  half  of  the  femur  to  which  it  is  attached)  and  its  nerve,  which  have 
been  carefully  separated  from  other  parts  and  removed  from  the  body. 


CHANGES    DURING   CONTRACTION.  455 

of  the  entire  muscle,  namely,  increase  in  thickness  and  diminution 
in  length.  A  change  in  the  position  and  relative  size  of  the 
singly  and  doubly  refracting  portions  of  the  muscle  element  has 
been  described,  and  some  authors  state  that  the  latter  increases 
at  the  expense  of  the  former  after  an  intermediate  period  in 
which  the  two  substances  seem  fused  together. 

Chemical  Changes. — During  the  contracted  condition,  the 
chemical  changes  which  go  on  in  passive  muscle  are  intensified, 
and  certain  new  chemical  decompositions  arise  of  which  not 
much  is  known. 

Active  muscle  takes  up  more  oxygen  than  muscle  at  rest,  as  is 
shown  by  the  facts  that,  during  active  muscular  exercise,  more 
oxygen  enters  the  body  by  respiration,  and  the  blood  leaving 
active  muscles  is  poorer  in  oxygen  than  when  the  same  muscles 
are  passive.  This  absorption  of  oxygen  may  be  detected  in  a 
muscle  cut  out  of  the  body,  but  a  supply  of  oxygen  is  not  neces- 
sary for  its  contraction,  since  an  excised  frog's  muscle  will  con- 
tract in  an  atmosphere  containing  no  oxygen.  From  this  it 
would  appear  that  a  certain  ready  store  of  oxygen  must  exist 
in  some  chemical  constituent  of  the  muscle  substance.  It  is 
possible  that  some  chemical  compound,  constantly  renewed  by 
the  blood,  is  the  normal  source  of  oxygen,  and  not  the  oxy- 
hsemoglobin. 

The  amount  of  COa  given  off  by  a  muscle  increases  in  its 
state  of  activity.  This  may  be  seen  («)  by  the  greater  elimina- 
tion from  the  lungs  during  active  muscular  exercise,  and  (/9)  by 
the  fact  that  the  venous  blood  of  a  limb,  when  the  muscles  are 
contracted,  contains  more  CO2  than  when  they  are  relaxed,  (j) 
The  increase  of  CO2  can  also  be  detected  in  a  muscle  removed 
from  the  body  and  kept  in  a  state  of  contraction,  f/5)  This 
increase  in  the  formation  of  CO2  takes  place  whether  there  is  a 
supply  of  oxygen  or  not,  (e)  and  the  quantity  of  CO2  given  off 
exceeds  the  quantity  of  oxygen  that  is  used  up.  So  that  it  is 
not  exclusively  from  the  newly-supplied  oxygen  that  the  CO2 
is  produced. 

Muscle  tissue,  when  passive,  is  neutral  or  faintly  alkaline ; 
during  contraction,  however,  it  becomes  distinctly  acid.  The 


456  MANUAL   OF   PHYSIOLOGY. 

litmus  which  it  changes  from  blue  to  red  is  permanently  altered, 
and  the  conclusion  follows  that  CO2  is  not  the  only  acid  that 
makes  its  appearance.  The  other  acid  is  sarcolactic  acid,  which 
is  constantly  present  in  muscle  after  prolonged  contraction,  and 
varies  in  amount  in  proportion  to  the  degree  of  activity  the 
muscle  has  undergone.  If  artificial  circulation  be  kept  up  in 
the  muscle,  the  quantity  of  sarcolactic  found  in  the  blood  is  very 
great.  It  varies  directly  with  the  CO2,  which  would  seem  to 
suggest  a  relationship  between  the  origin  of  the  two  acids. 

The  amount  of  glycogen  and  grape  sugar  is  said  to  diminish 
in  muscle  during  its  activity,  and  it  is  stated  that  sarcolactic 
acid  can  be  produced  from  these  carbohydrates  by  the  action  of 
certain  ferments. 

Active  muscle  contains  more  of  those  substances  than  can  be 
extracted  by  alcohol,  and  less  that  are  soluble  in  water  than 
passive  muscle. 

The  chemical  changes  which  take  place  during  muscle  con- 
traction are  probably  the  result  of  a  decomposition  of  some  car- 
bohydrates, in  which  the  albuminous  substances  do  not  take 
any  part  that  requires  their  own  destruction.  This  seems  sup- 
ported by  the  fact  that  the  increased  gas  exchange  in  muscle 
during  active  exercise  can  be  recognized  in  a  corresponding 
alteration  in  the  gas  exchange  in  pulmonary  respiration  ;  and 
there  seems  no  relation  between  muscular  labor  and  the  amount 
of  nitrogenous  waste,  as  estimated  by  the  urea  elimination, 
which  we  should  expect  if  muscular  activities  were  the  outcome 
of  a  decomposition  of  nitrogenous  (albuminous)  parts  of  the 
muscle  substance. 

The  chemical  changes,  then,  said  to  take  place  in  muscle 
during  its  contraction  are  :  — 

1.  The  contractile  substance,  which  is  normally  neutral  or 
faintly  alkaline,  becomes  acid  in  reaction,  owing  to  the  formation 
of  sarcolactic  acid. 

2.  More  oxygen  is  taken  up  from  the  blood  than  when  the 
muscle  is  at  rest.     This  using  up  of  oxygen  occurs  also  in  the 
isolated  muscle,  and  its  amount  appears  to  be  independent  of  the 
blood  supply. 


CHANGES   IN    ELECTRICAL   STATE.  457 

3.  The  extractives  soluble  in  water  decrease;  those  soluble  in 
alcohol  increase. 

4.  A  greater  amount  of  CO2  is  given  off,  both  in  the  isolated 
muscle  and  in  the  muscles  in  the  body,  and  the  change  in  the 
quantity  of  COj,  has  no  exact  relation  to  that  of  the  oxygen 
used. 

5.  A  diminution  is  said  to  occur  in  the  contained  glycogen, 
and  certainly  prolonged   inactivity  causes   an   increase  in    the 
amount  of  glycogen. 

6.  A  peculiar  muscle  sugar  makes  its  appearance. 

I.  Change  in  Elasticity. — The  elasticity  of  a  muscle  during  its 
state  of  contraction  is  less  than  in  the  passive  state.     That  is  to 
say,  a  given  weight  will  extend  the  same  muscle  more  if  attached 
to  it  while  contracted  (as  in  tetanus)  than  when  it  is  relaxed. 
The  contracted  muscle  is  then  more  extensible.     If  a  weight, 
which  is  just  over  the  maximum  load  the  muscle  can  lift,  be 
hung  from  it  and  the  muscle  stimulated,  it  should  become  ex- 
tended, because  the  change  to  the  active  state  lessens  its  elastic 
power,  while  it  cannot  contract,  being  over-weighted. 

II.  Electrical  Changes. — If  a    muscle,  in  connection  with  a 
galvanometer,  so  as  to  show  the  natural  current,  be  stimulated 
by  means  of  the  nerves,  a  marked  change  occurs  in  the  current. 
The  galvanornetric  needle  swings  toward  zero,  showing  that  the 
current  is  weakened  or  destroyed.     This  is  called  the  negative 
variation  of  the  muscle  current,  which  initiates  the  change  to  the 
active  condition.'  When  the  muscle  receives  but  a  momentary 
stimulus  sufficient  to   give  a  single  contraction,  this   negative 
variation  takes  place  in  the  current,  but,  owing  to  its  extremely 
short  duration,  the  galvanometric   needle  is   prevented  by  its 
inertia  from  following  the  change.     Only  the  most  sensitive  and 
well-regulated  instruments  show  the  electric  change  of  a  single 
contraction,  but  when  the  muscle  is  kept  contracted  by  a  series 
of  rapidly  repeated  stimulations  the  inertia  of  the  needle   is 
readily  overcome. 

Rheoscopic  Frog. — The  negative  variation  of  a  single  contrac- 
tion can  be  easily  shown  on  the  sensitive  animal  tissues.     For 
this  purpose  the  sciatic  nerve  of  a  frog's  leg  is  placed  upon  the 
39 


458  MANUAL   OF    PHYSIOLOGY. 

surface  of  the  gastrocnemius  of  another  leg,  so  as  to  pass  over 
the  middle  and  the  extremity  of  the  muscle.  When  the  second 
(stimulating)  muscle  is  made  to  contract,  its  negative  variation 
acts  as  a  stimulus  to  the  nerve  lying  on  it,  and  so  the  first 
(stimulated)  muscle  contracts.  Not  only  does  this  show  the 
negative  variation  of  a  single  contraction,  but  it  also  demon- 
strates that  the  continued  (tetanic)  contraction,  produced  by 
interrupted  electric  stimulation,  is  associated  with  repeated  nega- 
tive variations.  We  shall  see  that  the  continued  contraction  is 
brought  about  by  a  rapidly  repeated  series  of  stimulations,  so 
that  the  electric  condition  of  the  stimulating  muscle  undergoes 


FIG.  184. 


Diagram  illustrating  the  arrangement  in  the  Rheoscopic  Frog. 

A  =  stimulating  limb.  n= stimulated  limb.  The  current  from  the  electrodes  passes  into 
nerve  (N)  of  stimulating  limb  (A),  causing  its  gastrocnemius  to  contract.  Where- 
upon the  negative  variation  of  the  natural  current  between  +  and  —  stimulates  the 
nerve  (N'),  and  excites  the  muscles  of  B  to  action. 

a  series  of  variations.  The  contraction  of  the  stimulated  muscle, 
whose  nerve  lies  on  the  stimulating  muscle,  responds  to  the  elec- 
tric variations  of  the  stimulator,  and  contracts  synchronously 
with  it. 

If  an  isolated  part  of  a  muscle  be  stimulated,  the  contraction 
passes  from  that  point  as  a  wave  to  the  remainder  of  the  muscle. 
This  contraction  wave  is  preceded  by  a  wave  of  negative  varia- 
tion which  passes  along  the  muscle  at  the  rate  of  three  metres  per 
second  (the  same  rate  as  the  contraction  wave),  lasting  at  any 
one  point  .003  of  a  second,  so  that  the  negative  variation  is  over 


MUSCLE   CONTRACTION.  459 

before  the  contraction  begins,  for  the  muscle  requires  a  certain 
time,  called  the  latent  period,  before  it  commences  to  contract. 

The  origin  of  the  electric  currents  of  muscle  will  be  discussed 
with  nerve  currents,  to  which  the  reader  is  referred. 

III.  Temperature  Change. — Long  since  it  was  observed  in  the 
human  subject  that  the  temperatureof  muscles  rose  during  their  ac- 
tivity. In  frog's  muscle  a  contraction  lasting  three  minutes  caused 
an  elevation  of  .18°  C.    A  single  contraction  is  said  to  produce  a 
rise  varying  from  .001°  to  .005°  C.,  according  to  circumstances. 

The  production  of  heat  is  in  proportion  to  the  tension  of  the 
muscle.  When  the  inusoles  are  prevented  from  shortening,  a 
greater  amount  of  heat  is  said  to  be  produced. 

The  amount  of  heat  has  also  a  definite  relation  to  the  work 
performed.  Up  to  a  certain  point  the  greater  the  load  a  muscle 
has  to  move, the  greater  the  heat  produced;  when  this  maximum 
is  reached  any  further  increase  of  the  weight  causes  a  falling  off 
in  the  heat  production.  Repeated  single  contractions  are  said  to 
produce  more  heat  than  tetanus  kept  up  for  a  corresponding 
time. 

The  fatigue  which  follows  prolonged  activity  is  accompanied 
by  a  diminution  in  the  production  of  heat. 

IV.  Change  in  Form. — The  most  obvious  change  a  muscle 
undergoes  in  passing  into  the  active  state  is  its  alteration  in  shape. 
It  becomes  shorter  and  thicker.     The  actual  amount  of  shorten- 
ing varies  according  to  circumstances.      (a)  A  muscle  on  the 
stretch  when  stimulated  will  shorten  more  in  proportion  than  one 
whose  elasticity  is  not  called  into  play  before  contraction,  so  that 
a  slightly  weighted  muscle  shortens  more  than  an  unweighted  one 
with  the  same  stimulus.     (6)  The  fresher  and  more  irritable  a 
muscle  is,  the  shorter  it  will  become  in  response  to  a  given 
stimulus ;  and,  conversely,  a  muscle  which  has  been  some  time 
removed  from  the  body,  or  is  fatigued  by  prolonged  activity,  will 
contract  proportionately  less,      (c)   With   a  certain  limit,  the 
stronger  the  stimulus  applied  the  shorter  a  muscle  will  become. 
(d)  A  warm  temperature  augments  the  amount  of  shortening, 
the  amount  of  contraction  of  frogs'  muscles   increasing  up  to 
33°  C.     A  perfectly  active  frog's  muscle  shortens  to  about  half 


460  MANUAL   OF    PHYSIOLOGY. 

its  normal  length.  If  much  stretched  and  stimulated  with  a 
strong  current  it  may  contract  nearly  to  one-fourth  of  its  length 
when  extended.  Muscles  are  seldom  made  up  of  perfectly 
parallel  fibres,  the  direction  and  arrangement  varying  much  in 
different  muscles.  The  more  parallel  to  the  long  axis  of  the 
muscle  the  fibres  run,  the  more  will  the  given  muscle  be  able  to 
shorten  in  proportion  to  its  length. 

The  thickness  of  a  muscle  increases  in  proportion  to  its  short- 
ening during  contraction,  so  that  there  is  but  little  change  in 
bulk.  It  is  said,  however,  to  diminish  slightly  in  volume,  becom- 
ing less  than  y-^  smaller.  This  can  be  shown  by  making 
a  muscle  contract  in  a  bottle  filled  with  weak  salt  solution  so  as 
to  exclude  all  air,  and  to  communicate  with  the  atmosphere  only 
by  a  capillary  tube  into  which  the  salt  solution  rises.  The 
slightest  decrease  in  bulk  is  shown  by  the  fall  of  the  thin  column 
of  fluid  in  the  tube. 

Since  a  muscle  loses  in  elastic  force  and  gains  but  little  in 
density  during  contraction,  the  hardness  which  is  communicated 
to  the  touch  depends  on  the  difference  of  tension  of  the  semi- 
fluid contractile  substance  within  the  muscle  sheath. 

THE  GRAPHIC  METHOD  OF  RECORDING  MUSCLE  CONTRACTION. 
In  order  to  study  the  details  of  the  contraction  of  muscle,  the 
graphic  method  of  recording  the  motion  is  applied.  The  curve 
may  be  drawn  on  an  ordinary  cylinder  moving  sufficiently 
rapidly.  Where  accurate  time  measurements  are  required,  it  is 
better  to  use  one  of  the  many  special  forms  of  instruments, 
called  myographs,  made  for  the  purpose.  The  principle  of  all 
these  instruments  is  the  same ;  namely,  an  electric  current,  which 
passes  through  the  nerve  of  a  frog's  muscle  connected  with  the 
marking  lever,  is  broken  by  some  mechanism,  while  the  surface 
is  in  motion ;  the  exact  moment  of  breaking  the  contact  can  be 
accurately  marked  on  the  recording  surface,  by  the  lever  which 
draws  the  muscle  curve,  before  the  instrument  is  set  in  motion. 
The  rate  of  motion  is  registered  by  a  tracing  drawn  by  a  tuning 
fork  of  known  rate  of  vibration. 

In  order  that  the  muscle-nerve  preparation  may  not  be  injured 


PHASES   OF   A   SINGLE   CONTRACTION.  461 

by  the  tissues  becoming  too  dry,  it  is  placed  in  a  small  glass  box, 
the  air  of  which  is  kept  moist  by  a  damp  sponge.  This  moist 
chamber  is  used  when  any  living  tissue  is  to  be  protected  from 
drying. 

The  first  myograph  used  was  a  complicated  instrument  devised 
by  Helmholtz ;  in  which  a  small  glass  cylinder  is  made  to  rotate 
rapidly  by  a  heavy  weight,  and  when  a  certain  velocity  of  rotation 
is  attained,  a  tooth  is  thrown  out  by  centrifugal  force,  which  breaks 
the  circuit  of  the  current  passing  through  the  nerve  of  the  muscle. 
The  tendon  is  attached  to  a  balanced  lever,  at  one  end  of  which 
hangs  a  rigid  style  pressed  by  its  own  weight  against  the  glass 
cylinder.  When  the  circuit  is  broken  the  muscle  contracts, 
raises  the  lever,  and  makes  the  style  draw  on  the  smoked-glass 
cylinder. 

Fick  introduced  a  flat  recording  surface  moving  by  the  swing 
of  a  pendulum,  by  which  the  abscissa  is  made  a  segment  of  a  circle, 
and  not  a  straight  line,  and  the  rate  varies,  so  that  the  different 
parts  of  the  curve  have  varying  time  values.  The  curves  given 
in  the  following  woodcuts  are  drawn  with  the  Pendulum  Myograph. 

Du  Bois-Reymond  draws  muscle  curves  on  the  smoked  surface 
of  a  smaH'glass  plate  contained  in  a  frame,  which  is  shot  by  the 
force  of  a  spiral  spring  along  tense  wires,  and  on  its  way  breaks 
the  contact.  The  trigger  used  for  releasing  the  spring  sets  a 
tuning  fork  at  the  same  time  vibrating  (Spring  Myograph). 

SINGLE  CONTRACTION. 

In  response  to  an  instantaneous  stimulus,  such  as  occurs  in  the 
secondary  coil  on  breaking  the  primary  current,  a  muscle  gives 
a  momentary  twitch  or  spasm,  commonly  spoken  of  as  a  single 
contraction,  which  is  of  so  short  duration,  that  without  the  graphic 
method  of  recording  the  motion  we  could  not  appreciate  the 
phases  which  are  seen  in  the  curve. 

The  curve  drawn  on  the  recording  surface  of  a  pendulum 
myograph,  by  such  a  single  contraction,  is  represented  in  Fig. 
185.  The  short  vertical  stroke  on  the  abscissa,  or  base  line,  is 
drawn  by  touching  the  lever  when  the  muscle  is  in  the  uncon- 
tracted  state,  and  indicates  the  time  of  stimulation.  The  upper 


462  MANUAL   OF   PHYSIOLOGY. 

curved  line  is  drawn  by  the  lever  during  the  contraction  of  the 
muscle. 

In  such  a  curve  the  following  stages  are  to  be  distinguished  : — 

1.  A  short  period  between  the  moment  of  stimulation  and  that 
at  which  the  lever  begins  to  rise,  during  which  the  muscle  does 
not  move.     This  is  known  as  the  latent  period.     In  the  skeletal 
muscles  of  the  frog  this  period  lasts  nearly  .01  sec. 

2.  A  period  during  which  the  lever  rises,  at  first  slowly,  then 
more  quickly,  then  again  slowly,  until  it  ceases  to  rise.     This 
stage  has  been  called  the  period  of  rising  energy.     It  lasts  about 
.04  sec. 

3.  When  the  highest  point  is  attained  the  lever  commences 
to  fall,  at  first  slowly,  then  more  quickly,  and  at  last  slowly. 

FIG.  185. 


Curve  drawn  by  a  frog's  gastrocnemius  on  the  Pendulum  Myograph;  below  is  seen  the 
tuning-fork  record  of  the  time  occupied  by  the  contraction.  Parallel  to  the  latter  is 
the  abscissa.  The  little  vertical  mark  at  the  left  shows  the  moment  of  stimulation, 
and  the  distance  from  this  to  the  beginning  of  the  rise  of  the  curve  gives  the  latent 
period,  which  is  followed  by  the  ascent  and  descent  of  the  lever. 

There  is  then  no  pause  at  the  height  of  contraction.  The  stage 
of  relaxing  has  been  called  the  period  of  falling  energy.  It 
occupies,  when  quite  fresh,  about  the  same  time  as  the  second 
period,  viz.,  about  .04  sec. 

Thus,  a  stimulus  occupying  an  almost  immeasurably  short 
time  sets  up  a  change  in  the  molecular  condition,  which,  taking 
nearly  TV  sec.  to  run  its  course,  and  requiring  Ttnr  sec.  before  it 
exhibits  any  change  of  form,  then  in  Tfor  sec.  attains  the  maxi- 
mum height  of  contraction,  and,  without  waiting  in  the  con- 
tracted condition,  spends  Tfo  sec.  in  relaxing. 

The  latent  period  which  appears  in  a  single  contraction  curve 
drawn  by  a  muscle  stimulated  in  the  usual  way,  through  the 


PHASES   OF   A   SINGLE   CONTRACTION.  463 

medium  of  a  nerve,  is  not  entirely  occupied  by  preparatory 
changes  going  on  in  the  substance  of  the  muscle,  but  a  certain 
part  of  the  time  recorded,  as  latent  period  corresponds  to  the 
time  required  for  the  transmission  of  the  impulse  along  the 
nerve.  This  may  be  shown  by  stimulating  first  the  far  end  of  the 
nerve,  and  then  the  muscle  itself.  In  this  case  two  curves  will 
be  drawn,  having  different  latent  periods,  that  obtained  by  direct 
stimulation  of  the  muscle  being  shorter,  and  representing  the 
real  latent  period,  while  the  longer  one  includes  the  time  taken 
by  the  impulse  to  travel  along  the  piece  of  nerve  between  the 
electrodes  and  the  muscle. 

Wave  of  Contraction. — If  one  extremity  of  the  muscle  be 
stimulated  without  the  aid  of  the  nerve  (it  is  best  to  employ  a 
muscle  from  a  curarized  animal),  the  contraction  passes  along 
the  muscle  from  the  point  of  stimulation  in  a  wave  which  travels 
at  a  definite  rate  of  3-4  metres  per  sec.  in  a  frog,  and  4-5 
metres  per  sec.  in  a  mammal.  Reduction  of  temperature  and 
fading  of  vital  activity  cause  the  velocity  of  the  wave  to  be 
lessened,  until  finally  the  tissue  ceases  to  conduct ;  then  only  a 
local  contraction  occurs,  severe  stimulus  causing  simply  an  eleva- 
tion at  the  point  of  contact.  This  seems  analogous  to  the  idio- 
muscular  contraction,  which  marks  the  seat  of  severe  mechanical 
stimulation  after  the  general  contraction  has  ended. 

VARIATIONS   IN  THE   PHASES  OF  A  SINGLE  CONTRACTION. 

The  latent  period  varies  much  in  different  kinds  of  muscle,  in 
the  same  kind  of  muscle  of  different  animals,  and  in  the  same 
individual  muscle  under  different  conditions.  As  a  rule,  the 
slow-contracting  muscles  have  a  longer  latent  period.  Thus  the 
non-striated  slow-contracting  muscles  found  in  the  hollow  viscera 
have  a  latent  period  of  some  seconds.  The  striated  muscles  of 
cold-blooded  animals  have  a  longer  latency  than  the  same  kind 
of  muscle  in  birds  and  mammalia.  The  same  gastrocnemius  of 
a  frog  has  a  shorter  latent  period  when  strongly  stimulated,  or 
when  its  temperature  is  raised,  and  vice  versa. 

The  latent  period  is  considerably  lengthened  by  fatigue.  If 
the  weight  be  so  applied  that  it  does  not  extend  the  muscle  before 


464  MANUAL   OF  PHYSIOLOGY. 

contraction,  but  only  bears  on  it  the  instant  it  commences  to 
shorten,  the  duration  of  the  latent  period  increases  in  proportion 
to  the  weight  the  muscle  has  to  lift. 

The  duration  of  the  single  contraction  of  striated  muscle 
varies  in  different  cases  and  under  varying  circumstances.  With 
submaximal  stimulation  the  length  of  the  curve  increases  with 
the  strength  of  the  stimulation.  When  the  maximal  strength 
of  stimulus  (i.  e.,  that  exciting  a  maximal  contraction')  is  reached, 
no  further  lengthening  of  the  curve  takes  place. 

The  greatest  difference  is  observed  in  the  muscles  found  in  dif- 
ferent kinds  of  animals.  The  contraction  of  some  kinds  of  mus- 
cle tissue  (non-striated  muscle  of  mollusca,  for  example)  occupies 
several  minutes,  and  reminds  one  of  the  slow  movement  of 
protoplasm  ;  while  the  rapid  action  of  the  muscle  of  the  wing  of 

FIG.  186. 


Curv  es  drawn  by  the  same  muscle  in  different  stages  of  fatigue — A,  when  fresh  ;  B,  C, 
D,  E,  each  immediately  after  the  muscle  had  contracted  200  times.  Showing  that 
fatigue  causes  a  low,  long  contraction. 

a  horsefly  occurs  330  times  a  second.  Various  gradations  between 
these  extremes  in  the  rapidity  of  muscle  contraction  may  be 
found  in  the  contractile  tissues  of  different  animals.  The  fol- 
lowing table  gives  the  rate  of  contraction  of  some  insects'  mus- 
cles, which  may  help  to  show  the  extent  of  these  variations  : — 

Horsefly 330  contractions  per  second. 

Bee 190  " 

Wasp 110  "  " 

Dragonfly 28  "  '* 

Butterfly 9  «  « 

Among  the  vertebrata  the  duration  of  the  contraction  of  the 
skeletal  muscles  varies  considerably,  according  to  the  habits  of 
the  animal.  The  limb  muscles  of  the  tortoise  and  the  toad  take 


VARIATIONS    IN    THE   SINGLE   CONTRACTION.  465 

a  very  kng  time  to  finish  their  contraction  ;  other  muscles  of 

the  same  animals  act  more  quickly,  but  do  not  attain  the  rapidity 

of  contraction  of  the  skeletal  muscles  of  warm-blooded  animals. 

The  duration  of  a  single  contraction  of  the  same  muscle  is  also 

FIG.  187. 


Six  curves  drawn  by  the  same  muscle  when  stretched  by  different  weights.  Showing 
that  as  the  weight  is  increased  the  latency  becomes  longer  and  the  contraction  less  in 
height  and  duration. 

capable  of  considerable  variation.  It  seems  to  be  lengthened  by 
anything  that  leads  to  'an  accumulation  of  the  chemical  pro- 
ducts which  arise  from  muscle  activity.  Hence  fatigue  or  over- 
stimulation  causes  a  slow*contraction  (Fig.  186). 

FIG.  188. 


Curves  drawn  by  the  same  muscle  at  different  temperatures.  Showing  that  with  eleva- 
tion of  temperature  the  latency  and  the  contraction  become  shoiter.  (The  muscle 
had  been  previously  cooled.) 

Moderate  increase  of  temperature  greatly  shortens  the  time 
occupied  by  the  single  contraction  of  any  given  muscle.  Exces- 
sive heat  causes  a  state  of  continued  contraction. 


466  MANUAL   OF   PHYSIOLOGY. 

The  reduction  of  temperature  causes  a  muscle  to  contract  more 
slowly,  and,  when  extreme,  the  muscle  remains  contracted  long 
after  the  stimulus  is  removed. 

The  altitude  of  the  curve  which  represents  the  extent  of  the 

FIG.  189. 


Curves  drawn  by  the  same  muscle  while  being  cooled.    Showing  that  the  latency  and 
the  contraction  become  longer  as  the  temperature  is  reduced. 

contraction  varies  in  the  same  way  as  the  latent  period  and  the 
duration. 

MAXIMUM  CONTRACTION. 

The  extent  to  which  a  muscle  will  contract  depends  upon  the 
conditions  in  which  it  is  placed,  and  *  varies  with  the  load,  its 

FIG.  190. 


Pendulum  Myograph  tracings  showing  summation. 

1.  Curve  of  maximum  contraction  drawn  by  first  stimulus,  the  exact  time  of  applica- 
tion of  which  is  showa  by  the  small  upstroke  of  the  left  hand  of  the  base  line. 

2.  Maximum  contraction  resulting   from  second    simple    stimulation    given    at     the 
moment  indicated  by  the  other  small  upstroke. 

3.  Curve  drawn  as  the  result  of  double  stimulation  sent  in  at  an  interval  indicated  by 
the  distance  between  the  upstrokes,  showing  summation  of  stimulus  and  consequent 
increase  incontraction  over  the  "  maximum  contraction." 

irritability,  the  temperature,  and  the  force  of  the-  stimulus.  A 
fresh  muscle  at  the  ordinary  temperature,  with  a  medium  load, 
will  contract  more  and  more  as  the  intensity  of  the  current 


MAXIMUM    CONTRACTION.  467 

employed  increases.  There  is  a  limit  to  this  increase,  and  with 
comparatively  weak  stimulation  an  effect  is  produced  which 
cannot  be  surpassed  by  the  same  muscle  with  further  increment 
of  stimulus.  The  height  of  the  contraction  is  the  same  for  all 
medium  stimuli  while  the  muscle  is  fresh.  This  is  called  the 
maximum  contraction,  being  the  greatest  shortening  which  can  be 
produced  by  a  single  stimulus. 

Summation. — Each  time  a  muscle  receives  an  induction  shock 
of  medium  strength,  it  responds  with  a  "  maximal  contraction," 
but  this  is  not  the  maximum  amount  the  muscle  can  contract 
with  repeated  stimulation.  If  a  second  stimulus  be  given  while 
the  muscle  is  in  the  contracted  state,  a  new  maximum  contrac- 
tion is  added  to  the  contraction  already  arrived  at  by  the  muscle 
at  the  moment  of  the  second  stimulation.  If  stimulated  when 
the  lever  is  at  the  apex  of  the  curve,  the  sum  of  the  effect  pro- 
duced will  be  equal  to  two  maximum  contractions. 

If  applied  in  the  middle  of  the  period  of  the  ascent  or  descent 
of  the  lever,  a  second  stimulation  gives  rise  to  1*  maximum  con- 
tractions, and  so  on,  in  various  parts  of  the  curve,  a  new  maxi- 
mum curve  is  produced,  arising  from  the  point  at  which  the 
lever  is  when  the  second  stimulus  is  applied  (Fig.  190). 

During  the  latent  period  a  second  stimulation  produces  the 
same  effect,  but  the  summation  only  begins  at  the  end  of  the 
latent  period  of  the  second  contraction,  when  the  effect  of  the 
first  stimulus  is  as  yet  small.  It  is  difficult  to  demonstrate  the 
summation  when  the  stimuli  are  very  close,  but  if  the  second 
stimulus  come  after  an  interval  of  more  than  g-J^-  sec.,  summa- 
tion can  easily  be  appreciated. 

This  summation  of  effect  also  takes  place  when  the  stimulus  is 
insufficient  to  produce  a  maximum  contraction.  The  first  few 
weak  stimuli  give  rise  to  the  same  extent  of  contraction  as  if 
the  muscle  were  at  its  normal  length  at  the  time  of  each  succes- 
sive stimulation.  The  following  tracings  (Figs.  191-193)  show 
the  effects  of  repeated  stimulations  applied  at  the  various  periods 
indicated  by  the  numbers  on  the  abscissa  line. 


468  MANUAL   OF   PHYSIOLOGY. 

TETANUS. 

If  a  series  of  stimuli  be  applied  in  succession,  at  intervals  less 
than  the  duration  of  a  single  contraction,  a  summation  of  con- 
tractions occurs,  which  results  in  the  accumulation  of  effect  until 
the  muscle  has  shortened  to  about  one-half  of  the  length  it 

FIG.  191. 


Curve  of  tetanus  resulting  from  30  stimulations  per  second,  drawn  by  a  frog's  muscle  on 
a  drum,  the  surface  of  which  moves  1.5  centimeters  per  second.  The  stimulation 
commences  at  "  30,"  and  ceases  just  before  the  lever  begins  to  fall.  No  trace  of  the 
individual  contractions  of  which  the  tetanus  is  composed  can  be  recognized. 

attains  during  a  single  contraction,  or  about  one-fourth  the 
normal  length  of  the  relaxed  muscle;  it  then  remains  contracted 
to  the  same  extent  for  some  time,  and  does  not  shorten  further, 
though  the  stimulus  be  increased  in  rate  or  strength.  As  long 
as  the  stimuli  are  continued,  the  various  single  contractions 

FIG.  192. 


Curve  of  tetanus  composed  of  imperfectly  fused  contractions  resulting  from  12  stimu- 
lations per  second.  The  serrations  on  the  left  of  the  curve  indicate  the  individual 
contractions. 

caused  by  the  individual  shocks  are  fused  together  (Fig.  191)  ; 
but  if  the  intervals  between  the  stimuli  be  nearly  as  long  as  the 
time  occupied  by  a  single  contraction,  the  line  drawn  by  the 
lever  will  show  notches  indicating  the  apices  of  the  fused  single 
contractions  (Figs.  192  and  193). 


TETANUS.  469 

This  condition  of  summation  of  contractions  is  called  tetanus, 
and  is  said,  by  some,  to  be  the  manner  in  which  muscular  motion 
is  produced  by  the  action  of  the  nerves  in  obedience  to  the  will. 

With  from  fifteen  a  second  to  upwards  of  many  hundreds  of 
induced  shocks  one  can  produce  tetanus  in  a  frog's  muscle.  The 
lowest  rate  of  electric  stimulation  at  which  human  muscle  passes 
into  complete  tetanus  is  about  25  per  sec.  The  number  of  stim- 
uli required  varies  with  the  rate  of  contraction  of  the  muscle 
employed,  the  quick-contracting  bird's  muscle  requiring  70  per 
second,  while  the  exceptionally  slow-moving  tortoise  muscle 
only  requires  3  per  second.  According  to  some,  there  is  a  limit 
to  the  number  of  stimuli  which  will  cause  tetanus — 360  per 
second  is  named  as  the  maximum  for  a  certain  strength  of 

FIG.  193. 


Tetanus  produced  by  8  stimulations  per  second.  The  more  perfect  fusion  of  the  single 
contractions  shown  toward  the  end  of  the  curve  depends  on  the  altered  condition  of 
the  muscle. 

stimulus;  with  stronger  stimuli,  even  when  more  frequent, 
tetanus  occurs.  It  has  been  shown  that  many  thousand  stimuli 
per  second  can  cause  tetanus  even  with  very  weak  currents.  If 
tetanus  be  kept  up  for  some  seconds,  and  the  stimulation  be  then 
suddenly  stopped,  the  lever  falls  rapidly  for  a  certain  distance, 
but  the  muscle  does  not  quite  return  to  its  normal  length  for 
some  few  seconds.  This  residual  contraction  is  easily  overcome 
by  any  substantial  load.  If  kept  in  a  state  of  tetanus  by  weak 
stimulation,  after  some  time  the  muscle  commences  to  relax  from 
fatigue,  at  first  rapidly,  then  more  slowly.  This  falling  off  of 
the  tetanic  contraction  may  be  prevented  by  increasing  the 
stimulus. 


470  MANUAL   OF   PHYSIOLOGY. 

MUSCLE  TONE. 

Although  the  tracing  drawn  by  a  lever  attached  to  a  muscle 
in  tetanus  is  straight,  and  does  not  show  any  variation  in  the 
tension  of  the  tetanized  muscle,  some  variations  in  tension  must 
occur,  since  a  low  humming  sound  is  produced  during  contrac- 
tion. A  muscle  tone,  like  the  purring  of  a  cat,  can  be  heard  by 
applying  the  ear  firmly  over  any  large  muscle  (biceps)  while 
in  tetanus,  by  throwing  the  muscles  attached  to  the  orbit  and 
Eustachian  tube  into  powerful  action,  or  by  spasm  of  the  muscles 
in  mastication. 

The  number  of  vibrations  which  has  been  estimated  to  occur 
in  the  voluntary  contraction  of  human  skeletal  muscles  does  not 
produce  an  audible  note;  hence  it  has  been  supposed  that  the 
note  we  hear  has  been  an  overtone.  When  a  muscle  is  thrown 
into  tetanus  by  a  current  interrupted  by  a  tuning  fork,  a  tone  is 
produced  which  corresponds  with  that  of  the  fork  causing  the 
interruption  in  the  current  by  definite  vibrations,  which  regulate 
the  number  of  stimulations  the  muscle  receives.  If,  on  the 
other  hand,  a  contraction  of  the  muscle  be  brought  about  by 
stimulating  the  spinal  cord,  with  the  same  rate  of  breaking  the 
current,  the  normal  muscle  tone  is  produced,  as  if  the  contrac- 
tion were  the  result  of  a  nerve  impulse  coming  from  the  brain. 

There  is  no  satisfactory  proof,  however,  that  the  variation  in 
tension  of  the  continuous  contraction  of  voluntary  muscle  is 
strictly  rhythmical.  The  sensation  of  a  sound  like  the  muscle 
tone  is  produced  by  any  nearly  periodic  vibrations  of  less  rate 
than  25  per  second.  The  pitch  of  the  muscle  tone  varies  with 
the  tension  of  the  membrana  tympani.  Hence,  it  has  been  sug- 
gested that  it  corresponds  with  the  resonant  tone  proper  to  the 
membrane  of  the  drum  ;  which  may  be  evoked  by  any  trembling 
movements  of  the  muscle  fibres  due  to  slight  variations  in  the 
force  or  distribution  of  the  impulses  transmitted  by  the  motor 
nerves. 

IRRITABILITY  AND  FATIGUE. 

The  activity  of  the  muscle  tissue  of  mammalian  animals  is 
closely  dependent  upon  a  good  supply  of  nutrition,  and  if  its 
blood  current  be  completely  cut  off'  by  any  means  for  a  length 


FATIGUE.  ' 

of  time,  it  loses  its  power  of  contracting.  While  the  m 
remains  in  the  body,  and  is  kept  warm  and  moist  by  the  juices 
in  the  tissues,  it  will  live  a  very  considerable  time  without  any 
blood  flowing  through  it,  and  it  at  once  regains  its  contractility 
when  the  blood  stream  is  again  allowed  to  flow  through  its  ves- 
sels. This  is  seen  when  the  circulation  of  a  limb  is  brought  to  a 
standstill  by  means  of  a  tourniquet  or  a  tightly  applied  band- 
age. A  mammalian  muscle  soon  ceases  to  be  irritable  and  dies 
when  removed  from  the  body,  but  its  functional  activity  may  be 
renewed  by  passing  an  artificial  stream  of  arterial  blood  through 
its  vessels,  and  an  isolated  muscle  may  thus  be  made  to  contract 
repeatedly  for  a  considerable  time. 

On  the  other  hand,  the  muscle  of  a  cold-blooded  animal  will 
remain  alive  for  a  long  time — many  hours — if  kept  cool  and 
moist.  When  its  functional  activity  is  about  to  fade,  it  may  be 
revived  by  means  of  an  artificial  stream  of  blood  caused  to  flow 
through  its  vessels,  just  as  in  the  case  of  the  mammalian 
muscle. 

Common  experience  teaches  us  that  even  when  well  supplied 
with  blood  our  muscles  become  fatigued  after  very  prolonged 
exertion,  and  are  incapable  of  further  action.  This  occurs  all 
the  more  rapidly  when  anything  interferes  with  the  flow  of  blood 
through  them,  such  as  using  our  arms  in  an  elevated  position  : 
the  simple  operation  of  driving  in  a  screw  overhead  is  soon  fol- 
lowed by  pain  and  fatigue  in  the  muscles  of  the  forearm,  though 
the  same  amount  of  force  could  be  exerted  when  the  arms  are  in 
a  lower  posture,  without  the  least  feeling  of  fatigue. 

The  difficulties  of  experimenting  with  the  muscles  of  mammals 
make  the  frog  muscle  the  common  material  for  investigation, 
and  from  it  we  learn  the  following  facts : — 

1.  When  removed  from  the  body  and  deprived  of  its  blood 
supply,  the  muscle  of  a  cold-blooded  animal  slowly  dies  from 
want  of  nutrition.  If  it  be  placed  under  favorable  circumstances, 
and  allowed  perfect  rest,  it  may  live  twenty-four  hours.     If  it  be 
frequently  excited  to  action,  on  the  other  hand,  it  rapidly  loses 
its  irritability,  being  worn  out  by  fatigue. 

2.  From  a  muscle  removed  from  a  recently-killed  animal,  we 


472  MANUAL   OF   PHYSIOLOGY. 

learn,  that  even  without  a  supply  of  blood  the  muscle  tissue  is 
capable  of  recovering  from  very  well-marked  fatigue,  if  it  be 
allowed  to  rest  for  a  little  time,  so  that  the  muscle  has  in  itself 
the  material  requisite  for  the  recuperation. 

The  first  question  then  is,  What  causes  the  loss  of  irritability 
which  we  call  fatigue?  And  the  second  is,  By  what  means  is  the 
muscle  enabled  to  return  to  a  state  of  functional  activity?  We 
know  that  the  mere  life  of  a  tissue  must  be  accompanied  by  cer- 
tain chemical  changes  which  require  (a)  a  supply  of  fresh  mate- 
rial, and  (6)  the  removal  of  certain  substances  which  are  the 
outcome  of  the  tissue  change. 

In  the  case  of  muscle,  this  chemical  interchange  is  constantly 
but  slowly  going  on  between  the  contractile  substance  and  the 
blood.  When  the  muscle  contracts,  much  more  active  and  prob- 
ably different  changes  go  on  in  the  contractile  substance,  more 
new  material  being  required,  and  more  effete  matter  being  pro- 
duced. It  is  probable  that  the  accumulation  of  these  effete  mat- 
ters is  the  more  important  cause  of  the  loss  of  irritability  in  a 
muscle,  for  a  frog's  muscle,  when  quite  fatigued,  may  be  rendered 
active  again  by  washing  out  its  blood  vessels  with  a  stream  of 
salt  solution  of  the  same  density  as  the  serum  (.6  per  cent.  NaCl), 
and  thus  removing  the  injurious  "  fatigue  stuffs,"  as  they  have 
been  called.  It  is  found  that  a  very  minute  quantity  of  lactic 
acid  injected  into  the  vessels  of  a  muscle  destroys  its  irritability, 
and  brings  it  to  a  state  resembling  intense  fatigue.  Of  the  new 
materials  required  for  the  sustentation  of  muscle  irritability, 
oxygen  is  among  the  most  important,  though  its  supply  is  not 
absolutely  necessary  for  the  recuperation  of  a  partially  exhausted, 
isolated  frog's  muscle. 

The  slow  recovery  of  a  bloodless  muscle  from  fatigue  may  be 
explained  by  supposing  time  to  be  necessary  for  the  reconstruc- 
tion of  new  contractile  material,  and  probably,  also,  for  a  second- 
ary change  to  take  place  in  the  effete  materials,  by  which  they 
become  less  injurious. 

When  working  actively  the  muscles  require  an  adequate  sup- 
ply of  good  arterial  blood  in  order  to  ward  off  exhaustion  ;  and, 
as  already  explained  in 'speaking  of  the  vasomotor  influences,  a 


RIGOR   MORTIS.  473 

muscle  receives  a  greater  supply  of  blood  when  actively  con- 
tracting than  when  in  the  passive  state. 

The  irritability  of  a  muscle  and  the  rate  at  which  it  becomes 
exhausted  may  be  said  to  depend  upon  : — 

1.  The  adequacy  of  its  blood  supply  :  the  better  the  supply  of 
new  material  and  the  more  quickly  the  injurious  effete  materials 
are  removed,  the  more  work  a  muscle  can  do  without  becoming 
exhausted. 

•  2.  Temperature  has  a  marked  effect  on  the  irritability  of 
muscles,  as  well  as  upon  the  form  of  this  contraction.  Low  tem- 
peratures— approaching  5°  C. — diminish  the  irritability  of  a 
muscle,  but  do  not  seem  to  tend  toward  more  rapid  exhaustion. 
High  temperatures — approaching  30°  C. — increase  the  irritabil- 
ity, and  at  the  same  time  rapidly  bring  about  fatigue.  At  about 
35°  C.  an  isolated  frog's  muscle  begins  to  pass  into  heat  tetanus, 
and  permanently  loses  its  irritability. 

3.  Functional  activity  is  accompanied  by  an  increased  blood 
supply,  and  a  more  perfect  nutrition  of  the  muscles,  hence  activ- 
ity is  advantageous  for  their  growth  and  power ;  while,  on  the 
other  hand,  continued  and  prolonged  inactivity  causes  a  lower- 
ing of  the  nutrition  and  loss  of  irritability.  Thus,  when  the 
nerves  supplying  the  voluntary  muscles  are  injured,  there  is  con- 
siderable danger  of  atrophy  and  tissue  degeneration  of  the 
muscles ;  the  contractile  substance  becomes  replaced  by  fat 
granules.  This  degeneration  also  occurs  in  the  stump  when  a 
limb  is  amputated,  the  distal  attachments  of  the  muscles  having 
been  cut  they  cannot  act,  and  after  some  time  they  become  com- 
pletely atrophied,  so  that  muscle  tissue  can  hardly  be  recognized 
in  them. 

DEATH  RIGOR. 

The  death  of  muscle  tissue  is  associated  with  a  set  of  changes 
which,  in  some  respects,  resemble  those  observed  in  its  active 
state.  The  most  obvious  phenomenon  is  an  unyielding  contrac- 
tion, which  causes  the  stiffening  of  the  body  after  death.  Hence, 
it  is  called  rigor  mortis.  The  muscles  harden  ;  lose  their  elastic- 
ity, and  the  tissue  is  torn  if  forcibly  stretched.  When  isolated, 
the  muscle  is  seen  to  be  opaque,  and  its  reaction  is  found  to  be 
40 


474  MANUAL   OF   PHYSIOLOGY. 

distinctly  acid.  A  considerable  quantity  of  heat  is  developed 
during  the  progress  of  the  rigor.  The  electric  currents  alter  in 
direction  and  finally  disappear. 

The  period  at  which  rigor  comes  on  and  its  duration  depend 
on  (a)  the  state  of  the  muscles,  and  (6)  the  circumstances  under 
which  they  are  placed  at  the  time  of  death.  All  influences 
which  tend  to  cause  death  of  the  tissue  induce  early  rigor  of 
short  duration,  viz.,  (1)  Prolonged  activity— as  may  be  shown 
in  a  muscle  artificially  tetanized,  or  seen  in  an  animal  whose* 
death  was  preceded  by  intense  muscular  exertion — causes  rigor 
to  appear  almost  immediately,  and  to  terminate  rapidly.  (2) 
High  temperature  facilitates  the  production  of  rigor  in  dying 
muscles  ;  indeed,  a  temperature  not  much  exceeding  that  normal 
to  the  tissue  induces  rigor.  This  form  of  contraction,  which  is 
called  heat  rigor,  is  brought  about  in  mammalian  muscles  by  a 
temperature  of  about  £0°  C.,  and  in  frogs'  muscles  below  40°  C. 
If,  however,  the  temperature  of  a  muscle  be  suddenly  raised  to 
the  boiling  point,  it  is  killed,  and  the  chief  phenomena  of  rigor 
are  prevented  from  occurring.  (3)  Freezing  postpones  the 
changes  in  the  muscles  upon  which  rigor  depends.  (4)  Stretch- 
ing, or  any  mechanical  excitation  which  tends  to  injure  the  tissue, 
causes  it  to  pass  more  rapidly  into  rigor.  (5)  The  application 
of  water  and  of  a  number  of  chernica.  ubstances  cause  muscles 
quickly  to  pass  into  a  state  of  rigor  similar  to  that  which  ordi- 
narily follows  the  death  of  the  tissue.  (6)  Any  stoppage  in  the 
blood  current  normally  flowing  through  a  muscle,  after  some 
time  makes  it  pass  into  a  state  of  rigidity  like  rigor  mortis,  but 
this  may  be  removed  by  allowing  the  blood  to  flow  freely  again 
through  the  muscle. 

It  is  generally  admitted  that  rigor  mortis  depends  on  the  ten- 
dency of  the  muscle  plasma  to  coagulate  and  give  rise  to  myosin 
and  muscle  serum.  This  is,  in  most  respects,  comparable  with  the 
coagulation  of  the  blood,  and  may  also  depend  upon  the  action 
of  some  ferment,  of  which  there  is  no  lack  in  dead  muscle  tissue. 
Compare  the  paragraph  on  chemistry,  pp.  445,  446. 

Most  of  the  phenomena  of  the  process  of  muscle  rigor  remind 
us  of  the  changes  already  described  as  occurring  in  muscle,  when 


UNSTRIATED   MUSCLE.  475 

it  passes  from  the  passive  to  the  active  state.  Thus,  the  shorten- 
ing of  the  fibres,  the  evolution  of  heat,  and  the  chemical  changes 
may  be  said  to  be  identical  in  contraction  and  rigor  mortis. 
The  electrical  changes  are,  however,  very  transitory,  and  the 
rigor  is  accompanied  by  loss  of  elasticity  and  irritability. 
Opacity  of  the  tissue  marks  its  later  stages. 

Thus,  while  dying,  the  muscle  tissue  may  be  said  to  go  through 
a  series  of  events  analogous  to  those  which  would  occur  in  a 
prolonged  contraction  without  any  period  of  recuperation.  The 
idea  has  naturally  suggested  itself  to  the  minds  of  physiologists, 
that  the  active  state  of  muscle  depends  upon  chemical  changes 
which  are  the  initial  steps  in  the  coagulation  of  the  contractile 
substance,  when  the  muscle  is  dying.  The  muscle  tissue  is  sup- 
posed to  contain  a  special  proteid  of  extremely  intricate  and 
unstable  chemical  constitution,  which  is  constantly  undergoing 
slow  molecular  change,  and  which,  if  not  reintegrated  by  con- 
stant assimilation,  would  pass  into  coagulation.  Under  the  influ- 
ence of  stimuli  a  comparatively  sudden  and  intense  molecular 
disturbance  is  brought  about,  which  produces  shortening  of  the 
fibres,  and  the  same  chemical  changes  as  precede  the  coagulation. 
Before  the  stage  of  coagulation  appears  a  chemical  rearrange- 
ment takes  place,  the  result  of  which  is  the  reconstruction  of 
the  unstable  complex  proteid.  If  nutriment  be  withheld,  or  if 
the  stimulation  be  too  powerful,  the  recovery  cannot  take  place, 
and  we  find  the  muscle  passing  from  a  state  of  physiological 
contraction  to  one  of  intense  exhaustion,  and  then  to  coagula- 
tion and  death. 

UNSTRIATED  MUSCLE. 

So  far  reference  has  only  been  made  to  the  skeletal  muscles, 
the  fibres  of  which  are  marked  by  transverse  striations,  and 
whose  single  contraction  is  extremely  rapid  and  short.  The 
contractile  tissues  which  carry  on  the  movements  in  the  various 
organs  of  the  body  are  not  striated  fibres,  but,  as  has  been 
already  stated,  consist  of  elongated  flattened  cells  with  rod- 
shaped  nuclei.  They  occur  generally  in  the  form  of  sheets  or 
layers,  forming  coats  for  the  organs  in  which  they  lie.  Their 
single  contraction  is  slow  and  prolonged,  and  is  generally  trans- 


476  MANUAL   OF   PHYSIOLOGY. 

mitted  from  one  muscle  cell  to  another  as  a  kind  of  sluggish 
wave.  They  are  not  capable  of  passing  into  a  tetanic  state  of 
contraction,  like  striated  muscles. 

The  slowest  contraction  seems  to  be  that  of  the  muscle  cells 
in  the  walls  of  the  blood  vessels.  These  remain  in  a  state  of 
partial  contraction,  which  undergoes  a  brief  and  temporary 
rhythmical  relaxation.  The  most  forcible  aggregate  of  unstriated 
muscle  elements  is  met  with  in  the  uterus.  This  organ,  which 
has  very  exceptional  motorpowers  to  perform,  contracts  in  some- 
what the  same  way  as  the  muscles  of  the  blood  vessels,  but  more 
quickly,  and  with  longer  rhythmical  intervals  of  partial  relaxa- 
tion. The  muscular  wall  of  the  intestine,  and  the  iris,  are  among 
the  most  rapidly  contracting  smooth  muscles. 

The  chemical  properties  of  the  smooth  muscle  are  somewhat 
similar  to  those  of  striated  skeletal  muscles,  and  they  pass  into 
a  state  of  rigor,  while  dying,  which  seems  to  depend  on  the  same 
causes  as  the  rigor  mortis  already  described. 


APPLICATION   OF   SKELETAL   MUSCLES.  477 


CHAPTER  XXVI. 
THE  APPLICATION  OF  SKELETAL  MUSCLES. 

The  consideration  of  the  many  varieties  of  muscles,  and  the 
various  modes  in  which  they  are  attached  to  the  bones  that  they 
are  destined  to  move,  belongs  to  the  department  of  practical 
anatomy,  and  needs  no  mention  here.  As  a  general  but  by  no 
means  universal  rule,  a  muscle  has  one  attachment  which  is  fixed, 
commonly  spoken  of  as  its  origin,  and  a  second,  called  its  inser- 
tion, upon  which  it  acts  by  approximating  it  to  the  origin.  Mus- 
cles usually  pass  in  a  straight  line  between  their  two  attachments, 
but  sometimes  they  act  round  an  angle  by  sliding  over  a  pulley, 
or  by  means  of  a  small  bone  in  the  tendon,  like  the  patella. 

The  muscles  are  so  attached  that  they  are  always  slightly  on 
the  stretch,  and  thus,  at  the  moment  they  begin  to  contract,  they 
are  in  an  advantageous  position  to  bring  their  action  to  bear  on 
the  bones  which  they  move.  When  the  contraction  ceases,  the 
bones  are  drawn  back  to  their  former  position  without  any  sud- 
den jerk  or  jar. 

The  muscles  act  upon  the  bones  as  levers,  by  working  upon 
the  short  arm  of  the  lever,  so  that  more  direct  force  is  required 
on  the  part  of  a  muscle  than  the  weight  of  the  body  moved ;  but 
from  this  arrangement  considerable  advantages  are  gained,  viz., 
that  a  small  contraction  of  the  muscle  causes  an  extensive  excur- 
sion of  the  part  moved,  and  much  greater  rapidity  of  motion  is 
attained. 

All  the  three  orders  of  levers  are  met  with  in  the  movements 
of  the  different  bones  of  the  skeleton  ;  often,  indeed,  all  three 
varieties  are  found  in  the  same  joint,  as  the  elbow,  where  the 
simple  extension  and  flexion,  motions  of  the  biceps  and  triceps 
muscles  give  us  good  examples  (Fig.  194). 

The  first  order  of  lever  is  used  when  the  triceps  is  the  power 
and  draws  upon  the  olecrauon,  thus  moving  the  hand  and  fore- 
arm around  the  trochlea,  which  acts  as  the  fulcrum.  This  is 


478 


MANUAL   OF   PHYSIOLOGY. 


FIG.  194. 


shown  in  the  upper  diagram,  in  which  the  hand  is  striking  a  blow 

with  a  dagger. 

The  second  order  comes  into  play  when  the  hand,  resting  on  a 

point  of  support,  acts  as  the  fulcrum,  and  the  triceps  pulling  on 

the  olecranon  is  the  power  which  raises  the  humerus,  upon  which 

is  fixed  the  body  or  weight  (middle  diagram). 

The  third  order  may  be  exemplified  by  the  action  of  the  biceps 

in  ordinary  flexion  of  the  elbow.  Here  the  muscle,  which  is  the 
power,  is  placed  between  the  fulcrum — 
represented  by  the  lower  end  of  the 
humerus — and  the  weight  which  is  car- 
ried by  the  hand  (lower  diagram). 

The  various  groups  of  muscles  which 
are  so  arranged  as  to  assist  each  other 
when  acting  together,  are  called  syner- 
getic,  and  those  which,  when  contracting 
at  the  same  time,  oppose  each  other,  are 
called  antagonistic.  The  same  muscles 
may,  in  different  positions  of  a  joint  or 
in  combination  with  other  muscles,  have 
totally  different  actions,  at  one  time 
being  synergetic  and  at  another  antago- 
nistic. Thus,  the  sterno-mastoid  mus- 
cle may,  in  different  positions  of  the 
head,  either  bend  the  cranium  backward 
or  forward,  and  so  cooperate  with  two 
sets  of  muscles  which  are  definitely  an- 
tagonistic to  one  another. 


w 


Diagrams  showing  the  mode  of 
action  of  the  three  orders  of 
levers  (numbered  from  above 


JOINTS. 

The  unions  between  the  bones  of  the 
skeleton  are  very  varied  in  function  and 
character.  -They  may  be  classed  as : — 

1.  SUTURES,  in  which  the  bones  are  firmly  united  by  rugged 
surfaces  without  the  interposition  of  any  cartilage.  They  are 
practically  only  the  lines  of  union  of  different  bones,  which  grow 
together  to  form  a  single  bone. 


JOINTS.  479 

2.  SYMPHYSES,  in  which  two   bony  substances  are  strongly 
cemented  together  by  ligaments,  and  a  more  or  less  thick  adher- 
ent layer  of  fibro-cartilage,  are  joints  allowing  of  some  move  - 
ment,  which  is,  however,  very  limited. 

3.  ARTHROSES,  or  true  movable  joints,  such  as  are  commonly 
met  with  in  the  extremities.     They  are  characterized  by  a  syno- 
via! sac  lining  the  surrounding  ligaments,  and  two  smooth  sur- 
faces of  cartilage  which  cover  over  the  bony  extremities  taking 
part  in  the  articulation,  and  form  what  are  called  the  articular 
surfaces.     The  synovial  sac  is  strengthened  by  a  loose  membra- 
nous covering — the  capsular  ligament — which  is  attached  round 
the  edge  of  the  cartilages  next  to  the  periosteum,  which  here 
ceases. 

The  articular  surfaces  are  always  in  exact  and  close  contact, 
being  pressed  together  by  the  following  influences :  (1)  The 
elastic  tension  and  tonic  contraction  of  the  surrounding  muscles, 
which  exert  considerable  traction  on  them.  (2)  The  traction 
of  the  surrounding  ligaments,  which  in  some  cases  holds  the 
bones  firmly  together,  no  matter  what  their  relative  positions 
may  be.  This  can  be  well,  seen  in  the  knee  joint,  in  which  a 
comparatively  small  number  of  the  ligaments  suffice  to  keep  the 
articular  surfaces  in  contact.  (3)  The  atmospheric  pressure 
also  tends  to  hold  the  bones  in  close  apposition,  as  may  be  seen 
in  the  hip  joint,  which  is  not  easily  disarticulated,  even  when  all 
the  surrounding  structures  and  the  ligaments  have  been  severed. 

The  synovial  joints  may  be  classified  according  to  the  form  of 
their  surfaces,  or  their  mode  of  motion  as  follows  : — 

1.  Flat  articular  surfaces  held  together  by  a  short  rigid  cap- 
sule, allowing  of  but  very  slight  gliding  movement ;  examples  of 
this  form  of  joint  are  to  be  found  in  the  tarsus  and  the  articular 
processes  of  the  vertebra. 

2.  Hinge  joints,  in  which  the  surfaces  are  so  adapted  that  only 
one  kind  of  motion  can  take  place.     A  groove-like  cavity  in 
one  bone  fits  closely  and  glides  around  the  axis  of  a  roller  on  the 
other  bone,  while  the  sides  of  the  joint  are  kept  tightly  together 
by  means  of  strong  lateral  ligaments.     Examples  of  this  form  of 
joint  are  to  be  found  between  the  phalanges  of  the  digits  and  at 
the  humero-ulnar  joint. 


480 


MANUAL   OF   PHYSIOLOGY. 


3.  The  rotary  hinge,  or  pivot  joint,  in  which  a  part  moves 
round  the  axis  of  the  bone,  instead  of  the  axis  of  rotation  being 
at  right  angles  to  both  bones,  forming  the  joint  as  in  an  ordinary 
hinge.     Such  joints  are  seen  at  the  head  of  the  radius  and  at 
the  articulation  between  the  atlas  and  the  adontoid  process  of 
the  axis. 

4.  A  saddle-shaped  joint  is  a  kind  of  double  hinge,  in  which 
each  of  the  articulating  bones  forms  a  partial  socket  and  roller, 
and  hence  there  are  two  axes  of  rotation,  placed  more  or  less  at 
right  angles  one  to  the  other.     A  good  example  of  this  kind  of 
joint  occurs  between  the  thumb  and  one  of  the  wrist  bones.        .^- 

5.  Spiral  articulations  are  modifications  of  the  hinge,  in  which 
the  surface  of  the  roller  does  not  run  "  true,"  but  becomes  eccen- 
tric, so  that  the  surface  of  the  roller  forms,  really,  part  of  a 
spiral,  by  means  of  which  the  bone  articulating  with  it  is  forced 
away  from  the  central  axis  of  rotation  and  becomes  jammed,  as 
if  stopped  by  a  wedge.     The  best  example  of  this  is  the  knee. 
In  this  joint  the  axis  of  rotation  (c)  is  near  the  posterior  sur- 
faces of  the  bones,  and  passes  transversely  through  the  condyles 
of  the  femur,  the  surfaces  of  which  form  an  arc,  the  centre  cor- 
responding to  the  axis  of  motion.     In  ordi- 
nary flexion  the  head  of  the  tibia  (p)  moves 
on  the  arc  around  the  axes  so  as  to  partially 
relax  the  lateral  ligament  and  allow  of  some 
rotation  on  the  axis  of  the  tibia.     When  the 
head  of  the  tibia  moves  forward,  in  exten- 
sion (E),  it  becomes  wedged  against  the  ante- 
rior  part   of  the   articular   surface  of  the 
femur  (w),   which    presents   an    eccentric, 
spiral-like  curve,  departing  more  and  more 
from  the  centre  of  rotation  as  the  articular 
surface  of  the  tibia  proceeds  forward.     The 
effect  of  this  is,  that  in  extension  of  the  leg 
the  ligaments  are  made  tense,  and  the  bones 
are  firmly  locked  together.     Owing  to  the 

inequality  between  the  size  of  the  internal  and  external  con- 
dyles, the  axis  of  rotation  is  not  at  right  angles  to  the  axis  of  the 


FIG.  195. 


Diagram  of  the  action  of 

the  knee  joint. 
w  —  articular  surface  of 

femur. 
E  =  tibia  in  position  of 

extension. 
F  =  tibia  in  position  of 

flexion, 
c  =  centre  of  rotation. 


STANDING.  481 

femur,  but  is  at  such  an  angle  that  extreme  extension  causes  a 
slight  amount  of  outward  motion  of  the  leg. 

6.  In  the  ball  and  socket  joints — the  name  of  which  implies 
their  mechanism — the  most  varied  movements  occur.  (Hip  and 
shoulder.) 

STANDING. 

In  order  that  an  elongated  rigid  body  may  stand  upright,  it 
is  only  necessary  that  a  line  drawn  vertically  through  its  centre 
of  gravity  should  pass  within  its  basis  of  support,  and,  if  the 
latter  be  sufficiently  wide,  the  object  will  remain  permanently  in 
that  position.  The  human  body,  in  the  first  place,  is  not  rigid, 
and  in  the  second  place  the  basis  of  support  is  too  small  to 
insure  a  satisfactory  degree  of  steadiness.  The  act  of  standing 
must,  therefore,  be  accomplished  by  the  action  of  certain  mus- 
cles, which  are  employed  in  preventing  the  different  joints  from 
bending,  and  in  so  balancing  the  various  parts  of  the  body  as  to 
keep  the  whole  frame  from  toppling  over. 

In  order  to  economize  muscular  energy  while  standing,  we 
may  lock  the  more  important  joints,  and  thus  depend  rather  on 
the  passive  ligaments  than  upon  muscular  action  for  the  rigidity 
of  the  body.  With  this  object  we  are  taught  to  place  the  heels 
together,  turn  out  the  toes,  bring  the  legs  parallel  by  approxi- 
mating them,  and,  extending  the  knees  to  the  utmost,  to  straighten 
and  to  throw  back  the  trunk  so  as  to  render  tense  the  anterior 
hip  ligaments,  to  direct  the  face  straight  forward  so  as  to  bal- 
ance the  head  evenly,  and  to  let  the  arms  fall  by  the  sides. 

In  this  position,  as  a  soldier  stands  at  attention,  the  knee  and 
hip  joints  remain  fixed,  without  any  effort  on  the  part  of  the 
muscles,  but  it  is  far  from  being  the  most  comfortable  attitude 
one  can  assume  for  prolonged  standing,  and  hence  the  position 
known  best  by  the  order  "  stand  at  ease  "  is  adopted  if  more  com- 
plete rest  is  desired.  In  this  position  the  weight  of  the  body  is 
usually  allowed  to  rest  on  one  leg,  while  the  other  lightly  touches 
the  ground  to  form  a  kind  of  stay,  and  relieve  the  muscles  which 
surround  the  supporting  ankle  from  too  great  an  effort  of  bal- 
ancing. At  the  same  time  the  knee  is  extended,  and  the  pelvis 
becomes  somewhat  oblique,  so  as  to  bring  it  more  directly  over  the 
41 


482  MANUAL   OF   PHYSIOLOGY. 

head  of  the  femur.  In  ordinary  easy  standing,  the  joints  are 
not  usually  kept  locked  by  the  tension  of  the  liganientous  struct- 
ures, but  their  position  is  constantly  being  very  slightly  altered, 
so  as  to  vary  the  muscles  employed  in  preserving  the  balance  and 
thus  prevent  fatigue. 

The  joints  most  exercised  in  the  erect  posture  are  the  follow- 
ing :— 

1.  The  ankle  has  to  support  the  weight  of  the  entire  body,  while 
the  joint  is  neither  flexed  nor  extended  to  its  utmost,  and  cannot 
be  fixed  in  this  position  by  ligarnentous  arrangements.     The  foot 
being  placed  on  the  ground,  resting  on  the  heel  and  the  balls 
of  the  great  and  little  toes,  is  supported  in  an  arch-like  form  by 
strong  though  elastic  ligaments,  which  allow  but  little  motion  in 
the  numerous  joints.     The  bones  of  the  leg   can  move  in  the 
freest  way,  backward  or  forward,  over  the  articular  surface  of  the 
astragalus,  which  forms  the  roller  of  the  hinge,  lateral  motion 
being  prevented  by  the  malleoli.     The  line  passing  through  the 
centre  of  gravity  of  the  body  generally  falls  slightly  in  front 
of  the  axis  of  rotation  of  the  ankle  joint,  so  that  the  entire  body 
tends  to  fall  forward  at  the  ankles.   This  tendency  is  checked  by 
the  powerful  calf  muscles,  which,  attached  to  the  calcaneum  by 
means  of  the  strong  tendo-Achillis,  keep  the  parts  in  such  a  posi- 
tion that  an  exact  balance  is  almost  constantly  kept  up. 

2.  The  knee  joint,  when  completely  extended,  requires  no  mus- 
cular action  to  prevent  it  from  bending,  because  the  line  of 
gravity  then  passes  in  front  of  the  axis  of  rotation,  and  the 
weight  of  the  body  tends  to  bend  the  knee  backward.     This  is 
impossible,  on  account  of  the  strong  ligaments  which  exert  their 
traction  behind  the  axis  of  rotation.     As  a  rule,  these  ligaments 
are  not  put  on  the  stretch  in  this  way,  but  the  joint  is  held,  by 
muscular  power,  in  such  a  position  that  the  line  of  gravity  passes 
just  through,  or  very  slightly  behind,  the  axis  of  rotation  of  the 
joint,  so  that,  if  anything,  there  is  a  slight  tendency  for  the  knee 
to  bend.     This  is  completely  checked,  and  the  body  balanced,  by 
the  powerful  extensor  muscles  of  the  thigh. 

3.  In  the  hip  joints,  which  have  to  support  the  trunk  and  head, 
the  line  of  gravity  falls  just  behind  the  line  uniting  the  joints 


STANDING.  488 

when  the  person  is  perfectly  erect,  so  that  here  the  body  has  a 
tendency  to  fall  backward.  This  is  prevented  by  the  strong  ilio- 
femoral  ligament.  When,  however,  the  knee  is  not  straightened 
to  the  full  extent,  so  that  the  line  of  gravity  passes  through  or  a 
little  behind  the  axis  of  rotation  of  that  joint,  then  the  pelvis  is 
very  slightly  flexed  on  the  femora,  so  that  the  axis  of  the  joints 
lies  exactly  in  or  a  little  behind  the  line  of  gravity,  and  thus  the 
body  inclines  rather  to  fall  forward.  This  tendency  is  prevented 
by  the  powerful  glutei  muscles,  which  also  enable  us  to  regain 
the  erect  posture  after  bending  the  trunk  forward. 

The  motions  of  which  the  pelvis  and  vertebral  column  are 
capable  are  too  slight  to  deserve  attention  here.  The  vertebral 
column,  wedged  in  as  it  is  between  the  two  innominate  bones, 
may  be  taken,  together  with  the  pelvis,  as  forming  a  very  yield- 
ing and  elastic,  but  practically  jointless  pillar,  the  upper  part  of 
which  can  alone  be  bent  to  such  an  extent  as  to  require  mention 
in  discussing  the  mechanism  of  station. 

The  individual  joints  between  the  cervical  vertebra  permit  but 
a  slight  amount  of  movement  when  taken  separately,  but  by 
their  aggregate  motion  they  enable  considerable  extension  and 
flexion  of  the  neck  to  take  place.  These  motions  follow  so 
closely,  and  are  so  inseparably  associated  with  those  of  the  head 
on  the  upper  vertebra,  that  there  is  no  need  to  consider  them 
separately  from  the  latter. 

The  atlanto-occipital  joints  admit  of  some  little  lateral  movement, 
but  that  in  the  antero-posterior  direction  is  much  the  more  impor- 
tant, but  even  this  would  be  insignificant  were  it  not  associated 
with  the  movements  between  the  other  cervical  vertebrae. 

The  cranium  has  then  to  be  balanced  on  the  top  of  a  flexible 
column,  and  rests  immediately  in  a  kind  of  socket,  which  can 
move  as  a  double  hinge  around  two  axes  at  right  angles  one  to 
the  other.  The  vertical  line  from  the  centre  of  gravity  of  the 
cranium  must  vary  with  every  forward,  backward,  or  lateral 
movement  of  the  head  or  neck,  but  in  the  erect  posture  it  passes 
a  little  in  front  of  the  axis  of  rotation  of  the  atlanto-occipital 
joint,  and  somewhat  behind  the  curve  of  the  cervical  vertebrae,  so 
that  the  head  may  be  said  to  be  poised  on  the  apex  of  the  verte- 


484  MANUAL   OF    PHYSIOLOGY. 

bral  column,  with  some  tendency  to  fall  forward.  There  are  no 
ligamentous  structures  which  can  lock  the  joints  so  as  to  keep 
the  head  in  the  erect  position  ;  therefore,  without  the  aid  of  mus- 
cular force,  the  head  will  fall  forward  or  backward,  according 
to  the  position  it  may  be  in  when  the  muscles  suddenly  relax,  as 
happens  in  falling  asleep  in  an  upright  posture. 

From  the  foregoing  facts  it  will  be  seen  that  there  exists  a 
kind  of  coordinated  antagonism  at  work  in  ordinary  easy  stand- 
ing which  keeps  the  elastic,  pliable  body  upright,  without  the 
rigidity  adopted  when  standing  "  at  attention."  The  muscular 
action  is  more  exercised  when  we  are  not  on  steady  ground,  and 
varied  coordination  becomes  necessary ;  for  instance,  when  we  go 
on  board  ship  for  the  first  time.  Standing  then  takes  some  little 
time  to  become  easy,  and  requires  new  associations  of  move- 
ment. The  gastrocnemius  and  soleus  relax  the  ankle  in  a  degree 
just  proportionate  to  the  amount  of  flexion  of  the  knee  permitted 
by  the  quadriceps  extensor  cruris,  while,  simultaneously,  the  great 
gluteal  muscle  allows  the  body  to  incline  forward  so  as  to  keep 
its  centre  of  gravity  in  the  proper  relation  to  the  basis  of  sup- 
port. 

WALKING  AND  RUNNING. 

Walking  is  accomplished  by  poising  the  weight  on  one  foot 
and  then  tilting  the  body  forward  with  the  other,  which  is  then 
swung  in  front  and  placed  on  the  ground  to  prevent  falling. 
These  acts  are  performed  alternately  by  each  leg,  so  that  the 
"  swinging  limb  ' '  becomes  the  "  supporting  limb  "  of  the  next  step. 
The  swinging  leg  is  described  as  having  two  phases,  (1)  active, 
while  pushing  off  from  the  ground,  and  (2)  passive,  while  swing- 
ing forward  like  a  pendulum.  In  starting,  one  foot  is  placed 
behind  the  other,  so  that  the  line  of  gravity  lies  between  the 
two,  the  hindmost  limb  having  the  ankle  and  knee  a  little  bent. 
By  suddenly  straightening  these  joints  it  gives  a  "  push  off"  with 
the  toes  and  propels  the  body  forward,  so  as  to  move  it  around 
the  axis  of  motion  of  the  fixed,  or  supporting  ankle  joint.  At 
the  end  of  the  swing,  the  pendulous  leg  comes  to  the  ground,  and 
leaves  the  other  limb  in  the  attitude  ready  for  the  push  off. 
Thus,  on  level  ground  walking  is  carried  on  with  but  small  mus- 


WALKING   AND    RUNNING*  485 

cular  exercise ;  but  in  ascending  an  incline  or  going  upstairs, 
the  supporting  limb  has  to  elevate  the  body  at  each  step  by 
extending  the  knee  and  ankle  joints  by  the  thigh  extensors  and 
the  calf  muscles. 

Running  is  distinguished  from  walking  by  the  fact  that,  while 
in  the  latter  both  feet  rest  on  the  ground  for  the  greater  part  of 
each  pace,  in  the  former  the  time  that  either  foot  rests  on  the 
ground  is  reduced  to  a  minimum,  and  the  body  can  never  be 
said  to  be  balanced  on  either  leg,  so  that,  in  fact,  there  is  no  longer 
a  "support  limb."  The  legs  are  kept  in  a  semitiexed  position, 
ready  for  the  push  off  or  spring,  which  is  so  forcibly  carried  out 
that  the  body  is  propelled  through  the  air  without  any  support 
between  each  step,  and  has  a  constant  tendency  to  fall  forward. 
Thus,  an  interval  of  greater  or  less  duration,  according  to  the 
pace,  exists  during  which  both  the  feet  are  off  the  ground, 
because,  the  moment  either  foot  comes  to  the  ground,  it  at  once 
executes  a  new  spring  without  waiting  for  the  swing  of  the  other. 


486  MANUAL   OF   PHYSIOLOGY. 


CHAPTER  XXVII. 

VOICE  AND  SPEECH. 

The  human  voice  is  produced  by  an  expiratory  blast  of  air 
being  forced  through  the  narrow  opening  at  the  top  of  the  wind- 
pipe, called  the  glottis.  This  glottis,  which  lies  in  the  lower  part 
of  the  larynx,  is  bounded  on  each  side  by  the  edges  of  thin,  elas- 
tic, membranous  folds  that  project  into  the  air  passages.  These 
membranous  folds,  called  the  vocal  cords,  are  set  vibrating  by 
the  current  of  air  from  below,  and  in  turn  communicate  their 
vibrations  to  the  air  in  the  air  passages  situated  above  them. 

ANATOMICAL   SKETCH. 

The  vocal  apparatus  produces  sound  in  the  same  manner  as  a 
musical  instrument  of  the  reed-pipe  variety.  If  we  compare  it 
with  the  pipe  of  an  organ,  we  find  all  the  parts  of  the  latter 
represented.  The  lungs  within  the  moving  thorax  act  as  the 
bellows.  The  bronchi  and  trachea  are  the  supply  pipes  and  air 
box.  The  vocal  cords  are  the  vibrating  tongues ;  while  the  larynx, 
pharynx,  mouth  and  nose  act  as  the  accessory  or  resonating 
pipes.  The  blast  of  air  is  produced  and  regulated  by  the  respi- 
ratory muscles;  and  special  intrinsic  muscles  of  the  larynx 
change  the  conditions  of  the  vocal  cords  so  as  to  alter  the  pitch 
of  the  notes  produced.  Other  sets  of  muscles,  by  altering  the 
conditions  of  the  resonating  pipes,  give  rise  to  many  modifications 
in  the  vocal  tones,  and  thus  produce  what  is  called  speech. 

The  larynx,  which  may  be  regarded  as  the  special  organ  of 
voice,  is  made  up  of  four  cartilages,  viz.,  the  cricoid,  thyroid  and 
two  arytenoids,  jointed  together  so  as  to  allow  of  considerable 
motion.  Of  these  the  inferior,  the  crieoid,  is  attached  to  the 
trachea,  which  it  joins  to  the  others.  It  forms  a  ring,  which  is 
thin  in  front,  but  deep  and  thick  behind,  owing  to  a  peculiar 
projection  upward  of  its  posterior  part.  The  thyroid  consists  of 
two  side  wings  so  bent  as  to  form  the  greater  part  of  the  anterior 


ANATOMICAL   SKETCH. 


487 


Fro. 196. 


and  lateral  boundaries  of  the  voice  box,  and  can  be  felt  easily  in 
the  front  of  the  throat.  It  is  articulated  to  the  sides  of  the  cri- 
coid  by  its  two  inferior  and  pos- 
terior extremities,  so  that  the 
upper  part  of  the  cricoid  carti- 
lage can  move  backward  and 
forward.  The  arytenoid  carti- 
lages are  little  three-sided  pyra- 
midal masses  placed  on  the 
upper  surface  of  the  posterior 
part  of  the  cricoid,  to  which 
they  are  attached  by  a  loose 
joint.  They  are  so  placed  that 
one  surface  looks  inward,  the 
second  backward,  and  the  third 
forward  and  outward,  while  the 
inferior  surface  rides  on  the 
cricoid.  One  point  looks  for- 
ward, and  to  it  is  attached  the 
vocal  cord  on  each  side,  hence 
it  has  been  called  the  vocal 
process.  The  apex,  which  looks 
outward  and  backward,  gives 
attachment  to  some  of  the  in- 
trinsic muscles,  and  hence  has 
been  called  the  muscular  pro- 


cess. 


Anterior  half  of  a  transverse  vertical  section 
through  the  larynx  near  its  middle,  seen 
from  behind.  More  is  cut  away  on  the 
upper  part  of  the  right  side.  1.  Upper  di- 
vision of  the  laryngeal  cavity;  2.  Central 
portion ;  3.  Lower  portion  continued  into 
4,  trachea ;  e,  epiglottis ;  e',  its  cushion ; 
t,  thyroid  cartilage  seen  in  section,  vl, 
true  vocal  cord  at  the  rima  glottidis ;  s, 
ventricle  of  larynx?;  *•',  saccule.  (A.  Thom- 
son.) 


The  thyroid  cartilage  is  con- 
nected with  the  cricoid  below, 
and  with  the  hyoid  bone  above 

by  ligaments  and  tough  membranes,  which  hold  the  parts 
together,  fill  in  the  intervals,  and  complete  the  skeleton  of  the 
larynx. 

The  vocal  cords  are  composed  of  small  strands  of  elastic  tissue, 
which  are  stretched  between  the  anterior  processes  of  the  aryte- 
noid cartilages  and  the  inferior  part  of  the  thyroid,  where  they 
are  attached  side  by  side  to  the  posterior  surface  of  the  angle 


488 


MANUAL   OF    PHYSIOLOGY. 


formed  by  the  junction  of  the  two  lateral  parts  or  alse  of  the  thy- 
roid. The  raucous  membrane  which  lines  the  larynx  is  thin, 
and  closely  adherent  over  the  vocal  cords.  The 
surface  of  the  laryngeal  cavity  is  smooth  and 
even,  the  lining  membrane  passing  over  the 
cartilages  and  muscles  so  as  to  obliterate  all 
ridges  except  the  vocal  cords  and  two  others, 
less  sharply  defined,  called  the  false  vocal  cords, 
which  lie  parallel  to  and  above  the  true  vibrat- 
ing cords.  Between  these  is  the  cavity  known 
as  the  ventricle  of  the  larynx. 

MECHANISM  OF  VOCALIZATION. 
Shape  of  the  Opening  of  the  Glottis.  —  Taking 
the  thyroid  cartilage  as  the  fixed  base,  the  cri- 
coid  and  arytenoid  cartilages  undergo  move- 
ments which  bring  about  two  distinct  sets  of 
changes  in  the  glottis  and  its  elastic  edges, 
namely,  (1)  widening  and  narrowing  the  open- 
ing ;  (2)  stretching  and  relaxing  of  the  vocal 
cords.  During  ordinary  respiration  the  glottis 
remains  about  half  open,  being  slightly  wid- 
ened during  inspiration  (B').  During  forced 
inspiration  the  glottis  is  widely  dilated  by  mus- 
ingrams  taken  from  cular  action  (C').  If  an  irritating  gas  be 
view  ofatbegi°aSrynxC,  inspired,  the  glottis  is  tightly  closed  by  a  spas- 
tbe*po-  modic  action  of  certain  muscles,  so  that  the  true 
Jonrdlhand  !hj  vocal  cords  act  as  a  kind  of  valve. 


During  vocalization  the  glottis  is  formed  into 
a  narrow  chink  with  parallel  sides  (A'),  while 
Asilginag.cbink>as  in  the  cords  are  made  more  or  less  tense,  accord  - 
^ladon'of  a?ruiet  iuba~  ing  to  the  Pitcn  of  the  note  to  be  produced  ;  both 
C/ti!,n.  f°reed  inspira".these  changes  are  brought  about  by  muscular 

action. 

The  opening  of  the  chink  of  the  glottis  is  accomplished  chiefly 
by  a  muscle  called  the  posterior  crico-arytenoid,  which  passes 
from  the  posterior  surface  of  the  cricoid  cartilage  to  the  outer 


MECHANISM   OF   VOCALIZATION. 


489 


and  posterior  angle  of  the  arytenoids.  By  pulling  the  latter 
point  downward  and  backward  it  separates  the  arytenoid  carti- 
lages, particularly  at  their  anterior  extremity,  where  the  cords 
are  attached.  In  this  action  it  is  aided  by  a  small  muscle  con- 
necting the  posterior  surfaces  of  the  arytenoid,  namely,  the  pos- 
terior arytenoid,  which  tends,  when  the  two  arytenoid  cartilages 
are  held  apart,  to  rotate  them  so  that  the  vocal  processes  are 
separated. 

FIG. 199. 


Diagram  of  the  side  view  of  the  larynx, 
showing    the    position  of    the    vocal 
cords  (v).     (Huxley.) 
Ar.  Arytenoid  cartilage. 
Hy.  Hyoid  bone. 
Th.  Thyroid  cartilage. 
Cr.  Cricoid  cartilage. 
Tr.  Trachea. 

C.  th.  Crico-thyroid  muscle. 
Th.A.  Thyro-arytenoid  muscle. 
Ep.  Epiglottis. 


Diagram  of  the  opening  of  the  larynx  from 

above.    (Huxley.) 
Th.  Thyroid  cartilage. 
Cr.  Cricoid'cartilage. 
Ary.  Superior  extremities  of  the  arytenoid 

cartilages. 
V.  Vocal  cords. 

Th.A.  Thyro-arytenoid  muscles. 
C.a.l.  Lateral  crico-arytenoid  muscle. 
C.a.p.  Posterior  crico-arytenoid  muscle. 
A.r.p.  Posterior  arytenoid  muscle. 


The  narrowing  of  the  glottis  is  executed  by  the  lateral  crico- 
arytenoids  which  run  upward  and  backward  from  the  antero- 
lateral  aspect  of  the  cricoid  to  the  muscular  processes  of  the 
arytenoid  cartilages.  They  pull  the  muscular  processes  forward, 
and  thus  rotate  the  arytenoid  cartilages  so  as  to  approximate  the 
vocal  processes  to  one  another,  while  any  tendency  toward  pull- 
ing apart  the  bodies  of  the  cartilages,  owing  to  the  downward 
direction  of  the  muscle,  is  overcome  by  the  posterior  arytenoid 


490  MANUAL   OF   PHYSIOLOGY. 

muscle  and  those  muscular  bands  which  pass  from  the  posterior 
surface  of  the  arytenoid  cartilages  to  the  epiglottis  and  the 
upper  part  of  the  thyroid  cartilage,  the  external  thyro-arytenoid, 
and  the  thyro-ary-epiglottic  muscles  (Henle).  The  other  fibres, 
which  pass  directly  from  the  arytenoid  to  the  thyroid  cartilages 
— internal  and  external  thyro-arytenoid  muscles — in  the  same 
direction  as  the  vocal  cords,  complete  the  closure  by  helping  to 
press  together  the  vocal  processes,  and  by  approximating  the 
cords  themselves.  In  spasmodic  closure  of  the  glottis,  all  these 
latter  muscles  act  violently  together,  and  have  been  grouped  by 
Henle  as  the  constrictor  of  the  glottis. 

Relaxation  of  the  vocal  cords  accompanies  voluntary  closure 
of  the  glottis,  as  in  holding  the  breath,  when  the  false  vocal 
cords  are  said  to  have  a  valvular  action.  The  muscular  fibres, 
which  run  from  the  arytenoid  cartilages  to  the  thyroid,  nearly 
parallel  to  the  true  vocal  cords,  or  those  concerned  in  the  act  of 
relaxation  when  the  cords  are  active.  They  pull  forward  the 
arytenoid  cartilages,  and  at  the  same  time  draw  the  upper  part 
of  the  cricoid  slightly  forward.  These  muscles  have  the  all- 
important  action  of  adapting  the  edges  of  the  cords  and  the 
neighboring  surfaces  to  the  exact  shape  most  advantageous  to 
their  vibration. 

The  tightening  of  the  vocal  cords  is  caused  by  a  single  muscle, 
the  crico-thyroid,  which,  on  the  outer  side  of  the  larynx,  passes 
downward  and  forward  from  the  lower  part  of  the  thyroid  to  the 
anterior  part  of  the  cricoid  cartilage.  It  pulls  the  anterior  part 
of  the  cricoid  cartilage  upward,  causing  it  to  rotate  round  an 
axis  passing  through  its  thyroid  joints.  The  upper  part  of  the 
cricoid,  which  carries  the  arytenoids,  moves  backward,  the  attach- 
ments of  the  vocal  cords  are  separated,  and  the  membranes  are 
thus  put  on  the  stretch. 

The  requirements  necessary  for  the  production  of  voice  are 
the  following : — 

1.  Elasticity  of  the  vocal  cords  and  smoothness  of  their  edges  ; 
freedom  from  all  surface  irregularity,  such  as  would  be  caused 
by  thick  mucus  adhering  to  them,  or  by  any  abnormality. 

2.  The  cords  must  be  very  accurately  adjusted,  and  closely 


PROPERTIES   OF   THE   HUMAN   VOICE.  491 

approximated    together,   so    that    they   almost    touch    evenly 
throughout  their  entire  length. 

3.  The  cords  must  be  held  in  a  certain  degree  of  tension,  or 
their  vibration  cannot  produce  any  vocal  tone,  but  only  a  raucous 
noise. 

4.  The  air  must  be  propelled  through  the  glottis  by  a  forced 
expiration.     The  normal  expiratory  current  is  too  gentle  to  give 
the  necessary  vibration.     After  the  operation  of  tracheotomy, 
the  air  escapes  through  the  abnormal  opening,  and  sufficient 
pressure  cannot  be  brought  to  bear  on  the  cords,  so  no  vocal 
sound  can  be  produced,  and  the  person  speaks  in  a  whisper, 
unless  the  exit  of  air  through  the  tracheotomy  tube  is  prevented 
by  placing  the  finger  temporarily  upon  the  opening. 

PROPERTIES  OF  THE  HUMAN  VOICE. 

In  the  voice  we  can  recognize  the  properties  noted  in  other 
kinds  of  sound.     These  are  quality,  pitch  and  intensity. 

1.  The  quality  of  vocal  sounds  is  almost  endless  in   variety, 
as  is  shown  by  the  vocal  capabilities  of  different  individuals. 
The  quality  of  any  musical  sound  depends  upon  the  relative 
power  of  the  fundamental  tone,  and  of  the  overtones  that  accom- 
pany it.     The  less  the  fundamental  tone  is  disturbed  by  overtones, 
the  clearer  and  better  is  the  voice.     This  difference  in  quality  of 
the  human  voice  depends  upon  the  perfectness  of  the  elasticity, 
the  relation  of  thickness  to  length,  surface  smoothness,  and  other 
physical  conditions  of  the  cords  themselves,  and  the  exactitude 
with  which  the  muscles  can  adapt  the  surfaces.     For  singing 
well,  much  more  is  necessary  than  good  quality  of  tone,  which 
is  common  enough.     The  muscles  of  the  larynx,  thorax,  and 
mouth  must  all  be  educated  to  an  extraordinarily  high  degree. 

2.  The  pitch  of  the  notes  produced  in  the  larynx  depends 
upon — first,  the  absolute  length  of  the  vocal  cords.     This  varies 
with  age,  particularly  in  males,  whose  vocal  organs  undergo 
rapid  growth  at  puberty,  when  vocalization  is  uncertain  from  the 
rapid  changes  going  on  in  the  part ;  hence  the  voice  is  said  to 
crack.     The  vocal  cords  of  women  have  been  found  by  measure- 
ment to  be  about  one-third  shorter  than  those  of  men,  and  people 


492  MANUAL   OF   PHYSIOLOGY. 

with  tenor  voices  have  shorter  cords  than  basses  or  baritones. 
Secondly,  on  the  tension  of  the  cords :  the  tighter  the  vocal 
cords  are  drawn  by  the  crico-thyroid  muscles,  the  higher  the 
notes  produced  ;  and  the  well-known  singer  Garcia  believed  he 
observed  with  the  laryngoscope  the  vocal  processes  so  tightly 
pressed  together  as  to  impede  the  vibration  of  the  posterior  part 
of  the  cords,  and  by  this  means  they  could  be  voluntarily 
shortened. 

3.  Intensity  or  loudness  of  the  voice  depends  on  the  strength 
of  the  current  of  air.  The  more  powerful  the  air  blast  the 
greater  the  amplitude  of  the  vibrations,  and  hence  the  greater 
the  sound  produced.  The  narrower  the  chink  of  the  glottis, 
and  the  tighter  the  parallel  cords  are  stretched,  the  less  is  the 
amount  of  air  and  the  weaker  is  the  blast  required  to  set  them 
vibrating ;  and  vice  versa,  the  looser  the  cords  and  the  wider 
apart  they  are,  the  greater  the  volume  and  the  force  of  the  air 
current  .necessary  for  their  complete  vibration.  Hence  it  is  that 
an  intense  vibration  or  loud  note  can  be  produced  much  more 
easily  with  notes  of  a  high  pitch  than  with  very  low  notes,  and 
we  find  singers  choosing  for  their  telling  crescendo  some  note 
high  up  in  the  range  of  their  voice. 

The  human  voice,  including  every  kind,  extends  over  about 
three  and  a  half  octaves.  Of  this  wide  range  a  single  individual 
can  seldom  sing  more  than  two  octaves.  The  soprano,  alto,  tenor, 
and  bass  forming  a  descending  series,  the  range  of  each  one  ol 
which  considerably  overlaps  the  next  in  the  scale. 

During  the  ordinary  vocal  sounds,  the  air,  both  in  the  resonat- 
ing tubes  above  the  larynx  and  in  the  windpipe  coming  from 
below,  is  set  vibrating,  so  that  the  trachea  and  bronchi  act  as 
resonators  as  well  as  the  pharynx,  mouth,  etc.  This  may  be 
recognized  by  placing  the  hand  on  the  thorax,  when  a  distinct 
vibration  is  communicated  from  the  chest  wall.  Such  tones  are, 
therefore,  spoken  of  as  chest  notes.  Besides  the  chest  tones  of 
the  ordinary  voice,  we  can  produce  notes  of  a  higher  pitch  and  a 
different  quality,  which  are  called  head  notes,  since  their  produc- 
tion is  not  accompanied  by  any  vibration  of  the  chest  wall.  The 
physical  contrivance  by  means  of  which  this  falsetto  voice  is 


NERVOUS   MECHANISM    OF    THE    VOICE.  493 

brought  about  is  not  very  clearly  made  out.  The  following  are 
the  more  probable  views:  (1)  It  has  been  suggested  that  in 
falsetto  only  the  thin  edges  of  the  cords  vibrate,  the  internal 
thyro-arytenoid  muscles  keeping  the  base  of  the  cord  fixed; 
while  with  chest  tones  a  greater  surface  of  the  cord  is  brought 
into  play.  (2)  The  cords  are  said  to  be  wider  apart  in  falsetto 
than  in  chest  notes,  and  hence  the  trachea,  etc.,  ceases  to  act  as  a 
resonator.  (3)  Or  the  cords  may  be  arranged  so  that  only  one 
part  of  them,  the  anterior,  can  vibrate,  and  thus  they  act  as 
shortened  cords,  a  "  stop  "  being  placed  on  the  point  where  the 
vibrations  cease,  by  the  internal  thyro-arytenoid  muscle. 

The  production  of  a  falsetto  voice  is  distinctly  voluntary,  and 
is  probably  dependent  upon  some  muscular  action  in  immediate 
relation  to  the  cords,  for  it  is  always  associated  with  a  sensation 
of  muscular  exertion  in  the  larynx,  as  well  as  with  changes  that 
take  place  in  the  conformation  of  the  mouth  and  other  resonat- 
ing tubes. 

NERVOUS    MECHANISM    OF   VOICE. 

The  nervous  mechanism,  by  means  of  which  vocal  sounds  are 
produced,  is  among  the  most  complexly  coordinated  actions  that 
regulate  muscular  movements. 

Like  respiration,  vocalization  at  first  seems  a  simple  voluntary 
act,  sounds  of  various  kinds  being  produced  at  will  by  the  indi- 
vidual. No  doubt  the  respiratory  muscles,  which  work  the  bel- 
lows of  the  voice  organ,  are  under  the  control  of  the  will  so  long 
as  the  respiration  is  not  interfered  with.  The  mouth  and  throat 
muscles,  which  shape  the  resonating  tube,,  are  also  voluntary. 
But  the  intrinsic  muscles  of  the  larynx  are  only  voluntary  in  a 
certain  sense,  while  in  another  they  are  distinctly  involuntary,  as 
may  be  seen  in  spasm  of  the  larynx ;  for  they  are,  in  part  at 
least,  controlled  by  impulses  which  arise  at  the  organ  of  hearing 
and  pass  to  some  coordinating  centre,  which  arranges  the  finer 
muscular  movements  necessary  to  produce  a  certain  note.  When 
we  sing  a  note  just  struck  on  a  musical  instrument,  we  set  the 
expiratory,  the  mouth,  and  the  special  vocalizing  muscles  in 
readiness,  by  a  voluntary  act,  for  the  proper  application  of  the 
air  blast ;  but  the  exact  tuning  of  the  vocal  cords  is  accomplished, 


494  MANUAL   OF    PHYSIOLOGY. 

in  some  measure  at  least,  reflexly  by  impulses  arriving  from  the 
ear  at  a  special  coordinating  nervous  centre,  the  education  of 
which  is  in  advance  of  that  of  the  voluntary  centres,  and,  there- 
fore, can  only  be  controlled  by  the  latter  in  persons  specially 
educated  in  singing,  Some  persons  who  can  sing  a  given  note, 
with  promptness  and  exactitude,  without  any  effort,  would  find 
much  difficulty  in  overcoming,  by  volition,  the  accuracy  of  this 
perfect  reflex  mechanism.  In  fact,  a  person  with  a  naturally 
"  good  ear  "  finds  it  difficult  to  sing  out  of  tune,  even  if  he  try. 

Though  we  feel  that  we  have  command  over  the  pitch  of  the 
sounds  produced  in  the  larynx,  we  owe  much  of  our  accuracy  to 
the  aid  given  by  our  sound-appreciating  organs  and  the  nerve 
centres  in  connection  with  them. 

SPEECH. 

The  variations  in  vocal  sounds  which  give  rise  to  speech  are  not 
produced  in  the  larynx,  but  in  the  throat,  mouth  and  nose. 
When  unaccompanied  by  any  vocal  sound,  speech  only  gives  rise 
to  a  whisper;  but  when  a  vocal  tone  is  at  the  same  time  produced, 
we  have  the  ordinary  loud  speaking.  Since  vocal  tones  can  only 
be  produced  by  expiration,  so  we  can  only  speak  aloud  by  means 
of  an  expiratory  current  of  air ;  but  an  inspiratory  current  may 
be  made  to  give  rise  to  a  kind  of  whisper. 

Speech  is  composed  of  two  kinds  of  sounds,  in  one  of  which  the 
sounds  must  be  accompanied  by  a  vocal  tone,  and  are,  hence, 
called  "  vowels ; "  in  the  other  no  vocal  tone  is  necessary,  but 
changes  in  shape  take  place  in  the  resonating  chambers,  so  as  to 
give  rise  to  noises  called  consonants.  As  the  pronunciation  of 
the  consonants  is  always  accompanied  by  some  vowel  sound,  and 
as  the  difference  between  the  vowels  is  brought  about  by  changes 
in  the  shape  of  the  mouth,  the  distinction  between  the  two  sets  of 
sounds  is  rather  artificial  than  real. 

The  production  of  the  different  vowel  sounds  depends  upon 
such  a  change  being  brought  about  in  the  shape  of  the  mouth 
cavity  and  aperture,  that  a  resonator,  with  a  different  individual 
note,  is  formed  for  each  particular  word. 

The  sounds  called  consonants   are  caused  by  some  check  or 


SPEECH.  495 

impediment  being  placed  in  the  course  of  the  blast  of  air  issuing 
from  the  air  passages.  They  may  be  classified,  according  to  the 
part  at  which  the  obstruction  occurs,  as  follows : — 

1.  Labials,  when  the  narrowing  takes  place  at  the  lips,  as  in 
pronouncing  b,p,f,  v. 

2.  Dentals,  when  the  tongue  causes  the  obstruction  by  being 
pushed  against  the  hard  palate  or  the  teeth,  as  in  t,  d,  s,  I. 

3.  Gutturals,  when  the  posterior  part  of  the  tongue  moves 
toward  the  soft  palate  or  pharynx,  as  in  saying  k,  g,  gh,  ch,  r. 

Consonants  may  also  be  divided  into  different  groups,  accord- 
ing to  the  kind  of  movements  which  give  rise  to  them. 

1.  Explosives  are   produced   by   the  sudden  removal  of  the 
obstruction,  as  with  p,  d,  k. 

2.  Aspirates  are  continuous  sounds  caused  by  the  passage  of  a 
current  of  air  through  a  narrow  opening,  which  may  be  at  the 
lips,  as  in  /,  at  the  teeth  as  with  s,  or  at  the  throat  as  in  ch. 

3.  Eesonants  are  the  sounds  requiring  some  resonance  of  the 
vocal  cords,  and  the  air  current  is  suddenly  checked  by  closure 
of  the  lips,  as  in  m,  or  the  dental  aperture  as  in  n  or  ng. 

4.  Vibratory,  of  which  r  is  the  example,  requires  a  peculiar 
vibration  of  the  vocal  cords,  while  either  the  dental  or  the  gut- 
tural aperture  is  partially  closed. 


496  MANUAL   OF    PHYSIOLOGY. 


CHAPTER  XXVIII. 

GENERAL  PHYSIOLOGY  OF  THE  NERVOUS  SYSTEM. 
ANATOMICAL  SKETCH. 

The  nervous  system  includes  the  various  mechanisms  by  which 
the  distant  parts  of  the  body  are  kept  in  functional  relationship 
with  one  another.  By  it  the  condition  of  the  surroundings  and 
the  various  parts  of  the  body  are  communicated  to  a  central 
department  (cerebro-spinal  axis)  which  in  turn  regulates  and 
controls  the  activities  of  the  various  organs. 

It  is  made  up  of  two  varieties  of  tissue,  both  of  which  pos- 
sess special  vital  properties.  The  one,  nerve  fibres,  composed  of 
thread-like  strands  of  protoplasm,  connects  the  elements  of  the 
other,  nerve  corpuscles,  which  form  the  peripheral  or  central  ter- 
minals of  the  fibres.  Nerve  fibres  are  simply  special  conducting 
agents,  having  at  one  extremity  a  special  terminal,  or  nerve  cell, 
for  sending  impulses,  and  at  the  other  end  a  nerve  cell  for 
receiving  the  impulses.  These  terminal  organs,  between  which 
the  nerve  fibres  pass,  are  the  agents  which  determine  the  direc- 
tion in  which  the  impulse  is  to  travel  along  the  nerve.  The 
sending  organ  may  be  at  the  peripheral  end  of  the  nerve,  and 
the  receiver  in  the  nerve  centres,  as  in  the  case  of  an  ordi- 
nary cutaneous  nerve,  which  carries  impulses  from  the  skin  to 
the  brain  ;  or  the  sending  organ  may  be  at  the  centre,  and  the 
receiving  organ  at  the  periphery,  as  in  the  case  of  the  nerves 
conveying  impulses  from  the  brain  to  the  muscles. 

The  former  kind  of  nerves  are  called  afferent,  carrying  cen- 
tripetal impulses,  and  the  latter  efferent,  carrying  centrifugal  im- 
pulses. Nerves  are  capable  of  carrying  impulses  in  either  direc- 
tion, as  has  been  proved  by  cutting  the  afferent  lingual  and  the 
efferent  hypoglossal  nerves,  and  causing  the  proximal  end  of  the 
former  to  unite  with  the  distal  end  of  the  latter,  which  is  dis- 
tributed to  the  muscles  of  the  tongue.  When  the  union  has 
taken  place,  a  stimulus  applied  to  the  upper  part  which  was  nor- 


NERVOUS    TISSUES. 


497 


mally  afferent,  or  sensory,  carries  motor  impulses  to  the  muscles, 
i.  e.,  acts  as  an  efferent  nerve. 

Protoplasm,  though  not  formed  into  fibres,  can  conduct  impulses, 
as  is  seen  in  the  transmission  of  an  impulse  in  textures  and 
animals  which  seem  to  have  no  special  conducting  elements  or 
nerve  fibres.  Thus,  in  the  hydra  all  the  cells  act  as  nerves,  and 
in  the  higher  animals  an  impulse,  producing  a  wave  of  contrac- 
tion, can  pass  from  one  muscle  cell  to  the  other  directly,  as  is  seen 
in  the  ureter,  or  in  the  heart  of  cold-blooded  animals. 


FIG.  200. 


FIG.  201. 


Highly-magnified  view  of  three  medul- 
lated  and  two  npn-medullated  nerve 
fibres  of  frog,  stained  with  osraic  acid, 
which  makes  the  medullary  sheath 
black. 

N.— Nodes  of  Ranvier— where  the  axis 
cylinder  can  be  seen  to  pass  the  gap  in 
the  medullary  sheath. 


Transverse  section  of  nerve  fibres,  show- 
ing the  axis  cylinders  cut  across,  and 
looking  like  dots  surrounded  by  a  clear 
zone,  which  is  the  medullary  sheath. 
Neuroglia  separates  the  fibres  into 
bundles. 


The  only  essential  part  of  a  nervous  conductor  is  a  delicate 
protoplasmic  fibril.  Single,  thin,  thread-like  fibrils  are  found 
carrying  impulses  in  the  nerve  centres.  In  the  nerves  distributed 
about  the  body,  one  does  not  meet  these  single  protoplasmic 
threads  (except  where  the  fibrils  are  interwoven  to  form  terminal 
networks,  as  seen  in  the  cornea),  but  the  fibrils  are  clustered 
together  in  large  bundles,  so  as  to  make  one  nerve  fibre.  In  the 
peripheral  nerves  this  bundle  of  protoplasmic  fibrils  is  covered, 
42 


498  MANUAL   OF   PHYSIOLOGY. 

and  is  called  the  axis  cylinder  of  the  nerve  fibre.  In  some  nerve 
fibres  there  is  but  one  very  thin  transparent  covering,  termed  the 
primitive  sheath,  while  in  others  there  is  a  thick  layer  of  doubly- 
refracting  fluid  inside  the  primitive  sheath,  in  immediate  contact 
with  the  fibrils  of  the  axis  cylinder.  This  is  called  the  medullary 
sheath,  or  white  substance  of  Schwann,  because  its  peculiar  refrac- 
tive properties  make  it  look  white  when  viewed  in  a  direct  light. 
As  the  nerves  have  or  have  not  this  medullary  sheath,  they  have 
been  termed  "  white  "  or  "  gray."  The  former  are  by  far  the 
most  plentiful,  since  they  make  up  the  greater  part  of  the  ordinary 
nerves,  while  the  gray  fibres  only  predominate  in  the  sympathetic 
nerve  and  its  ramifications,  and  parts  of  the  special  sense 
organs. 

An  ordinary  nerve,  then,  is  made  up  of  a  large  number  of 
fibres,  held  together  by  connective  tissue,  each  fibre  containing 
a  vast  number  of  fibrils  within  its  sheath. 

FUNCTIONAL  CLASSIFICATION. 

Nerve  fibres  may  be  classified,  according  to  their  function,  in 
the  following  way: — 

I.  AFFERENT  NERVES,  which  bear  impulses  from  the  surface 
to  the  nervous  centres.     These  may  be  further  divided  into: — 

(a)  Sensory  nerves,  when  the  impulse  they  convey  gives  rise  to 
a  "  perception."  The  perceptions  may  be  the  special  sensations 
which  are  transmitted  from  the  organs  of  special  sense,  or  those  of 
general  sensation,  giving  rise  to  pleasure  or  pain. 

(6)  Excito-reflex  nerves  communicate  impulses  to  central  nerve 
elements,  and  give  rise  to  some  action,  without  exciting  mental 
perception.  Such  nerves  regulate  the  viscera.  According  to  the 
result  of  the  excitation  arising  from  their  impulse,  they  are 
termed  excito-motor,  excito-secretory,  and  excito-inhibitory,  etc. 

(c)  Mixed  nerves  act  as  sensory  and  reflex  nerves;  these  are  the 
most  numerous,  the  sensory  or  reflex  action  depending  upon  the 
condition  of  the  nerve  centres. 

II.  EFFERENT  NERVES,  which  carry  impulses  from  the  centres 
to  the  various  organs  throughout  the  body.     According  to  the 
effect  produced  by  their  excitation,  they  are  termed: — 


MODE   OF   INVESTIGATION.  499 

(a)  Motor,  conveying  impulses  to  muscles  and  exciting  them  to 
contract. 

C/5)  Secretory,  the  stimulation  of  which  calls  forth  the  activity 
of  a  gland. 

(f)  Inhibitory,  when  they  check  or  prevent  some  activity  by 
the  impulses  which  they  carry. 

(#)  Vasomotor  nerves,  which  regulate  the  contraction  of  the 
muscular  coat  of  the  blood  vessels. 

(e)  Trophic,  thermic,  electric  nerves  are  also  to  be  named,  the 
two  former  being  of  doubtful  existence,  and  the  latter  being  only 
found  in  those  animals  which  are  capable  of  emitting  electric 
discharges,  such  as  electric  fishes. 

III.  INTERCENTRAL  NERVES  act  as  bonds  of  union  between 
the  several  ganglion  cells  of  the  nervous  centres,  which  are  con- 
nected, in  a  most  elaborate  manner,  one  with  the  other.  The 
terminals  of  these  fibres  are  possibly  both  receiving  and  direct- 
ing agents,  and  the  delicate  strands  of  protoplasm  communi- 
cating between  them  probably  convey  impulses  in  different 
directions,  but  of  this  we  can  have  no  definite  knowledge, 
although  such  a  supposition  would  aid  us  in  forming  a  mental 
picture  of  the  manner  in  which  the  wonderfully  complete  inter- 
central  communications  are  accomplished. 

MODE  OF  INVESTIGATION. 

In  order  to  understand  the  functions  of  the  different  nerves  a 
knowledge  of  their  central  connections  and  their  peripheral  dis- 
tribution is  necessary.  But  anatomical  research,  unaided  by  ex- 
perimental inquiry,  does  not  suffice  to  determine  their  function. 

The  procedure  adopted  in  testing  the  function  of  a  nerve  is  the 
following:  The  nerve  is  exposed  and  cut,  and  it  is  observed 
whether  there  be  any  loss  of  sensation  or  muscular  paralysis  in 
the  part  to  which  it  passes.  The  end  connected  with  the  centres 
is  spoken  of  as  the  central  or  proximal  endj  and  that  leading  to 
the  distribution  of  the  nerve  is  called  the  peripheral  or  distal 
end.  Each  of  these  cut  ends  is  then  stimulated,  and  the  results 
are  observed.  If  the  nerve  be  purely  motor,  stimulation  of  the 
proximal  end  will  yield  no  result,  but  when  the  distal  end  is 


500  MANUAL   OF   PHYSIOLOGY. 

irritated,  movements  follow.  If,  on  the  other  hand,  it  be  a  sen- 
sory nerve,  stimulation  of  the  distal  end  gives  no  result,  and  that 
of  the  proximal  end  produces  signs  of  pain. 

CHEMISTRY  OF  NERVE  FIBRES. 

The  axis  cylinder  of  nerves  is  probably  composed,  as  already 
mentioned,  of  protoplasm ;  further  than  that  nothing  is  known 
of  its  chemical  properties.  The  medullary  sheath  yields  certain 
substances  which  are  related  to  the  fats,  and  can  be  extracted 
with  ether  and  chloroform.  Among  these  is  the  peculiar  com- 
pound nitrogenous  fat,  lecithin,  containing  phosphorus,  also 
cholesterin,  cerebin,  and  kreatin. 

ELECTRIC  PROPERTIES  OF  NERVES. 

Like  muscle,  nerves  may  be  regarded  as  having  a  state  of  rest 
and  a  state  of  activity,  but  the  two  states  are  not  obvious  in  the 
same  striking  way  as  they  are  in  muscle,  nor  do  we  know  much 
of  the  physical  properties  of  nerve.  While  at  rest,  however,  it 
shows  electric  phenomena  similar  to  those  which  have  already 
been  described  as  belonging  to  muscle  tissue.  These  electrical 
currents  are  contemporaneous  with  the  life  of  the  nerve,  and 
they  undergo  the  same  variation  as  occurs  in  muscle  when  the 
nerve  passes  into  the  active  state ;  that  is,  when  it  transmits  an 
impulse. 

The  so-called  natural  current  of  nerve  is  practically  the  same 
as  that  of  muscle,  passing  in  the  nerve  to  the  central  part  from  the 
cut  extremities  of  the  fibre  ;  that  is  to  say,  the  current  passes 
through  the  galvanometer  from  the  electrode  leading  from  the  mid- 
dle of  the  nerve,  to  that  applied  to  the  extremity.  The  electro- 
motive force  of  a  small  nerve  is  much  less  than  that  of  a  muscle. 
In  a  frog's  sciatic  it  has  been  estimated  to  be  0.02  of  a  Daniell 
cell.  The  natural  current  of  the  frog's  nerve  is  said  to  increase 
in  intensity  in  proportion  to  the  increase  in  temperature  up  to 
about  20°  C.,  after  which  it  decreases. 

Experiments  on  nerve  currents  must  be  carried  on  with  all  the 
precautions  mentioned  in  speaking  of  muscle  currents,  and  with 
the  non-polarizable  electrodes  there  figured  (page  448). 


NERVE   STIMULI.  501 

THE  ACTIVE  STATE  OF  NERVE  FIBRES. 

Nerves  pass  into  a  state  of  activity  in  response  to  a  variety  of 
stimuli,  but  their  active  condition  cannot  be  readily  recognized, 
because  the  only  change  we  can  detect  in  the  nerve  is  that  which 
takes  place  in  the  electric  state.  If  it  be  connected  with  its 
terminals,  we  learn  when  a  nerve  is  carrying  an  impulse  from 
the  results  occurring  in  them  on  stimulation.  In  the  case  of  an 
afferent  nerve,  we  get  evidence  of  a  sensation,  and  when  the 
nerve  is  efferent,  for  example,  bearing  impulses  from  the  centres 
to  the  muscles,  we  judge  of  the  state  of  activity  of  the  nerve  by 
the  muscle  contraction.  For  experimental  purposes  we  use  the 
nerve  and  the  muscle  of  a  frog.  This  nerve-muscle  preparation  is 
made  from  the  leg  of  a  frog  :  the  sciatic  nerve  is  carefully  pre- 
pared from  the  thigh  and  abdominal  cavity  without  being 
dragged  or  squeezed,  and  the  gastrocnemius  is  separated  from  all 
its  attachments  except  that  to  the  femur,  about  two-thirds  of 
which  bone  is  left,  so  that  the  preparation  may  be  fixed  in  the 
clamp.  In  fact,  the  method  used  for  the  direct  stimulation  of 
muscle  is  also  employed  for  the  study  of  the  excitability  of  nerve 
fibres. 

NERVE  STIMULI. 

Besides  the  normal  physiological  impulse  which  comes  from 
the  cells  in  connection  with  the  nerve  fibres,  a  variety  of  stimuli 
may  excite  their  active  state.  These  nerve  stimuli  differ  little 
from  those  which  are  found  to  affect  muscle,  when  applied  directly 
to  that  tissue.  They  may  be  enumerated  as  follows: — 

1.  Mechanical  Stimulation. — Almost  any  mechanical  impulse, 
applied  to  any  part  of  a  nerve,  causes  its  excitation.     The  stimulus 
must  have  a  certain  degree  of  intensity,  and   definite,  though  it 
may  be  of  very  short  duration.      If  mechanical  stimuli  be  fre- 
quently applied  to  a  nerve  in   the  same  place,  the  irritability 
of  the  part  is  soon  destroyed ;  but  if  fresh  parts  of  the  nerves  be 
stimulated,  at  each  application  the  nerve  passes  into  a  state  of 
tetanus,  as  shown  by  the  contraction  of  the  muscle  to  which  it  is 
supplied. 

2.  Chemical  Stimulation. — Loss  of  water  by  the  tissue  of  the 
nerve,  whether  this  be  caused  by  evaporation,  or  facilitated  with 


502  MANUAL   OF   PHYSIOLOGY. 

blotting  paper,  exposure  over  sulphuric  acid,  or  the  addition  of 
solutions  of  high  density,  such  as  syrup,  glycerine,  or  strong  salt 
solution.  The  application  of  strong  metallic  salts  or  acids ;  or 
alcohol  and  ether,  also  a  solution  of  bile  irritates  nerves;  weak 
alkalies,  except  ammonia,  which  has  no  effect  on  nerve,  although 
it  acts  on  muscle  when  applied  directly  to  that  tissue. 

3.  Thermic  stimulation  occurs  when  sudden  changes  are  brought 
about,  approaching  either  of  the  extreme  temperatures  at  which 
the  nerve  can  act ;  i.  e.,  near  5°  or  50°  C. 

4.  Electric  stimulation  is  by  far  the  most  important  for  physi- 
ologists, being  the  most  easily  applied  and    regulated,  and  the 
least  injurious  to  the  nerve  tissue.    As  was  mentioned  with  respect 
to  muscle,  any  sufficiently  rapid  change  of  intensity  in  an  electric 
current  passing  through  a  nerve  causes  the  molecular  changes  we 
call  excitation,  as  shown  by  the   muscle  contracting,  and   the 
natural  electric  currents  of  the  nerve  undergoing  variation.    The 
less  the  absolute  intensity  of  the  current,  the  greater  the  effect 
caused  by  any  given  change  in  intensity.     The  muscle  of  a  nerve- 
muscle  preparation  contracts,  when  a  weak  constant  current,  say 
from  a  single  small  Daniell  cell,  is  suddenly  allowed  to  pass 
through  the  nerve.     This  is  done  by  placing  a  part  of  the  nerve 
in  the  circuit,  which  is  made  complete,  by  closing  a  key,  when 
the  stimulation  is  to  be  applied.     This  form  of  stimulation  is 
called  a  making  shock.     While  the  current  is  allowed  to  pass 
through  the  nerv£,  little  or  no  effect  is  produced,  if  the  battery  be 
quite  constant.      On  breaking  the  circuit,  by  opening  the  key, 
the  current  suddenly  ceases,  and  another  contraction  occurs;  this 
is  called  the  breaking  shock.     At  each  making  and  breaking  of 
the  constant  current,  a  stimulus  is  applied  to  the  nerve,  and  trans- 
mitted to  the  muscle,  and  it  has  been  found  that  a  weaker  current 
suffices  to  bring  about  a  contraction  when  applied  to  the  nerve, 
than  when  applied  directly  to  the  muscle. 

If  a  strong  constant  current  be  allowed  to  pass  through  a  con- 
siderable length  of  a  nerve  for  some  little  time,  and  the  circuit  be 
then  suddenly  broken,  instead  of  a  single  contraction,  tetanus  of 
the  muscle  results.  This  breaking  tetanus  (Hitter's  tetanus)  is  easily 
produced  when  the  positive  pole  or  anode  is  next  the  muscle. 


NERVE   STIMULI.  503 

Sometimes,  in  particular  conditions  of  the  nerve,  and  with  certain 
strengths  of  stimulation,  a  making  tetanus  also  occurs,  but  more 
rarely  and  only  when  the  negative  pole  is  next  the  muscle. 

When  a  constant  current,  such  as  we  get  directly  from  a 
Daniell  cell,  is  used,  that  part  of  the  nerve  between  the  stimu- 
lating points,  through  which  the  current  passes,  is  found  not  to  be 
equally  affected  throughout  its  entire  length,  but  one  single  point 
is  stimulated  whence  the  impulse  spreads.  This  point  may  be 
where  either  of  the  poles  is  in  contact  with  the  nerve ;  and,  further, 
the  stimulus  starts  from  a  different  pole,  according  as  the  circuit 
is  made  or  broken.  With  a  making  shock  the  stimulation  takes 
place  at  the  negative  pole  or  cathode,  and  with  a  breaking  shock 
at  the  positive  pole  or  anode.  That  is  to  say,  the  point  where 
the  current  leaves  the  nerve  is  affected  at  the  make,  and  the 
point  where  the  current  enters  the  nerve  is  affected  at  the  break 
of  the  current. 

It  has  been  found  that,  other  things  being  equal,  the  making 
shock  is  a  more  powerful  stimulus  than  the  breaking  shock ;  i.  e., 
a  weak  current  will  sooner  cause  a  contraction  when  the  circuit 
is  made  than  when  it  is  broken. 

This  fact,  that  the  impulse  starts  from  the  anode  in  a  breaking 
shock,  is  proved  by  means  of  the  breaking  tetanus  just  alluded  to. 
It  has  been  found  that  when  the  anode  is  next  to  the  muscle  the 
breaking  tetanus  is  more  marked  and  lasts  longer  than  when  the 
anode  is  further  from  the  muscle  than  the  cathode.  When  the 
cathode  is  nearer  to  the  muscle  than  the  anode,  section  of  the  nerve 
between  these  points  during  stimulation  stops  the  contraction  at 
once,  and  no  breaking  tetanus  occurs,  because  the  point  from 
which  the  stimulus  comes  is  cut  off  from  the  muscle.  Intra-polar 
section  has  no  effect  if  the  anode  be  next  the  muscle,  and  the 
tetanus  proceeds  in  a  normal  way,  because  the  active  pole 
remains  in  continuity  with  the  muscle.  That  the  stimulus 
occurs  at  the  cathode  in  making  a  current,  may  be  demonstrated 
by  the  fact  that  it  takes  a  certain  measurable  time  for  the 
impulse  to  travel  along  the  nerve.  If  the  cathode  be  placed  as 
far  as  possible  from  the  muscle  and  the  anode  quite  near  it,  the 
contraction  after  a  breaking  shock,  when  the  stimulus  starts  from 


504  MANUAL   OF    PHYSIOLOGY. 

the  anode,  will  occur  sooner  than  that  which  follows  the  making 
shock,  when  the  stimulus  starts  from  the  cathode,  because  the 
impulse  has  a  less  distance  of  nerve  to  traverse  in  the  former 
case. 

In  most  experiments  on  nerve,  a  constant  current,  i.  e.,  one 
coming  directly  from  a  battery,  is  seldom  used,  because  there 
is  no  ready  means  of  regulating  or  varying  the  strength  of  the 
stimulation.  The  instantaneous  current  induced  in  one  coil  of 
wire — the  secondary  coil — by  the  making  or  breaking  of  a  cur- 
rent passing  through  another  coil — the  primary  coil— is  more 
effective  and  suitable  for  physiological  purposes.  It  must  be 
remembered,  however,  that  the  induced  current  is  both  a  rise 
and  fall  of  electric  current,  i.  e.,  a  make  and  break ;  but 
the  duration  of  the  two  changes  is  so  small  (circa,  .00004") 
that  they  only  act  as  a  single  stimulus.  As  there  is  no  current 
in  the  secondary  coil  while  a  constant  current  is  kept  passing 
through  the  primary,  of  course  the  induced  current  cannot  be 
used  for  experiments  relating  to  the  making  and  breaking 
shocks.  The  strength  of  the  induced  current  being  approxi- 
mately in  inverse  proportion  to  the  square  of  the  distance  between 
the  two  coils — moving  the  secondary  away  from  the  primary 
coil  gives  a  ready  means  of  varying  and  regulating  the  strength 
of  the  stimulus,  without  any  special  care  being  devoted  to  the 
exact  strength  of  the  element  used.  ' 

Du  Bois-Reymond's  Inductorium  is  the  instrument  commonly 
used  in  physiological  laboratories.  In  it  the  secondary  coil  can 
be  moved  away  from  the  primary  on  a  graduated  slide,  and  the 
primary  current  may  be  made  to  pass  through  a  magnetic  inter- 
rupter so  as  to  cause  a  rapid  succession  of  breaks  and  makes,  and 
thus  give  a  series  of  stimulations,  one  after  another,  which  is 
necessary  to  produce  tetanus.  A  drawing  and  further  descrip- 
tion of  the  instrument  will  be  found  at  pages  453,  454. 

VELOCITY  OF  NERVE  FORCE. 

It  has  already  been  stated  that  nerve  fibres  are  capable  of 
conducting  impulses  in  either  direction — from  or  to  the  nervous 
centres.  The  position  and  character  of  the  terminal  organs 


VELOCITY  OF  NERVE  FORCE.  505 

* 

determines  the  direction  in  which  the  nerve  impulse  usually  pro- 
duces results.  In  the  ordinary  peripheral  nerves  there  are  gen- 
erally both  kinds — efferent  and  afferent  fibres,  carrying  impulses 
in  different  directions. 

When  we  reflect  that  the  passage  of  an  impulse  along  a  nerve 
is  brought  about  by  a  molecular  change  in  the  axis  cylinder,  we 
are  at  once  struck  with  the  rapidity  with  which  impressions  are 
transmitted  from  one  part  of  the  body  to  another.  This  velocity 
is,  however,  only  relatively  great.  When  we  compare  it  with  the 
velocity  of  the  electric  current  or  of  light,  we  at  once  see  how 
much  slower  the  rate  of  nerve  impulse  is,  and  that  it  may  be 
compared  with  rates  of  motion  commonly  under  our  observation. 
To  take  every-day  examples — viz.,  nine  metres  per  second  is 
about  the  rate  at  which  a  quick  runner  can  accomplish  his  100 
yards;  race  horses  can  gallop  about  15  metres  a  second  for  a 
mile  or  so ;  a  mail  train  at  full  speed  travels  about  30  metres  a 
second,  and  the  velocity  of  nerve  force  has  been  estimated  to  be 
in  cold-blooded  animals  27  metres  per  second  ;  and  in  man  about 
33  metres  per  second.  So  that  the  intercommunications  between 
man's  brain  and  the  various  parts  of  his  body  only  travel  about 
the  same  rate  as  an  express  train,  and  about  twice  as  fast  as  the 
quickest  horse  can  gallop. 

Different  methods  may  be  employed  for  the  measurement  of  the 
rate  of  transmission  of  nerve  force.  The  simplest  is,  with  a  good 
myograph,  such  as  described  in  Chap,  xxv,  p.  462,  to  make  a 
muscle  draw  two  curves,  one  over  the  other,  in  one  of  which  the 
stimulation  is  applied  to  the  nerve  close  to  the  muscle,  and  in  the 
other  as  far  as  possible  away  from  it.  The  difference  in  dura- 
tion of  the  latent  period  in  the  two  curves,  shown  by  the  tuning- 
fork  tracing,  corresponds  to  the  time  taken  by  the  impulse  to 
travel  along  the  part  of  the  nerve  between  the  two  points  of 
stimulation,  the  length  of  which  can  be  directly  measured ;  and 
hence  the  velocity  of  the  impulse  estimated. 

Utilizing  the  fact  that   the   extent  of  the   deflexion  of  the 

needle  of  a  galvanometer  is  in  proportion  to  the  duration  of  a 

current  of  known  strength  passing  through  it  for  a  short  time, 

an  accurate  measurement  of  the  difference  in  time  of  remote 

43 


506  MANUAL   OF   PHYSIOLOGY. 

and  near  stimulation  of  a  nerve  may  be  made.  By  a  special 
mechanism  the  time-measuring  current  is  sent  through  the  gal- 
vanometer at  the  same  moment  that  the  stimulating  current  goes 
through  the  nerve,  and  the  instant  the  muscle  begins  to  contract, 
it  breaks  the  current  passing  through  the  galvanometer,  so  that 
this  time-measuring  current  lasts  only  from  the  moment  when  the 
nerve  is  stimulated  until  the  muscle  begins  to  contract 

THE  ELECTRIC  CHANGE  IN  NERVE. 

Negative  Variation. — The  natural  current  of  a  nerve,  like  that 
of  muscle,  undergoes  a  diminution  at  the  moment  the  nerve  is 
stimulated  ;  this  is  termed  the  negative  variation.  It  occurs  with 
any  other  form  of  stimulation  as  well  as  when  an  electric  shock 
is  used,  so  it  is  not  dependent  on  an  escape  of  the  stimulating 
current.  In  the  case  of  a  single  stimulation,  the  negative  varia- 
tion is  so  rapidly  over — lasting  only  .0005  sec.,  that  the  inertia 
of  the  needle  of  the  galvanometer  prevents  the  change  in  the 
current  being  indicated.  In  tetanus,  however,  it  makes  a  decided 
impression  on  the  galvanometric  needle.  The  strength  of  the 
negative  variation  depends  on  the  condition  of  the  nerve  and  the 
strength  of  the  stimulus  ;  being  stronger  when  the  nerve  is  fresh 
and  irritable  and  has  a  good  natural  current,  and  when  a  strong 
stimulus  is  applied. 

The  negative  variation  of  the  natural  currents  passes  along 
the  nerve  from  the  point  of  stimulation  in  both  directions,  just 
as  does  the  nerve  impulse  ;  and  with  a  galvanometer  the  electric 
change  may  be  traced  from  the  nerve  to  the  muscle.  It  has  also 
been  shown  that  the  negative  variation  travels  along  the  nerve 
at  the  same  velocity  as  the  impulse  ;  namely,  about  27  metres 
per  second.  Further,  this  rate  is  said  to  be  influenced  in  the 
same  way  by  the  passage  of  a  constant  current  through  the  nerve 
(to  be  presently  described)  as  is  the  impulse  derived  from 
stimulus.  These  points  seem  to  lead  to  the  belief  that  the  nerve 
impulse  and  the  negative  variation  are  closely  related.  This 
peculiar  electric  change  and  its  accompanying  impulse  pass 
along  the  nerves  as  a  kind  of  wave  of  activity,  the  speed  and 
duration  of  which  we  know  to  be  27  metres  per  sec.  and  .0005 


ELECTROTONUS,  507 

of  a  sec.  respectively ;  the  length  of  the  wave  we  therefore  cal- 
culate to  be  about  18  millimetres. 

ELECTKOTONUS. 

If  one  of  two  wires  leading  to  a  galvanometer  be  applied 
to  the  centre,  and  the  other  to  the  end  of  a  nerve,  so  as  to  indi- 
cate the  natural  current,  and  at  the  same  time  another  part  of 
the  nerve  be  placed  in  the  circuit  of  a  constant  current  from  a 
battery,  when  the  circuit  of  the  constant  (now  called  polarizing) 
current  is  completed,  a  change  is  found  to  take  place  in  the 
natural  current.  This  is  called  electrotonus.  Instead  of  the 

FIG.  202. 


Diagram  to  illustrate  Electrotonus. 

N.  N'.  Portion  of  Nerve  G.  G'.  Galvanometers.  D.  Battery  from  which  polarizing  cur- 
rent can  be  sent  into  nerve  by  closing  key  K.  The  direction  of  the  polarizing  and 
electrotonic  currents  is  indicated  by  the  arrows,  and  is  seen  to  be  the  same. 

natural  currents  from  the  centre  to  the  end  of  the  nerve,  a  cur- 
rent is  found  to  pass  through  the  entire  length  of  the  nerve  in 
the  same  direction  as  the  polarizing  current  from  the  battery. 
This  electrotonic  current  is  not  proportional  to  the  strength  of 
the  natural  currents,  and  is  to  be  recognized  when  the  latter  are 
no  longer  to  be  found.  It  is  stronger  with  a  strong  polarizing 
current,  and  is  most  marked  in  the  immediate  neighborhood  of 
the  poles,  fading  gradually  away  as  one  passes  to  the  remoter 
parts  of  the  nerve.  The  electrotonic  state  is  not  to  be  attributed 
to  an  escape  of  the  constant  polarizing  current,  because  it  decreases 


508  MANUAL   OF   PHYSIOLOGY. 

gradually  with  the  waning  of  the  physiological  activity  of  the 
nerve,  and  ceases  at  the  death  of  the  nerve  long  before  the 
tissue  has  lost  its  power  of  conducting  electric  currents.  It 
has  been  shown  that  a  ligature  applied  to  the  nerve  so  as  to 
destroy  its  physiological  continuity,  but  not  its  power  of  carry- 
ing electric  currents,  prevents  the  passage  of  the  electrotonic 
current  to  the  part  of  the  nerve  which  is  thus  separated. 

The  condition  of  the  portion  of  the  nerve  near  the  anode  is 
found  to  differ  somewhat  from  that  near  the  cathode,  and  hence 
it  is  found  convenient  to  speak  of  the  region  of  the  anode  being 
in  the  aneledrotonic,  and  that  of  the  cathode  being  in  the  catelec- 
trotonic  condition.  A  certain  time  appears  to  be  required  for 
the  production  of  electrotonus,  a  current  of  less  duration  than 
.0015  of  a  second  we  are  unable  to  detect  the  electrotonic  state. 
The  negative  variation  must,  therefore,  have  passed  away  before 
the  electrotonus  has  commenced. 

IRRITABILITY  OF  NERVE  FIBRES. 

The  irritability  of  nerves  varies  according  to  certain  conditions 
and  circumstances.  While  uninjured  in  the  body,  the  irritability 
of  a  nerve  depends  upon — 

1.  A  supply^of  blood  sufficient  to  supply  nutriment,  and  to 
carry  off  any  injurious  effete  matters  that  may  be  produced  by 
its  molecular  changes. 

2.  A  suitable  amount  of  rest.    Prolonged  activity  causes  fatigue 
and  loss  of  irritability,  no  doubt  from  the  same  causes  mentioned 
as  bringing  about  fatigue  in  muscles.     The  chemical  changes 
taking  place  in  nerves  have  not  yet,  however,  been  made  out 
with  any  degree  of  accuracy. 

3.  Uninjured  connection  with  the  nerve  centres.     When  a  spinal 
nerve  is  cut,  the  part  connected  with  the  periphery  rapidly  under- 
goes degenerative  changes  which  seem  to  depend  upon  faulty 
nutrition,  since  they  are  accompanied  by  structural  changes — 
fatty  degeneration.  This  appears  to  commence  in  a  very  short  time 
after  the  section — often  in  about  three  to  five  days.     The  part  of 
the  nerve  remaining  in  direct  connection  with  the  cord  retains 
its  irritability  for  a  very  much  longer  time. 


IRRITABILITY   OF    NERVE    FIBRES. 


509 


In  the  artificial  stimulation,  by  means  of  electric  shocks  applied 
to  the  nerve  of  a  cold-blooded  animal,  there  are  many  minor  con- 
ditions which  have  considerable  influence  on  the  irritability,  as 
evidenced  by  the  response  given  by  the  attached  muscle  to  weak 
stimuli.  The  more  important  of  these  are: — 

1.  Temperature  changes.  In  the  case  of  a  frog's  nerve,  a  rise 
of  temperature  to  32°  C.  causes  an  increase  in  its  excitability. 
Also  a  fall  of  temperature  below  zero  tends  to  make  the  nerve 
more  easily  excited.  Both  these  conditions  have,  however,  a  very 
fleeting  effect,  for  the  nerve  soon  dies  at  the  temperatures  named, 
and,  probably,  the  increased  irritability  is  only  to  be  taken  as  a 
sign  of  approaching  death.  It  thus  appears  that  a  medium  tem- 
perature is  the  optimum  for  nerve  work. 

FIG.  203. 


Diagram  illustrating  the  variations  of  irritability  of  different  parts  of  a  nerve  during  the 
passage  of  polarizing  currents  of  varying  strength  through  a  portion  of  it. 

A  =  Anode ;  B  =  Cathode ;  AB  =  Intra-polar  district ;  yl  =  Effect,  of  weak  current ;  y*  = 
Effect  of  medium  current;  y3  =  Effect  of  strong  current. 

The  degree  of  change  effected  in  the  irritability  of  the  part  is  estimated  by  the  distance 
of  the  curves  from  the  straight  line.  The  part  of  curve  below  the  line  corresponds  to 
decrease,  that  above  to  increase  of  irritability.  W^here  the  curves  cross  the  line  is  called 
the  indifferent  point.  With  strong  currents  this  approaches  the  cathode.  (From  Foster, 
after  Pflllger.) 

2.  The  part  of  the  nerve  stimulated  is  also  said  to  have  some 
effect  on  the  result  of  a  given  strength  of  stimulus.    The  further 
from  the  muscle,  the  more  powerful  the  contraction  produced, 
other  things  being  equal.     So  that  the  impulse  is  supposed  to 
gather  force  as  it  goes,  as  in  the  case  of  a  falling  body,  and  hence 
has  been  spoken  of  as  the  avalanche  action  of  nerve  impulse. 

3.  A  new  section  of  the  nerve  is  said  to  increase  its  irritability, 
as  does,  indeed,  any  slightly  stimulating  influence,  such  as  drying, 
arid  chemical  or  mechanical  meddling  of  any  kind.     This  increase 


510  MANUAL   OF   PHYSIOLOGY. 

in  irritability  probably  depends  upon  injurious  changes  going  on 
in  the  nerve,  as  the  influences  just  alluded  to  lead  to  complete 
loss  of  excitability,  if  carried  too  far. 

4.  The  eledrotonic  state.  The  most  remarkable  changes  in  the 
excitability  of  a  nerve  are  those  brought  about  by  the  action  of 
a  constant  current  passing  through  the  nerve,  so  as  to  set  up  the 
conditions  just  described  as  anelectrotonus  and  catelectrotonus. 


FIG.  204. 


Diagram  to  show  the  meaning  of  the-  terms  ascending  and  descending  currents,  used 

in  speaking  of  the  law  of  contraction.     The  end  of  the  vertebral  column,  sciatic 

nerves  and  calf  muscles  of  a  frog  are  shown. 
The  arrows  indicate  the  direction  of  the  ascending  current,  A,  on  the  left,  and  the  descending 

current,  D,  on  the  right,  according  as  the  positive  pole  of  the  battery,  c,  is  below  or 

above. 

The  irritability  of  the  nerve  is  increased  in  the  region  near  the 
cathode,  and  is  diminished  in  the  neighborhood  of  the  anode. 

The  increase  of  irritability  is  in  proportion  to  the  intensity  of 
the  catelectrotonic  and  the  decrease  in  proportion  to  the  intensity 
of  the  anelectrotonic  state.  Thus,  the  increase  is  most  marked 
in  the  immediate  neighborhood  of  the  cathode,  and  fades  with 
the  distance  from  the  negative  pole ;  and  similarly,  the  decrease 


LAW   OF    CONTRACTION.  511 

is  strongest  at  the  anode,  and  becomes  less  and  less  as  it  passes 
away  from  the  positive  pole.  In  the  same  way,  in  the  part  of  the 
nerve  between  the  two  poles — the  intra-polar  region — the  decrease 
and  increase  of  irritability  become  less  marked  toward  the  middle 
point  between  the  cathode  and  the  anode,  so  that  here  we  find  an 
unaffected  part,  which  has  been  called  the  indifferent  point. 

It  is  a  remarkable  fact  that  this  indifferent  point  is  not  always 
midway  between  the  two  poles,  but  decreases  its  distance  from  the 
cathode  in  proportion  as  the  polarizing  current  is  made  stronger. 
That  is  say,  with  strong  polarizing  currents  the  indifferent  point 
is  near  the  cathode  (B) ;  with  weak  currents  it  lies  near  the 
anode  (A)  (Fig.  203). 

Besides  becoming  less  irritable  in  proportion  as  the  polarizing 
current  becomes  more  powerful,  the  anelectrotonic  region  of  the 
nerve  loses  its  ability  to  conduct  impulses,  and  may  finally,  with 
a  very  strong  current,  even  when  applied  for  a  short  time, 
become  quite  incapable  of  conducting  an  impulse. 

If  the  polarizing  current  be  now  opened,  so  as  to  stop  its  pass- 
age through  the  nerve,  and  remove  the  anelectrotonic  and  the 
catelectrotonic  states,  a  kind  of  rebound  occurs  in  the  condition 
of  both  the  altered  regions,  and  the  part  which  has  just  ceased 
to  be  catelectrotonic,  and  was,  therefore,  over-irritable,  becomes, 
by  a  kind  of  negative  modification,  very  much  lowered  in  its 
irritability;  while,  on  the  other  hand,  the  anelectrotonic  part,  by 
a  positive  rebound,  becomes  more  excitable  than  in  its  normal 
state.  The  rebound  over  the  line  of  normal  irritability  lasts 
a  very  short  time  ;  but  as  we  shall  see  presently,  it  is  of  greater 
duration  than  the  passage  of  the  negative  variation  along  the 

nerve. 

THE  LAW  OF  CONTRACTION. 

Upon  the  foregoing  facts,  and  others  already  mentioned — viz., 
that  the  impulse  starts  in  the  nerve  from  different  poles  and  with 
different  force,  with  a  making  and  a  breaking  shock — depends 
the  law  of  contraction,  which  would  be  difficult  to  understand 
without  bearing  in  mind  all  these  interesting  points. 

It  was  found  that,  with  the  same  strength  of  stimulation,  not 
only  were  different  degrees  of  contraction  produced  with  making 


512  MANUAL   OF   PHYSIOLOGY. 

and  breaking  shocks,  but  also  that,  other  things  being  similar,  a 
different  result  followed  when  the  current  was  sent  through  the 
nerve  in  an  upward  direction  (i.  e.,  from  the  muscle),  and  when 
it  was  sent  in  a  downward  direction  (i.  e.,  toward  the  muscle). 
The  stimulating  current  is  spoken  of,  in  the  former  case,  as  an 
ascending  current,  and  in  the  latter  as  a  descending  current. 
The  following  is  a  tabular  view  of  the  law  of  contraction  :  — 


Weak  Stimulation. 
Medium 
Strong           " 

ASCENDING  CURRENTS. 

DESCENDING  CURRENTS. 

Make  =  Contraction. 
Break=  No  Response. 

Make  —  Contraction. 
Break=No  Response. 

Make  =  Contraction. 
Break=  Contraction. 

Make  =  Contract  ion. 
Break=  Contraction. 

Make  =  No  Response. 
Break=  Contraction. 

Make  =  Contraction. 
Break=  No  Response. 

To  explain  this  law,  the  following  points  must  be  kept  in 
view: — 

1.  In  a  breaking  shock,  it  is  the  disappearance  of  anelectrotonus 

which  causes  the  stimulation  to  start  from  the  anode. 

2.  In  a  making  shock,  it  is  the  appearance  of  catelectrotonus 

which  causes  the  stimulation  to  start  from  the  cathode. 

3.  With  the  same  current  the  make  is  more  powerful  than 

the  break. 

4.  Anelectrotonus   causes  reduction  of  irritability    and    con- 

ductivity of  the  nerve. 

5.  Catelectrotonus  causes  increase  of  irritability  and  conduc- 

tivity of  the  nerve. 

6.  With   ascending   currents  the  part  of  the  nerve  next  the 

muscle  is  in  a  state  of  reduced  functional  activity  (ane- 
lectrotonus). 

7.  With  descending  currents  the  part  of  the  nerve  next  the 

muscle  is  in  a  state  of  exalted  activity  (catelectrotonus). 

8.  The  reduction  or    exaltation  of  activity  is  much  greater 

with  strong  currents. 

That  only  making  shocks  cause  contraction  with  very  weak 
currents,  simply  depends  on  the  greater  efficacy  of  the  entrance  of 
catelectronus  into  the  nerve,  which  causes  the  making  stimulation 


NERVE  CORPUSCLES  OR  TERMINALS.          513 

That  contraction  follows  in  all  four  cases,  with  medium  stimu- 
lation, is  explained  by  assuming  that  the  depression  of  the 
functional  activity  of  the  nerve  is  not  sufficient  to  affect  its 
conductivity. 

The  want  of  response  to  a  making  shock,  in  the  case  of  the 
strong  descending  current,  depends  upon  the  fact  that  the  part 
of  the  nerve  near  the  muscle,  around  the  anode,  is  in  a  state  of 
lowered  activity,  and  is,  therefore,  unable  to  conduct  the  impulse 
which  has  to  pass  through  this  region  from  the  cathode,  where 
the  stimulation  takes  place,  in  order  to  reach  the  muscle. 

The  absence  of  contraction  at  the  breaking  of  a  strong 
descending  current,  is  caused  by  the  same  lowering  of  the  con- 
ductivity of  the  nerve  between  the  point  of  stimulation  and  the 
muscle,  because  at  the  cessation  of  strong  catelectrotonus,  the 
region  near  the  cathode  rebounds  from  exalted  to  depressed 
activity,  and  at  the  moment  of  stimulation  the  greater  part  of 
the  intra-polar  region  is  an  electrotonic. 

The  special  function  of  nerve  fibres  may  be  briefly  stated  to 
be  their  power  of  rapidly  intercommunicating  between  distant 
parts.  The  axis  cylinder  has  undergone  a  special  development, 
by  which  it  is  enabled  to  conduct  impulses  much  more  quickly 
than  ordinary  protoplasm.  Each  muscle  tissue  transmits  im- 
pulses about  thirty  times  more  slowly  than  a  nerve  fibre.  A 
highly-organized  animal  body,  without  nerve  fibres,  would  be  in 
a  worse  condition  than  a  highly-organized  state  without  a  tele- 
graphic or  even  a  postal  system. 

NERVE  CORPUSCLES  OR  TERMINALS. 

These  are  the  real  actors  in  the  nervous  operations,  while  the 
fibres  are  merely  their  means  of  communicating  with  one  another. 
One  set  of  terminals  is  placed  on  the  surface  of  the  body  and  is 
adapted  to  the  reception  of  the  various  external  influences  which 
are  brought  to  bear  on  it  from  without  by  its  surroundings. 
These  receivers  of  extrinsic  stimuli  are  necessarily  much  varied, 
so  as  to  be  capable  of  appreciating  all  the  different  kinds  of 
stimulation  presented  to  them.  They  are  either  distributed  over 
the  entire  surface  so  as  to  meet  with  general  mechanical  and 


514  MANUAL   OF   PHYSIOLOGY. 

thermic  changes,  or  they  are  further  specialized  for  the  reception 
of  luminous,  sonorous,  odorous  or  gustatory  impulses.  In  the 
latter  cases  the  special  terminals  are  collected  into  one  part,  and 
form  complex  organs,  which  will  be  described  presently  in  the 
chapters  on  the  special  senses. 

Another  set  of  terminals  is  placed  in  the  deeper  textures, 
wher«3  they  act  as  local  distributing  agents;  such  as  the  nerve 
plates  on  skeletal  muscles,  and  the  ganglionic  networks  in  the 
wall  of  the  intestine.  In  many  instances,  however,  the  exact 
mode  of  connection  between  the  nerve  and  the  protoplasm  of 
the  tissue  elements,  to  which  it  bears  impulses,  has  not  been  sat- 
isfactorily made  out.  In  the  remaining  class  of  nerve  terminals 
the  cells  are  grouped  together  so  as  to  form  larger  and  smaller 

FIG.  205. 


n 

Tactile  nerve  endings,  composed  of  small  capsules,  in  which  the  black  axis  cylinder  of 
the  nerve  (a),  and  (n)  meets  with  many  protoplasmic  units. 

colonies,  and  more  definitely  deserve  the  name  of  nerve  or  gan- 
glion cells.  These  are  the  central  terminals,  and  are  placed 
either  in  the  cerebro-spinal  axis,  or  in  swellings  of  the  nerves 
called  sporadic  ganglia. 

Of  these  nerve  cells  there  are  many  varieties,  all  of  which 
have  the  following  characteristics.  The  cells  are  of  considerable 
size  and  have  processes  branching  off  from  them,  by  means  of 
which  they  communicate  with  the  nerve  fibres.  These  processes 
may  be  single  or  many,  hence  they  are  spoken  of  as  uni-,  bi-,  or 
multi- polar  cells,  etc.  The  nucleus  is  commonly  very  distinct, 
and  contains  a  well-marked  nucleolus.  The  abundant  proto- 
plasm, which  is  usually  contained  in  a  delicate  cell  wall,  is  in 


THE    FUNCTIONS   OF    NERVE   CELLS. 


515 


direct  connection  with  the  axis  cylinder  of  the  nerve  fibres,  with 
which  it  communicates  by  means  of  thin  strands  of  protoplasm 
that  pass  out  from  the  cell  by  the  processes.  A  delicate  striation 
of  the  protaplasm  may  sometimes  be  recognized,  indicating  the 
course  of  the  nerve  fibrils  as  they  run  into  the  cells  from  the 
processes. 

FIG.  206. 


Multipolar  cells  from  the  anterior  gray  column  of  the  spinal  cord  of  the  dog-fish  (a) 
lying  in  a  texture  of  fibrils  :  (6)  prolongation  from  cells  ;  (c)  nerve  fibres  cut  across. 
(Cadial.) 

THE  FUNCTIONS  OF  NERVE  CELLS. 

Any  mass  of  living  protoplasm,  such  as  an  amceba,  can  receive 
extrinsic  stimuli,  which  affect  directly  its  conditions,  and  though 
the  impression  may  be  very  localized  in  its  application,  yet  all 
the  parts  of  the  cell  participate  in  the  sensation,  and  probably 
take  part  in  the  resulting  movements. 

Besides  those  acts  of  which  we  can  recognize  the  cause,  many 
others  occur  in  amoebae  which  we  are  not  able  to  trace  to  any 
definite  cause  other  than  the  energies  derived  from  its  special 
powers  of  assimilation.  We  say  that  not  only  can  an  amceba 
feel  local  stimulation,  transmit  the  impulse  to  remoter  parts  of 
its  body,  and  respond  by  movement  to  the  stimulus,  but  it  can 
also  initiate  impulses  which  appear  as  motions,  etc.,  as  the  result 
of  intrinsic  processes  of  a  chemical  nature.  We  may  conclude 


516  MANUAL   OF   PHYSIOLOGY. 

from  this  fact  alone  that  automatic  action  is  one  of  the  proper- 
ties of  protoplasm  derived  from  its  proper  chemical  activities. 

In  the  nerve  centres  of  all  the  more  complex  animals  we  find 
that  each  of  these  kinds  of  action  is  distributed  to  different  vari- 
eties of  cells,  and  thus  an  important  division  of  labor  takes  place. 
The  first  act  is  performed  by  a  wonderfully  elaborate  set  of 
special  organs  adapted  to  the  reception  of  the  various  extrinsic 
impulses  or  sensations  from  without.  The  excitation  is  then  sent 
by  nerve  fibres  to  another  group  of  central  nerve  cells,  which  are 
apparently  employed  solely  in  receiving  the  stimuli  from  the 
peripheral  organs,  and  then  distributing  the  impulses  to  their 
neighbors,  which  can  direct,  modify,  analyze,  classify,  redistri- 
bute, or  check  the  impulses,  so  that  the  higher  nerve  cells  may 
have  less  work,  and  at  the  same  time  lose  none  of  the  advantage 
that  is  to  be  gained  from  the  income  derived  from  stimulus  com- 
ing from  without.  Connected  with  the  last  group  is  another,  the 
nerve  cells  which  lie  out  of  the  reach  of  the  ordinary  peripheral 
impulses,  but  are  capable  of  developing  within  themselves  ener- 
gies, and  can  initiate  impulses  with  no  other  aid  than  that  of 
their  nutrition  and  the  chemical  changes  resulting  from  their 
assimilation. 

These  impulses  are  distributed  to  the  peripheral  active  tissues, 
muscles,  glands,  etc.,  probably  through  the  medium  of  other  sets 
of  cells  analogous  to  the  last  group  situated  in  the  nerve  centres 
as  well  as  to  the  local  distributors  which  act  as  unions  between 
the  other  textures  and  the  nerve  fibres. 

The  functions  of  nerve  cells  which  form  centres  of  action  may 
be  classified  thus : — 

1.  REFLEXION. — Many  cells  are  capable  of  reflecting  impulses 
received  from  an  afferent  nerve ;  that  is  to  say,  they  send  it  by 
an  efferent  nerve  to  some  active  tissue,  such  as  a  muscle  or  gland. 
This  kind  of  direction  is  spoken  of  as  a  simple  reflex  action.  For 
instance,  if  a  grain  of  red  pepper  be  placed  on  the  tongue,  an 
impulse  soon  travels  from  the  peripheral  receiving  terminal,  along 
an  afferent  nerve  to  its  central  terminal,  which  reflects  the  im- 
pulse to  the  efferent  nerve,  going  to  the  salivary  gland,  and  the 
result  is  an  increased  secretion  of  saliva. 


FUNCTIONS   OF   THE   NERVE   CELLS.  517 

2.  COORDINATION. — There  are  but  few  reflex  acts  that  do  not 
require  the  cooperation  of  several  cells,  and  these  work  together 
in  an  orderly  manner,  the  resulting  activity  being  well  arranged 
and  usually  adapted  to  some  purpose.  The  first  act  of  the  receiv- 
ing cells  of  a  reflex  centre  must  then  be  to  distribute  and  direct 
the  impulse  into  those  channels  which  lead  to  groups  of  cells 
capable  of  sending  impulses  in  an  orderly  and  definite  direction. 
This  directing  and  arranging  power  is  spoken  of  as  coordination, 
and  probably  is  an  attribute  common  to  all  nerve  cells. 

3.  AUGMENTATION. — The  force  of  the  reflected  efferent  impulse 
bears  a  direct  relation  to  the  afferent  impulse  as  determined  by 
the  strength  of  the  stimulus.     Thus,  if  the  amount  of  pepper  on 
the  tongue  be  much  increased,  not  only  is  the  flow  of  saliva 
greater,  but  the  excitation  spreads  from  one  central  cell  to  another 
until  the  neighboring  centres  are  affected.     Thus,  we  often  find 
the  lachrymal  glands  are  influenced  by  very  strong  stimulation 
of  the  tongue,  and  pour  out  their  secretion,  as  is  said,  "  in  sym- 
pathy "  with  the  mouth  glands.     But  the  amount  of  the  afferent 
impulse  is  not   the  only  factor  in  determining  the  energy  of 
response  to  be  reflected  along  the  efferent  channels.     Some  nerve 
cells  have  a  distinct  power  of  increasing  the  amount  of  response 
to  a  given  stimulus.     When  an  irritant  falls  near  the  mucous 
membrane  in  the  neighborhood  of  the  laryngeal  opening,  a  very 
different  result  is  produced.     The  greater  response  to  an  equal 
stimulus  in  such  cases  probably  depends  rather  on  a  peculiar 
augmenting  power  of  some  central  cells  than  upon  any  special 
local  mechanisms. 

4.  INHIBITION. — Under  certain  conditions,  which  will  be  more 
fully  explained  presently,  nerve  cells  appear  to  have  the  power 
of  restraining  the  activity  of  other  cells  or  tissues,  of  checking 
their   receptive  or  executive   power,  or  lessening  the   impulse 
reflected  so  as  to  produce  less  effect;  this  is  called  inhibition. 

5.  AUTOMATISM. — Nerve  cells  are  supposed  to  have  the  power 
of  originating  activity,  i.  e.,  discharging  impulses  without  receiv- 
ing any  exciting  impulses  from  other  nervous  agencies  that  we 
can  find  out.     Examples  may  be  found  among  those  carrying  on 
operations  which  require  to  be  of  a  more  or  less  permanent  kind, 


518  MANUAL   OF    PHYSIOLOGY. 

such  as  the  partial  contraction  of  the  muscle  cells  of  the  arteries. 
Automatic  actions  are  sometimes  classified  as  those  acting  continu- 
ously and  those  that  undergo  rhythmical  changes.  If  carefully 
examined,  most  of  the  so-called  constant  automatic  actions  will 
be  found  to  show  traces  of  rhythmic  relaxation.  The  centre 
governing  respiratory  movement  is  an  example  of  an  automatic 
group  of  cells.  Impulses  are  discharged  from  it  even  when  the 
connections  with  all  the  afferent  nerves  which  influence  it  nor- 
mally are  cut  off,  and  it  has  no  other  excitant  than  the  warm 
blood  supplying  it  with  nutriment.  Respirations  are,  however, 
normally  regulated  by  a  reflex  mechanism,  the  channels  of  which 
reside  in  the  vagus  nerve. 

In  the  nerve  cells  we  must  also  seek  mental  activity,  under  which 
term  may  be  considered  perception,  volition,  thought  and  memory. 
It  is  very  difficult  to  allocate  the  due  proportions  of  reflexion, 
coordination,  augmentation,  inhibition,  automatism,  etc.,  requisite 
for  the  development  of  mental  faculties.  In  all  probability, 
what  we  call  mental  operations  are  related  to  activities  called 
forth  as  the  resultant  of  a  long  series  of  external  and  internal 
excitations,  modified  by  intrinsic  nutritive  influences,  acting  upon 
innumerable  groups  and  complex  associations  of  nerve  cells,  the 
general  outline  of  whose  function  and  tendency  of  action,  char- 
acter, has  been  rough  hewn  by  hereditary  transmission. 


SPINAL   NERVES.  519 


CHAPTER  XXIX. 

SPECIAL  PHYSIOLOGY  OF  NERVES. 
SPINAL  NERVES. 

The  thirty-one  pairs  of  nerves  which  leave  the  vertebral  canal 
by  the  openings  between  the  vertebrae  are  called  spinal  nerves, 
in  contradistinction  to  the  cranial  nerves,  which  pass  through  the 
base  of  the  skull.  They  are  attached  to  the  spinal  marrow  by 
two  bands,  the  anterior  and  posterior  roots,  which  unite  together 
in  the  intervertebral  canal  to  form  the  trunk  of  the  nerve.  Just 
before  the  junction  of  the  two  roots  the  posterior  one  is  enlarged 
by  a  ganglionic  swelling. 

The  spinal  nerves  are  all  "  mixed  nerves,"  that  is  to  say,  they 
contain  both  efferent  and  afferent  fibres;  but  these  two  sets  of 
fibres  are  separate  in  the  roots  of  each  nerve,  the  posterior  root 
containing  only  afferent,  and  the  anterior  only  efferent  fibres. 
The  spinal  nerves  are  thus  joined  to  the  spinal  marrow  by  two 
nervous  cords,  each  one  of  which  is  functionally  distinct.  About 
seventy  years  ago  Charles  Bell  discovered  that  the  anterior  roots 
were  motor,  and  the  posterior  sensory  channels.  Hence,  the  an- 
terior are  commonly  spoken  of  as  the  motor  roots,  and  the  poste- 
rior as  the  sensory  roots  of  the  spinal  nerves.  The  experiments  to 
show  this  difference  are  simple,  but  require  delicate  manipulation. 

If  the  anterior  roots  of  the  nerves  supplying  the  hind  leg  of  a 
recently-killed  frog  be  divided,  the  muscles  of  the  limb  are  cut 
off  from  the  centres  in  the  spinal  cord,  and  the  leg  hangs  limply, 
and  does  not  move  if  pinched  when  the  frog  is  suspended  ; 
whereas  the  limb  on  the  sound  side,  upon  which  the  anterior 
roots  are  intact,  will  move  energetically  when  the  motionless  one 
is  irritated.  If  the  distal  ends  of  the  divided  anterior  roots  be 
stimulated,  the  muscles  of  the  paralyzed  limb  are  thrown  into 
action  ;  but  stimulation  of  the  proximal  end  gives  no  result.  If 
the  two  webs  of  this  frog  be  compared,  the  blood  vessels  running 


520  MANUAL    OF    PHYSIOLOGY. 

across  the  transparent  part  of  the  web  on  the  injured  side  will  be 
found  to  be  fuller  than  those  in  the  web  of  the  other  limb,  but  if 
the  distal  ends  of  the  motor  roots  be  stimulated,  the  dilated  blood 
vessels  return  to  their  normal  calibre.  By  these  experiments 
we  are  shown  that  efferent  fibres  carrying  impulses  to  the  mus- 
cular walls  of  the  vessels  are  contained  in  the  anterior  roots  of 
the  spinal  nerve,  together  with  fibres  to  the  skeletal  muscles. 

Posterior  Roots. — The  fact  that  when  the  leg  on  the  side  where 
the  anterior  roots  have  been  severed  is  stimulated,  the  animal 
moves  the  other,  is  sufficient  to  show  that  the  sensory  connections 
between  its  surface  and  the  cord  are  not  destroyed  by  cutting 
those  anterior  roots ;  and  we  may  conclude — taking  the  other 
facts  just  mentioned  into  account — that  the  afferent  fibres  are 
situated  in  the  posterior  roots. 

We  can  confirm  this  result  by  cutting  the  posterior  roots  on 
one  side  of  a  recently-killed  frog,  and  repeating  the  stimulation 
of  the  feet. 

Pinching  the  limb  whose  posterior  roots  are  cut,  gives  rise  to 
no  response,  because  the  impulses  cannot  reach  the  spinal  cord  ; 
but  stimulation  of  the  sound  foot  causes  obvious  movements  of 
both  legs.  This  shows  that  the  section  of  the  posterior  roots  of 
one  limb  cuts  off  the  afferent  (sensory)  communication  on  the 
side  operated  on,  but  that  the  efferent  (motor)  impulses  can  pass 
freely  to  the  muscles,  even  when  the  posterior  roots  are  divided, 
for  the  limb  moves  on  pinching  the  other  foot.  If  the  proximal 
ends  of  the  cut  posterior  roots  be  stimulated,  motions  are  pro- 
duced showing  that  the  centres  in  the  spinal  cord  are  influenced 
by  the  afferent  impulses  carried  by  those  posterior  roots.  If  the 
distal  ends  of  the  cut  roots  be  stimulated  no  movement  results. 

Recurrent  Fibres. — It  has  been  sometimes  found  that  stimula- 
tion of  the  anterior  roots  seemed  to  cause  pain,  as  shown  by  the 
motion  of  other  parts  besides  those  to  which  this  root  was  dis- 
tributed ;  and  it  was  believed  that  some  sensory  fibres  must  run 
in  the  anterior  roots.  But  it  has  been  found  that  if  the  corre- 
sponding posterior  roots  be  cut  these  signs  of  pain  when  the  ante- 
rior roots  are  stimulated  are  not  shown.  From  this  it  has  been 
concluded  that  the  apparent  sensory  channels  of  the  motor  roots 


SPINAL   GANGLIA. 


521 


are  nothing  more  than  some  sensory  fibres  which  pass  from  the 
nerve  trunk  a  little  way  up  the  motor  root,  and  then  turn  back 
and  descend  again  to  the  junction  of  the  roots,  whence  they  pass 
along  the  posterior  root  to  the  cord.  These  fibres  are  named  the 
"  recurrent  sensory  fibres,"  and  the  recurrent  sensibility  of  the 
anterior  roots  is  not  regarded  as  any  serious  departure  from  Bell's 
law. 

The  course  of  the  secretory,  etc.,  nerves  probably  follows 
that  of  the  motor  channels  at  their  exit  from  the  cord.  Their 
peripheral  distribution,  and  that  of  the  vasomotor  nerves,  are 


FIG.  207. 


FIG.  208. 


Two  cells  from  the  former 
seen  under  a  high  power, 

Section  through  spinal  ganglion  of  a  cat,  showing  ganglion  cells      showing  the  fine  proto- 
interspersed  between  the  fibres.    (Low  power.)  plasm  Dere  and  lhere  re, 

tracted  from  the  cell  wall. 


intimately  connected  with  the  sympathetic  system,  and  will  be 
considered  further  on. 

Of  the  function  of  the  ganglia  on  the  posterior  roots  of  the 
spinal  nerves  but  little  is  positively  known.  There  is  no  evidence 
of  their  being  centres  of  reflex  action,  nor  can  they  be  shown  to 
possess  any  marked  automatic  activity.  From  the  fact  that 
when  a  mixed  nerve  is  divided  the  end  cut  off  from  the  ganglion 
degenerates  after  a  few  days,  these  ganglia  are  supposed  to 
preside  over  the  nutrition  of  the  tissue  of  the  nerve  itself.  And 
if  the  roots  be  cut,  that  part  of  the  posterior  root  attached  to 
44 


522  MANUAL   OF   PHYSIOLOGY. 

the  cord  degenerates,  while  the  piece  connected  with  the  ganglion 
is  well  nourished.  This  is  not  the  case  if  the  anterior  root  be 
divided,  but,  on  the  contrary,  that  portion  next  the  cord  is  well 
nourished,  while  that  connected  with  the  posterior  root  is  degen- 
erated. 

It  would  thus  appear  that  the  trophic  function  of  the  ganglia 
is  restricted  to  the  sensory  nerves,  while  the  nutrition  of  the 
motor  nerves  is  provided  for  by  nervous  centres  situated  higher  up. 

THE  CRANIAL  NERVES. 

The  nerves  which  pass  out  through  the  foramina  in  the  base 
of  the  skull  must  be  considered  separately,  as  the  function  of 
each  of  them  shows  some  peculiarity.  Some  are  exclusively 
nerves  of  special  sense,  some  are  simple,  being  purely  motor  in 
function,  while  others  are  exceedingly  complex,  containing  many 
kinds  of  fibres.  They  may  be  taken  in  the  order  of  their  func- 
tional relationships,  motor  and  mixed.  Those  which  relate  to  the 
special  senses  will  be  considered  in  future  chapters. 

III.— THE  MOTOR  OCULI  NERVE. 

The  nerves  of  the  third  pair  are  efferent,  being  the  chief  motor 
nerves  of  the  eyes.  They  arise  from  the  gray  matter  on  the 
floor  and  roof  of  the  aqueduct  of  Sylvius,  pass  out  of  the  brain 
substance  near  the  pons  from  between  the  fibres  of  the  peduncle, 
and  run  between  the  posterior  cerebral  and  superior  cerebellar 
arteries.  They  pass  into  the  orbits  in  two  branches,  and  are 
distributed  to  the  following  orbital  muscles:  (1)  elevator  of  the 
eyelid,  (2)  the  superior,  (3)  inferior,  and  (4)  internal  recti,  and 
(5)  the  inferior  oblique.  They  also  contain  fibres  which  carry 
efferent  impulses  to  (1)  the  circular  muscle  of  the  iris,  and  to 
(2)  the  ciliary  muscle.  The  latter  branches  reach  the  eye  by  a 
short  twig  from  the  inferior  oblique  branch,  which  goes  to  the 
ciliary  ganglion,  and  thence  enters  the  ciliary  nerves. 

The  action  of  the  orbital  muscles  is,  in  the  main,  under  the 
control  of  the  will,  though  they  afford  good  examples  of  peculiar 
coordination  and  involuntary  association  of  movements.  The 
reflex  contraction  of  the  pupil  by  the  action  of  the  circular  muscle 


CRANIAL   NERVES.  523 

(sphincter  papillae)  is  a  bilateral  act,  the  afferent  impulse  of 
which  originates  in  the  retina,  passes  along  the  optic  nerves,  and 
is  transmitted,  from  the  corpora  quadrigemina,  to  both  the  third 
nerves.  The  central  extremities  of  the  third  nerves  must  have 
an  intimate  connection  with  each  other  and  with  the  optic  nerves, 
for  the  diminution  in  size  of  both  pupils  follows  accurately  the 
increase  in  intensity  of  the  light  to  which  even  one  of  the  retinae 
is  exposed.  In  retinal  blindness  and  after  section  of  the  optic 
nerve  the  pupil  becomes  dilated  from  loss  of  the  retinal  excita- 
tion. The  action  of  the  ciliary  muscle  may  be  said  to  be  volun- 
tary, since  we  can  voluntarily  focus  our  eyes  for  near  or  far  objects. 
Contraction  of  the  sphincter  pupillse  and  of  the  internal  rectus  is 
associated  with  the  contraction  of  the  ciliary  muscle  in  accommo- 
dation. 

Section  of  the  third  nerve  within  the  cranium  gives  rise  to  the 
following  group  of  phenomena  :  (1)  Drooping  of  the  upper  lid 
(Ptosis).  (2)  Fixedness  of  the  eye  in  the  outer  angle  (Luscitas). 
(3)  Dilatation  and  immobility  of  pupil  (Mydriasis).  (4)  Ina- 
bility to  focus  the  eye  for  short  distances. 

IV.— THE  TROCHLEAR  NERVE. 

This  thin  nervous  filament  arises  under  the  Sylvian  aqueduct, 
and  passes  into  the  superior  oblique « muscle,  to  which  it  carries 
voluntary  impulses,  which  are  involuntarily  associated  with  those 
of  the  other  muscles  moving  the  eyeball.  Paralysis  of  this  muscle 
causes  no  very  obvious  impairment  in  the  motions  of  the  eyeball 
when  the  head  is  held  straight,  but  it  is  accompanied  by  double 
vision,  so  there  must  be  some  displacement  of  the  eyeball. 
When  the  head  is  turned  on  one  side  the  eye  follows  the  position 
of  the  head  instead  of  being  held  in  its  primary  position.  In 
paralysis  of  this  nerve  a  double  image  is  seen  only  when  looking 
downward,  and  the  image  on  the  affected  side  is  oblique  and 
below  that  seen  by  the  sound  eye. 

VI.— THE  ABDUCTOR  NERVE  OP  THE  EYE. 
This  arises  in  the  floor  of  the  fourth  ventricle,  and  appears 
just  below   the   pons   Varolii.     It   is   the  motor  nerve  of  the 


524  MANUAL   OF   PHYSIOLOGY. 

external  rectus  muscle  of  the  eye.     Paralysis  or  section  of  it 
causes  inward  squint. 

VII.— (PORTIO  DURA)  MOTOR  NERVE  OF  THE  FACE. 

This  nerve  arises  from  a  gray  nucleus  in  the  floor  of  the  fourth 
ventricle.  It  passes,  with  the  other  part  of  the  seventh  (portio 
mollis)  or  auditory  nerve,  into  the  internal  auditory  meatus  of 
the  temporal  bone.  It  first  passes  out  toward  the  hiatus,  then 
turns  at  a  right  angle  to  form  a  knee-like  swelling  (geniculate 
ganglion),  and  then  runs  backward  along  the  top  of  the  inner 
wall  of  the  drum,  and  passing  downward  through  a  special 
canal  in  the  bone,  comes  out  at  the  stylo-mastoid  foramen,  and 
finally  spreads  out  on  the  side  of  the  face.  It  is  essentially  an 
efferent  nerve,  being  partly  motor  and  partly  secretory,  though 
its  connections  have  caused  afferent  functions  to  be  ascribed  to 
it.  Its  distribution  may  be  thus  briefly  summarized  : — 

(i.)  Motor  Fibres. — (1)  To  the  muscles  of  the  forehead,  eyelids, 
nose,  cheek,  mouth,  chin,  outer  ear  and  the  platysma,  which  may 
be  grouped  together  as  the  muscles  of  expression.  (2)  To  some 
muscles  of  mastication,  viz.,  buccinator,  posterior  belly  of  digas- 
tric, and  the  stylohyoid — all  the  foregoing  being  supplied  by 
external  branches — while  in  the  temporal  bone  it  gives  a  branch 
to  (3)  the  stapedius  muscle,  and  also  a  branch  from  the  geniculate 
ganglion,  named  the  great  superficial  petrosal  nerve,  which,  after 
a  circuitous  course,  is  supplied  to  the  elevator  and  azygos  muscles 
of  the  palate  and  uvula. 

(ii.~)  Secretory  Fibres.— (1)  To  the  parotid  gland  by  the  small 
superficial  petrosal  nerve,  which  sends  a  branch  to  the  otic  gan- 
glion, whence  the  fibres  pass  to  the  auriculo-temporal  nerve,  and 
then  on  to  the  gland.  (2)  To  the  submaxillary  gland  by 
the  chorda  tympani,  which,  after  having  traversed  the  tympanum, 
leaves  the  ear  by  a  fissure  at  its  anterior  extremity,  then  joins 
the  lingual  branch  of  the  fifth  to  separate  from  it  and  pass  into 
the  submaxillary  ganglion,  which  lies  in  close  relation  to  the 
gland  (compare  Figs.  64  and  65). 

(m.)  Vasomotor  or  vaso-inhibitory  influences  are  chiefly  con- 
nected with  the  motor  and  secretory  functions,  since  dilatation  of 


CRANIAL   NERVES.  525 

the  vessels  of  muscles  and  glands  accompanies  the  motion  and 
secretion  that  follows  stimulation  of  the  nerves  going  to  them. 

(t'v.)  The  following  afferent  impulses  are  said  to  travel  along 
the  track  of  the  portio  dura  and  its  branches:  (1)  Special 
taste  sensations,  which  are  chiefly  located  in  the  chorda  tympani 
branch,  may  be  explained  by  the  branches  of  communication 
which  pass  from  the  trunk  and  petrous  ganglion  of  the  glosso- 
pharyngeal  to  the  portio  dura  at  its  exit  from  the  foramen,  or 
by  the  connection  in  the  drum  of  the  ear  between  the  tympanic 
branch  of  the  glosso-pharyngeal  and  the  geniculate  ganglion  of 
the  portio  dura  through  the  lesser  superficial  petrosal  nerve. 
(2)  Ordinary  sensations,  which  are  also  located  in  the  chorda 
tympani,  are  said  to  traverse  this  nerve  in  an  afferent  direction 
until  it  comes  near  the  otic  ganglion,  when  the  sensory  fibres 
leave  the  chorda  and  pass  to  the  inferior  division  of  the  fifth 
nerve  through  the  otic  ganglion. 

Injury  of  the  facial  nerve  in  any  of  the  deeper  parts  of  its 
course  gives  rise  to  the  striking  group  of  symptoms  known  as 
facial  paralysis,  the  details  of  which  are  too  long  to  be  given 
here.  When  it  is  remembered  that  muscles  aiding  in  expres- 
sion, mastication,  deglutition,  hearing,  smelling,  and  speaking  are 
paralyzed,  and  that  taste,  salivary  secretion,  and  possibly  ordi- 
nary sensation  are  impaired,  one  can  form  some  idea  of  the  com- 
plex pathological  picture  such  a  case  presents. 

V.— N.  TRIGEMINUS,  OR  TRIFACIAL  NERVE. 
This  nerve  transmits  both  efferent  and  afferent  impulses  carried 
by  two  different  strands  of  fibres.  The  motor  part,  which  arises 
from  a  gray  nucleus  in  the  floor  of  the  fourth  ventricle,  is  much 
the  smaller  of  the  two,  and  has  been  compared  to  the  anterior 
root  of  a  spinal  nerve.  The  large  sensory  division  springs  from 
a  very  extensive  tract,  which  can  be  traced  from  the  pons  Varolii 
through  the  medulla  to  the  lower  limit  of  the  olivary  body,  and 
on  to  the  posterior  cornua  of  the  spinal  marrow.  This  set  of 
fibres  has  been  linked  to  the  posterior  root  of  a  spinal  nerve, 
being  somewhat  analogous  to  it  in  origin,  function,  and  the  fact 
that  there  is  a  large  ganglion  on  it  within  the  cranium. 


526  MANUAL   OF   PHYSIOLOGY. 

The  distribution  and  peripheral  connections  of  this  nerve  are 
somewhat  complicated,  and  should  be  carefully  studied  when  the 
manifold  functions  of  its  branches  are  being  considered.  The 
various  impulses  conveyed  by  the  trifacial  nerves  may  be  thus 
enumerated: — 

(1)  EFFERENT  FIBRES. 

1.  Motor. — To  the  muscles  of  (1)  mastication,  viz.,  temporal 
masseters,  both  pterygoids,  mylohyoid,  and  the  anterior  part  of 
the  digastrics;  (2)  to  the  tensor  muscle  of  the  soft  palate ;  and 
(3)  to  the  tensor  tympani.     (4)  In  some  animals  (rabbit)  nerve 
filaments  are  said  to  pass  to  the  iris,  reaching  the  eyeball  by  the 
ciliary  ganglion. 

2.  Secretory. — The  efferent  impulses  which  stimulate  the  cells 
of  the  lachrymal  gland  to  increased  action  pass  along  the  branches 
of  the  ophthalmic  division  of  this  nerve. 

3.  Vasomotor. — The  nerves  governing  the  muscles  of  the  blood 
vessels  of  the  eye,  of  the  lower  jaw,  and  of  the  mucous  membrane 
of  the  cheeks  and  gums. 

4.  Trophic. — On  account  of  the  impairment  of  nutrition  of  the 
eye  and  mucous  membrane  of  the  mouth,  which  occurs  after 
injury  of  fifth  nerve,  it  is  said  to  carry  fibres  which  preside  over 
the  trophic  arrangements  of  these  parts. 

(2)  AFFERENT  FIBRES. 

1.  Sensory. — All  the  divisions  of  the  trifacial  nerve  may  be 
said  to  be  connected  with  cutaneous  nerves,  by  which  the  ordinary 
sensory  impulses  are  carried  from — (1)  the  entire  skin  of  the  face, 
and  the  anterior  surface  of  the  external  ear ;  (2)  from  the  external 
auditory  meatus;  (3)  from  the  teeth  and  periosteum  of  the  jaws, 
etc.;  (4)  from  the  mucous  membrane   lining   the  cheeks,  floor 
of  the  mouth,  and  anterior  part  of  the  tongue;  (5)  from  the  lining 
membrane  of  the  nasal  cavity;  (6)  from  the  conjunctiva,  ball 
of  the  eye,  and  orbit  generally;  (7)  and  from  the  dura  mater 
including  the  tentorium. 

2.  Excito-motor. — Some  of  the  fibres  which   have  just   been 
enumerated  as  carrying  ordinary  sensory  impressions  have  special 
powers  of  exciting  coordinated  reflex  motions.    Thus  the  sensory 


AFFERENT    FIBRES.  527 

fibres  from  the  conjunctiva  and  its  neighborhood  are  the  afferent 
channels  in  the  common  reflex  acts  of  winking  and  closing  the 
eyelids ;  and  the  fibres  from  the  nasal  mucous  membrane  excite 
the  involuntary  act  of  sneezing. 

3.  Excito-secretory. — As  in  the  case  of  reflex  movements,  secre- 
tion may  be  excited  reflexly.     Fibres  carry  afferent  impulses  to 
the  medulla  from  the  anterior  part  of  the  tongue,  and  excite 
activity  of  the  salivary  glands.     Stimulation  of  the  mucous  mem- 
brane of  the  nose  or  eye  causes  impulses  to  pass  to  the  secretory 
centre  of  the  lachrymal  glands,  which  are  frequently  thus  reflexly 
excited. 

Intense  stimulation  of  almost  any  of  the  afferent  nerves  may 
excite  these  reflex  phenomena.  Thus  the  most  stoic  person  will 
experience  active  secretions  of  saliva  and  lachrymal  fluid,  as  well 
as  spasmodic  closure  of  the  lids  during  the  extraction  of  a  tooth. 
Even  the  bold  use  of  a  blunt  razor  will  cause  tears  to  flow  down 
the  cheeks,  by  sending  excito-secretory  impulses  along  the 
branches  of  the  inferior  and  superior  maxillary  division  of  this 
nerve. 

4.  Tactile  impulses  are  appreciated  by  the  anterior  part  of  the 
tongue  with  remarkable  delicacy,  and  are  conveyed  by  the  lingual 
branch  of  the  fifth  nerve ;  most  of  the  cutaneous  fibres  are  also 
capable  of  receiving  tactile  stimulation. 

5.  Taste. — The  tastes  appreciated  by  the  anterior  part  and  the 
edges  of  the  tongue  are  carried  by  fibres  which  lie  in  the  periph- 
eral branches  of  this  nerve.     These  belong  chiefly,  if  not  alto- 
gether, to  the  chorda  tyrnpani,  and  leave  this  lingual  branch  of 
the  fifth  to  join  the  seventh  nerve  on  their  way  to  the  trunk  of 
the  glosso-pharyngeal. 

There  are  four  ganglia  in  close  relation  to  the  branches  of  the 
fifth  nerve  which  have  certain  points  of  similarity,  and  may, 
therefore,  be  considered  together,  although  their  positions  show 
that  they  are  engaged  in  the  performance  of  very  different 
functions. 

We  have  not  yet  been  able  to  ascertain  the  value  of  these  little 
points  of  junction  of  motor,  sensory,  vasomotor,  and  secretory 
fibres,  because,  so  far,  we  are  unable  to  attribute  to  the  cells  of 


528  MANUAL   OF   PHYSIOLOGY. 

the  ganglia  either  reflecting  or  controlling  action,  or  any  auto- 
matic power. 

They  all  have  efferent  (motor  and  secretory)  and  afferent  (sen- 
sory) connections  with  the  nervous  centres,  and  also  connections 
with  the  main  channels  of  the  sympathetic  nerves.  These  are 
spoken  of  as  the  roots  of  the  ganglia.  Their  little  branches  are 
generally  mixed  nerves. 

THE  CILIAEY  OR  OPHTHALMIC  GANGLION. 
This  ganglion  lies  in  the  orbit.  It  has  three  roots,  which  come 
from — (1)  the  inferior  oblique  branch  of  the  third  nerve,  by  a 
short  slip,  which  forms  the  motor  root ;  (2)  from  the  nasal  branch 
of  the  ophthalmic  division  of  the  fifth,  and  (3)  from  the  caro- 
tid plexus  of  the  sympathetic.  The  branches  go  mostly  to  the 
ball  of  the  eye,  and  may  be  divided  into  afferent  and  efferent. 
The  afferent  are  sensory  branches,  connecting  the  cornea  and  its 
neighboring  conjunctiva  with  the  centres.  The  efferent  fibres 
go  to  the  iris  and  cause  dilatation  of  the  pupil  (coming  mostly 
from  the  sympathetic),  and  the  vasomotor  fibres  going  to  the 
choroid  coat,  iris,  and  retina. 

THE  SPHENO-PALATINE  OR  NASAL  GANGLION. 
This  lies  on  the  second  division  of  the  fifth  nerve,  from  which 
it  gets  its  sensory  root.  Its  motor  root  comes  from  the  seventh 
by  the  great  superficial  petrosal  nerve,  and  its  sympathetic  root 
from  the  carotid  plexus  by  the  branch  joining  this  nerve.  These 
enter  the  ganglion  together,  and  are  spoken  of  as  the  vidian 
nerve.  Afferent  (sensory)  impulses,  from  the  greater  part  of  the 
nasal  cavity,  pass  through  this  ganglion.  Its  efferent  branches 
are — (1)  motor  to  the  elevator  of  the  soft  palate  and  azygos 
uvulae ;  (2)  vasomotor,  which  comes  from  the  sympathetic  ;  and 
(3)  secretory,  which  supply  the  glands  of  the  cheek,  etc. 

OTIC  OR  EAR  GANGLION. 

The  otic  ganglion  lies  under  the  foramen  ovale,  where  the 
interior  division  of  the  fifth  comes  from  the  cranium.  Its  roots 
are — (1)  motor ;  (2)  sensory,  from  the  inferior  division  of  the  fifth ; 


SPINAL   ACCESSORY   NERVES.  529 

and  (3)  sympathetic,  made  up  of  a  couple  of  fine  filaments  from 
the  plexus,  around  the  meningeal  artery.  By  its  branches  it 
communicates  with  the  seventh,  chorda  tympani,  and  sends  fila- 
ments to  the  parotid  gland. 

THE  SUBMAXILLARY  GANGLION. 

This  is  on  the  hyoglossus  muscle  in  close  relation  to  the  lin- 
gual branch  of  the  fifth,  from  which  it  gets  a  sensory  root.  The 
chorda  tympani  passes  to  the  ganglion,  carry  ing  efferent  impulses 
through  it  to  the  gland.  Its  sympathetic  branches  come  from 
the  plexus  around  the  facial  artery. 

VIII.— THE  GLOSSO-PHARYNGEAL  NERVE. 

This  nerve  forms  part  of  the  eighth  pair,  and  springs  from  the 
floor  of  the  fourth  ventricle  above  the  nucleus  of  the  vagus.  It 
is  a  mixed  nerve,  the  functions  of  which  may  be  thus  classified: — 

Afferent  fibres,  which  are  of  various  kinds,  viz.: — 

(1)  Sensory  fibres,  carrying  impulses  from   the  anterior  sur- 
face of  the  epiglottis,  the  base  of  the  tongue,  the  soft  palate,  the 
tonsils,  the  Eustachian  tube  and  tympanum. 

(2)  Excito-motor. — This  nerve  excites  important  reflex  move- 
ments in  swallowing  and  vomiting,  when  a  stimulus  is  applied  to 
the  glosso-palatine  arch. 

(3)  Excito- secretory,  the  stimulation  of  the  back  of  the  tongue 
gives  rise  to  a  copious  flow  of  saliva  by  means  of  reflex  action. 

(4)  Taste  sensations  are,  for  the  most  part,  carried  by  this 
nerve ;  they  are  conveyed  from   special  nerve  endings  in   the 
back  of  the  tongue  (see  Taste). 

The  efferent  fibres  are  not  so  varied,  being  simply  motor  to  the 
middle  constrictor  of  the  pharynx,  stylo-pharyngeus,  elevator  of 
the  soft  palate,  and  the  azygos  uvuhe. 

THE  SPINAL  ACCESSORY  NERVES. 

These  also  form  part  of  the  eighth  pair  of  nerves,  and  arise 

from  the  medulla  oblongata  and  spinal  cord,  as  low  down  as  the 

seventh  cervical  vertebra.     The  lower  fibres  leave  the  lateral 

columns  at  their  posterior  aspect,  and  then  run  up  between  the 

45 


530  MANUAL   OF   PHYSIOLOGY. 

denticulate  ligament  and  the  posterior  roots  of  the  spinal  nerves 
to  enter  the  cranial  cavity.  On  their  way  out  of  the  cranium 
they  divide  into  two  parts,  one  of  which  becomes  amalgamated 
with  the  vagus,  and  the  other  passes  down  the  side  of  the  neck 
as  the  motor  nerve  of  the  sterno-mastoid  and  trapezius  muscles. 
Physiologically,  it  may  be  compared  with  the  anterior  root  of  a 
spinal  nerve,  and  the  part  accessory  to  the  vagus  probably  sup- 
plies that  nerve  with  most  of  its  motor  branches. 

THE  VAGUS  NERVE. 

The  vagus  arises  from  the  lower  part  of  the  floor  of  the  fourth 
ventricle,  and  is  connected  with  many  of  the  important  groups  of 
nerve  cells  in  this  neighborhood. 

The  functions  of  its  widely-distributed  fibres  may  be  thus 
briefly  stated : — 

(A)  The  EFFERENT  FIBRES  may  be  divided  into — 

1.  Motor-nerve  channels,  going  to  a  great  portion  of  the  alimen- 
tary tract  and  air  passage ;  the  following  muscles  getting  their 
motor  supply  from  the  branches  of  the  vagus — the  pharyngeal 
constrictors,  some  muscles  of  the  palate,  oesophagus,  stomach  and 
greater  part  of  the  small  intestine.  Motor  impulses  also  pass 
along  the  trunk  of  the  vagus — though  leaving  the  cord  by  the 
roots  of  the  accessory  nerve — to  the  intrinsic  muscles  of  the 
larynx ;  these  fibres  lie  in  the  inferior  or  recurrent  laryngeal 
nerve,  except  that  to  the  crico-thyroid,  which  lies  in  the  superior 
laryngeal  branch.  The  tracheal  muscle  and  the  smooth  muscle 
of  the  bronchial  walls  are  also  under  the  control  of  the  pul- 
monary branches  of  the  vagus. 

2.  Vasomotor  fibres  are  said  to  be  supplied  to  the  stomach  and 
small  intestine.     These  fibres  are  probably  derived  from  some  of 
the  numerous  connections  with  the  sympathetic. 

3.  Inhibitory  impulses  of  great  importance  for  the  regulation  of 
the  forces  of  the  circulation  pass  along  the  vagus  to  the  ganglia 
of  the  heart.     As  already  explained  in  detail  (see  page  280), 
these  fibres  are  always  acting,  as  shown  by  the  fact  that  section 
of  the  vagi  causes  a  considerable  quickening  of  the  heart  beat. 
On  the  other  hand,  if  the  distal  end  of  the  cut  vagus  be  stimu- 


VAGUS   NERVE.  531 

lated,  the  heart  beats  more  slowly,  and  in  some  animals  may 
come  to  a  standstill  in  a  condition  of  relaxation. 

(B)  The  AFFERENT  FIBRES,  still  more  widely  spread,  are  im- 
portant for  the  functions  of  the  various  viscera.  They  are  : — 

1.  Sensory  fibres  carrying  impulses  from  the  pharynx,  oesopha- 
gus, stomach  and  intestine,  and  from  the  larynx,  trachea,  bronchi 
and  lungs  generally.     The  pneumonia  which  follows  section  of 
the  vagi  depends  on — (1)  the  removal  of  sensibility,  and  the  ease 
with  which  foreign  matters  can  enter  the  air  passages ;  or  (2) 
the  violent  breathing  necessary  when  the  motor  nerves  of  the 
larynx   are    cut;  or    (3)  the   injury  of    trophic   or    vasomotor 
fibres. 

2.  Excito-motor  nerves. — There  is  no  nerve  that  can  be  com- 
pared with  the  vagus  in  the  variety  of  reflex  phenomena  in 
which  it  participates.     Afferent  fibres  in  this  nerve  cause  spasm 
of  the  muscles  of  the  glottis  and  thorax,  and  govern  the  respira- 
tory rhythm,  preside  over  inhalation  of  air  and  excite  the  expira- 
tory muscles.     Thus,  irritation  of  the  mucous  membrane  at  the 
root  of  the  tongue,  the  folds  of  the  epiglottis,  larynx,  trachea 
or  bronchi,  causes  spasmodic  fits  of  coughing.     Irritation  of  the 
pharyngeal  or  the  gastric  fibres  gives  rise,  by  reflex  stimulation, 
to  the  act  of  vomiting. 

Stimulation  of  the  proximal  cut  end  of  the  trunk  of  the  vagus 
causes  inspiratory  effort  and  cessation  of  breathing  movements 
in  the  position  of  inspiration.  Stimulation  of  the  central  cut 
end  of  the  superior  laryngeal  branch  causes  reflex  spasm  of  the 
muscles  of  the  larynx  and  a  fixation  of  the  expiratory  muscles  in 
the  position  of  expiration.  The  fibres  which  regulate  the  res- 
piratory rhythm  consist  of  two  sets,  probably  passing  from  the 
lungs  to  the  inspiratory  and  expiratory  centres,  and  causing 
each  to  act  before  its  ordinary  automatism  would  transmit  any 
discharge  of  impulse  to  the  thoracic  muscles. 

In  the  laryngeal  branches  are  fibres  which  bear  centrifugal 
impulses  to  the  vasomotor  centres  in  the  medulla,  and  excite 
them  to  action.  These,  which  may  be  grouped  with  the  excito- 
motor  channels,  are  spoken  of  as  " pressor  "  fibres,  from  the  influ- 
ence they  exert  upon  the  pressure  of  the  blood  in  the  arteries. 


532  MANUAL   OF   PHYSIOLOGY. 

3.  Excito -inhibitory  fibres  pass  from  the  heart  to  the  vasomotor 
centre.     Stimulation  of  these  fibres,  which  take  somewhat  dif- 
ferent courses  in  different  animals,  checks  the  tonic  action  of  the 
vasomotor   centre,    and   greatly   reduces    the    blood    pressure. 
Hence  these  fibres  form  the  depressor  nerve.     Its  terminals  in  the 
heart  are  stimulated  by  distention  of  that  organ;  and  the  vaso- 
motor  centre  is  thereby  inhibited,  the  arteries  dilate  and  the 
blood  pressure  falls  so  that  the  over-filled  heart  can  empty  itself. 

4.  Excito-secretory  fibres. — Stimulation  of  the  gastric  endings  of 
the  vagus  causes  not  only  gastric,  but  also  salivary  secretion, 
which  occurs  as  a  precursor  of  gastric  vomiting. 

Section  of  both  vagi  in  the  neck  causes  the  death  of  the  animal 
within  a  day  or  two  after  the  operation,  and  the -folio  wing 
changes  may  be  observed  while  it  lives :  1.  The  heart  beat  is 
much  quicker,  as  shown  by  the  increased  pulse  frequency.  2. 
The  rate  of  breathing  is  very  much  slower.  3.  Deglutition  is 
difficult,  the  food  easily  passing  into  the  air  passages  through  the 
insensitive  larynx. 

Section  of  the  superior  laryngeal  nerves  is  followed  by  slight 
slowness  of  breathing,  loss  of  sensibility  in  the  larynx,  entrance 
of  food  into  the  air  passages,  chronic  broncho-pneumonia  and 
death. 

Section  of  the  inferior  laryngeal  nerves  gives  rise  to  the  same 
final  result,  because  the  muscles  of  the  larynx  are  paralyzed,  and 
closure  of  the  glottis  is  impossible.  A  change  in  voice  follows 
the  section  or  injury  of  even  one  inferior  laryngeal,  as  may  often 
be  seen  in  man  from  the  effect  of  the  pressure  of  an  aneurism. 

IX.— HYPOGLOSSAL  NERVE. 

This  nerve  appears  in  the  furrow  between  the  olivary  body 
and  the  anterior  pyramid,  on  a  line  with  the  anterior  roots  of 
the  spinal  nerves.  It  corresponds  with  the 'anterior  roots  in 
function,  being  a  purely  motor  nerve.  It  bears  impulses  to  the 
muscles  of  the  tongue,  and  others  attached  to  the  hyoid  bone. 

Some  sensory  fibres  lie  in  its  descending  branch,  but  these  are 
probably  derived  from  the  vagus  or  tri facial  nerves,  with  which 
its  branches  inosculate. 


HYPOGLOSSAL   NERVE.  533 

It  is  also  said  to  contain  the  vasotnotor  fibres  of  the  tongue. 

Section  of  the  nerves  causes  paralysis  of  the  muscles  of  the 
tongue ;  when  this  is  unilateral,  the  tongue  inclines  to  the  injured 
side,  while  being  protruded  from  the  mouth  ;  but,  while  being 
drawn  in,  it  passes  to  the  sound  side.  This  is  easily  understood 
when  it  is  borne  in  mind  that  the  two  acts  depend  upon  the 
intrinsic  muscles  of  the  tongue,  bringing  about  an  elongation  or 
shortening  of  the  organ  respectively. 


534  MANUAL   OF    PHYSIOLOGY. 


CHAPTER  XXX. 

SPECIAL  SENSES. 

It  has  been  pointed  out  that  the  afferent  or  sensory  nerves 
receive  impressions  at  the  surface  of  the  body,  and  carry  the  im- 
pulses to  nerve  cells  in  the  brain,  where  they  give  rise  to  sensation. 
The  afferent  nerves  are  the  means  by  which  the  mind  becomes 
acquainted  with  occurrences  in  the  outer  world,  and  also  the 
channels  along  which  the  impulses  pass  to  reflex  nerve  centres 
whence  they  are  sent  to  different  parts,  without  causing  any  sen- 
sation in  the  nerve  cells  of  the  sensorium. 

The  ordinary  sensory  nerves  are  in  such  relationship  to  the 
surface  that  they  are  affected  by  slight  mechanical  and  thermal 
stimuli,  which  throw  them  into  activity  and  send  impulses  to  the 
brain.  But  we  are  capable  of  appreciating  many  other  impres- 
sions besides  those  excited  by  the  ordinary  sensory  nerves.  We 
feel  the  character  of  a  surface  by  touch,  and  we  distinguish 
between  degrees  of  heat  and  cold,  when  the  difference  is  far  too 
slight  to  act  as  a  direct  nerve  stimulus.  We  can  appreciate 
light,  of  which  no  degree  of  intensity  is  capable  of  exciting  a 
nerve  fibre  to  its  active  state,  or  of  stimulating  an  ordinary  nerve 
cell  in  the  least  degree.  We  recognize  the  delicate  air  vibrations 
called  sound,  which  would  have  no  effect  on  an  ordinary  nerve 
ending.  We  can  also  distinguish  several  tastes;  and,  finally,  we 
are  conscious  of  the  presence  of  incomprehensibly  small  quanti- 
ties of  subtle  odors  floating  in  the  air.  When  the  amount  of 
the  substance  is  too  small  to  be  recognized  even  by  spectrum 
analysis,  which  detects  extraordinarily  minute  quantities,  we  can 
perceive  an  odor  by  our  olfactory  organs. 

There  must,  then,  be  a  special  apparatus  for  the  reception  of 
each  of  these  impressions,  in  order  that  the  nervous  system  may 
be  accessible  to  such  slender  influences.  In  fact,  special  mechan- 
isms must  exist  by  means  of  which  the  quality  of  a  surface, 
heat,  light,  sound,  taste  and  odor  are  enabled  to  act  as  nerve 


SPECIAL   SENSES.  535 

stimuli.  These  nerve  terminals  are  known  as  the  special  sense 
organs,  the  physiology  of  which  is  at  the  same  time  the  most 
difficult  and  most  interesting  branch  of  study  in  Biological 
Science. 

The  nerve  fibres  which  carry  the  impulses  from  the  various 
organs  of  special  sense  do  not  differ  from  other  nervous  cords,  so 
far  as  their  structure  and  capabilities  are  concerned.  The  special 
peripheral  end  organs  are  connected  with  nerve  cells  in  the 
brain,  the  sole  duty  of  which  is  to  receive  impulses  from  a  special 
sense  organ  and  distribute  them  to  the  brain  centres,  so  that  they 
may  cause  a  special  sensation.  By  whatever  means  a  nerve  trunk 
from  a  special  sense  organ  be  stimulated,  its  impulse  excites  the 
special  sensation  usually  arising  from  stimulation  of  the  special 
organ  to  which  it  belongs.  Thus,  electric  stimulation  of  nerves 
in  the  tongue  causes  a  certain  taste;  mechanical  or  other  stimu- 
lation of  the  optic  nerve  trunk  gives  rise  to  the  sensation  of 
flashes  of  light,  and  a  distinct  odor  may  be  caused  by  the  pres- 
ence of  a  bony  growth,  pressing  upon  the  olfactory  nerve. 

The  capability  of  the  nerve  centres  connected  with  the  nerves 
of  special  sense  to  give  rise  to  a  special  sensation,  is  called  their 
specific  energy.  And  the  special  influence,  light,  sound,  etc., 
which  alone  suffices  to  excite  the  special  peripheral  terminal,  and 
which  the  given  terminal  alone  can  convert  into  a  nerve  stimulus, 
may  be  called  its  specific  or  adequate  stimulus. 

Although  we  habitually  think  of  the  sensation  as  if  coming 
from  the  surface  where  the  stimulus  is  applied,  it  is  really  only 
developed  in  the  centres  in  the  brain.  Thus  we  say  we  feel  with 
our  skin,  hear  with  our  ears,  and  see  with  our  eyes,  etc.,  whereas 
these  are  only  the  parts  from  which  the  nerve  impulses,  giving 
rise  to  the  specific  energy,  pass  to  the  feeling,  hearing  or  seeing 
regions  of  our  cerebral  cortex.  This  is  obvious  from  what  has 
been  already  said  of  the  nerve  fibres  of  the  special  sense  organs. 
If  the  nerve  be  cut,  no  sensation  is  excited,  though  adequate 
stimulus  reach  the  organ  of  special  sense;  and  if  a  stimulus  be 
applied  to  the  nerve  trunk,  a  similar  sensation  is  produced,  as  if 
the  specific  stimulation  had  operated  on  the  special  nerve  terminal 
from  which  these  fibres  habitually  carried  impulses.  This  periph- 


536  MANUAL   OF   PHYSIOLOGY. 

eral  localization  of  cutaneous  sensations  is  really  accomplished  in 
the  mind,  just  as,  by  a  mental  act  of  a  different  character,  the 
impressions  communicated  by  the  eye  are  projected  into  the  space 
about  us  in  our  thoughts,  instead  of  being  referred  to  the  retina, 
or  thought  of  as  being  produced  in  the  eye  itself.  This  power  of 
the  sensorium  to  localize  impressions  to  certain  points  of  the  skin, 
and  to  project  into  space  the  stimulation  caused  by  the  light 
reflected  from  distant  objects,  so  as  to  get  a  distinct  and  accurate 
idea  of  their  position,  is  the  result  of  experience  and  habit,  which 
teach  each  individual  that  when  a  certain  sensation  is  produced, 
it  means  the  stimulation  of  a  certain  point  of  the  skin,  and  that 
the  objects  we  see  are  not  in  our  eyes,  where  the  impulses  start, 
but  at  some  distance  from  us.  We  learn  this  from  a  long  series 
of  unconscious  experiments  carried  on  in  our  early  youth  by 
movements  of  the  eyes  with  cooperation  of  the  hands.  Even  the 
sensations  which  arise  in  the  various  centres  of  the  sensorium,  as 
the  result  of  internal  or  central  excitations,  are,  from  habit, 
attributed  to  external  influences,  and  thus  we  have  various  hallu- 
cinations and  delusions,  such  as  seeing  objects  or  hearing  sounds 
which  may  only  depend  on  the  excitation  of  certain  groups  of 
cells  in  the  cortex  of  the  brain. 

The  sensations  produced  in  our  nerve  centres  as  the  result  of 
the  afferent  impulses  corning  from  our  special  sense  organs  give 
rise  to  a  form  of  knowledge  called  perception.  Each  perception 
helps  to  make  up  our  knowledge  of  the  outer  world  and  of  our- 
selves. •  Without  this  power  of  perception  we  could  have  no 
notion  of  our  own  existence  and  no  ideas  of  our  surroundings;  in 
fact,  we  should  be  cut  off  from  all  sources  of  knowledge  and  be 
idiots  by  deprivation  of  all  intelligence  from  without. 

A  complete  special  sense  apparatus  may  be  said  to  be  made  up 
of  the  following  parts: — 

1.  A  special  nerve  ending,  only  capable  of  being  excited  by  a 
special  adequate  stimulus. 

2.  An  afferent  nerve  to  conduct  the  impulses  from  the  special 
end  organ  to  the  nerve  centre. 

3.  Central  nerve  cells,  capable  by  specific  energy  of  translating 
the  nerve  impulse  into  a  sensation,  which  is  commonly  referred  to 
some  local  point  of  the  periphery. 


SKIN    SENSATIONS.  537 

4.  Associated  nerve  centres,  capable  of  perceiving  the  sensa- 
tions, forming  notions  thereon,  and  drawing  conclusions  from  the 
present  and  past  perceptions,  as  to  the  intensity,  position,  quality, 
etc.,  of  the  external  influence. 

SKIN  SENSATIONS. 

The  sensations  arising  from  many  impulses  coming  from  the 
skin  are  grouped  together  under  the  name  of  the  Sense  of  Touch. 
This  special  sense  may  be  resolved  into  a  number  of  specific  sen- 
sations, each  of  which  might  be  considered  as  a  distinct  kind  of 
feeling,  but  is  usually  regarded  as  simply  giving  different  quali- 
ties to  the  sensations  excited  by  the  skin.  These  sensations  are: 
(1)  Tactile  Sensation,  or  sensation  proper,  by  means  of  which  we 
appreciate  a  very  gentle  contact,  recognize  the  locality  of  stimu- 
lation, arid  judge  of  the  position  and  form  of  bodies;  (2)  the 
sense  of  pressure ;  and  (3)  the  sense  of  temperature. 

The  variety  of  perceptions  derived  from  the  cutaneous  surface, 
and  the  large  extent  of  surface  capable  of  receiving  impressions, 
make  the  skin  the  most  indispensable  of  the  special  sense  organs, 
though  we  value  this  source  of  our  knowledge  but  little.  If  we 
could  not  place  our  hands  as  feelers  on  near  objects  to  investigate 
their  surfaces,  etc.,  we  should 

FIG.   209. 

lose  an  important  source  of 
information  that  has  contrib- 
uted largely  to  our  visual 
judgment.  We  think  we 
know  by  the  look  of  a  thing 
what  we  originally  learned 
by  feeling  it.  If  our  conjunc- 
tiva did  not  feel,  we  should 
miss  its  prompt  warning,  and 
our  voluntary  movements  _ 

Drawing  from  a  section  of  injected  skin,  snow- 

COllld    not    protect    Our    eyes       ing  threei*pill*fth«  central  one  containing 

.  a  tactile  corpuscle  (a),  which  is  connected 

from     many    unseen    injuries       with  a  medullated  nerte,  and  those  at  each 

*  side  are  occupied  by  vessels.    (Cadiat.) 

that  normally  never  trouble 

us.     If  the  skin  were  senseless,  it  would  require  constant  mental 

effort  to  hold  a  pen,  and  our  power  of  standing  and  walking 


538 


MANUAL   OF   PHYSIOLOGY. 


would  be  most  seriously  impaired.  And  how  utterly  cut  off  from 
the  outer  world  should  we  be,  were  we  incapable  of  feeling  heat 
and  cold. 

NERVE  ENDINGS. 

Although  the  end  organs  of  the  nerves  of  the  skin  are  the 
simplest  of  all  those  belonging  to  the  apparatus  of  special  sense, 
yet  we  have  a  very  imperfect  knowledge  of  their  immediate  rela- 
tionships to  the  different  qualities  of  touch  impressions.  We  know 
of  several  different  nerve  endings  apparently  adapted  to  the 
reception  of  certain  impressions,  but  of  the  exact  kinds  of  stimuli 
that  affect  these  different  terminals  we  are  ignorant. 


FIG.  210. 


FIG.  211. 


End  bulb  from  human  conjunctiva, 
treated  with  osraic  acid,  showing  cells 
of  core.  (Longworth.) 

a,  Nerve  fibre ;  6,  nucleus  of  sheath  ; 
c,  nerve  fibre  within  core ;  d,  cells  of 
core. 


Tactile  corpuscle  from  a  duck's  tongue, 
containing  two  tactile  cells  between 
which  lies  the  tactile  disc.  (Izquier- 
do.) 


The  peripheral  terminals  of  the  sensory  nerves,  like  the  other 
special  sense  organs,  are  usually  composed  of  modified  epithelial 
cells,  into  close  relation  to  which  the  axis  cylinders  of  nerves  can 
be  traced.  They  may  be  thus  enumerated  : — 

1.  The  Touch  corpuscles  (Meissner)  are  egg-shaped  bodies  situ- 
ated in  the  papillae  of  the  true  skin,  underlying  directly  the  epi- 
thelial cells  of  the  rete  mucosum.  They  occupy  almost  the  entire 
papilla.  The  nerve  fibres  seem  to  be  twisted  around  the  corpus- 
cle in  a  spiral  mariner,  while  the  axis  cylinders  enter  the  body, 
and  the  covering  of  the  nerve  becomes  amalgamated  with  its 
outer  wall.  The  touch  corpuscles  vary  in  size  in  different  parts 


NERVE    ENDINGS. 

of  the  skin  ;  usually  being  larger  where  the  papillae  in  which 
,hey  lie  are  well  developed.  The  axis  cylinders  are  said  to  end 
n  swellings  called  tactile  cells. 

2.  End  bulbs  (Krause)  are  smaller  than  the  last  and  are  less 
generally  distributed  over  the  surface  of  the  body,  being  localized 
;o  certain  parts.  They  are  chiefly  found  in  the  conjunctiva  and 
mucous  membranes  of  the  mouth  and  external  generative  organs. 
They  consist  of  a  little  vesicle  containing  some  fluid  ;  a  few  large 
nucleated  cells.  The  axis  cylinder  terminates  between  the  cells, 


FIG.  212. 


Drawing  of  termination  of  nerves  on  the  surface  of  the  rabbit's  cornea,  a,  Nerve  fibre 
of  sub-epithelial  network;  6,  Fine  fibres  entering  epithelium  ;  c,  Intra-epithelial  net- 
work. (Klein.) 

;he  membrane  which  forms  the  vesicle  of  the  bulb  being  fused 
with  the  sheath  of  the  nerve.  Many  different  shapes  and  varie- 
,ies  of  these  bodies  have  been  described,  but  there  seems  to  be 
no  definite  morphological  or  physiological  distinction  between 
the  varieties. 

3.  Touch  cells  (Merkel),  found  in  the  deeper  layers  of  the  epi- 
dermis of  man  as  well  as  in  the  tongues  of  birds,  are  large  cells 
of  the  epithelial  type  with  distinct  nuclei  and  nucleoli.  Fre- 
quently they  are  grouped  together  in  masses  and  surrounded  by 


540  MANUAL   OF    PHYSIOLOGY. 

a  sheath  of  connective  tissue;  in  which  condition  they  resemble 
touch  corpuscles. 

4.  Free  nerve  endings  occur  on  the  surface  of  the  epithelium  of 
the  mucous  membranes,  and  are  seen  on  the  surface'of  the  cornea 
(Cohnheim).     Here  delicate,  single  strands  of  nerve  fibrils  can 
be  seen  after  gold  staining,  passing  between  the  epithelial  cells 
and  ending  at  the  surface  in  very  minute  blunted  points  or  knobs. 

Naked  nerve  fibrils  have  also  been  traced  into  the  deeper 
layers  of  the  epidermis  of  the  skin,  where  they  end  among  the  soft 
cells  of  the  mucous  layer,  either  in  branched  cell-like  bodies 
(Langerhans)  or  delicate  loops  (Kanvier). 

In  the  subcutaneous  fat  tissue  and  in  parts  remote  from  the 
surface  some  sensory  nerves  terminate  in  large  bodies,  easily  visi- 
ble to  the  naked  eye,  called — 

5.  Pacinian  Corpuscles. — They  are  ovoid  bodies  made  up  of  a 
great  number  of  concentrically-arranged  layers  of  material,  o 
varying  consistence,  with  a  collection  of  fluid  in  the  centre,  ir 
which  an  axis  cylinder  ends.     There  is  no  doubt  that  they  an 
the  terminals  of  afferent  nerves,  but  if  they  are  connected  witt 
the  sense  of  touch,  which  is  doubtful  from  their  distribution,  it 
unknown  to  what  special  form  of  sensation  they  are  devoted 
From  their  comparatively  remote  relation  to  the  skin,  lying  som( 
distance  beneath  it  and  not  in  it,  like  the  other  endings  men 
tioned,  they  are  probably  connected  with  the  appreciation  of  pres 
sure  sensations  rather  than  those  more  properly  called  tactile. 

The  sense  of  touch  must  be  carefully  distinguished  from  ordi 
nary  sensibility  or  the  capability  of  feeling  pain,  which  is  not  ; 
special  but  a  general  sensation,  and  is  received  and  transmitte< 
by  different  nerve  channels.  This  we  know  from  the  facts,  tha 
the  mucous  passages  in  general  can  receive  and  transmit  painfu 
but  not  tactile  impressions,  and  that  in  the  spinal  cord  the  sec 
sory  and  tactile  impulses  are  probably  conveyed  by  distinc 
tracts.  Certain  narcotic  poisons  destroy  ordinary  sensation  with 
out  removing  the  sense  of  touch.  This  effect  is  also  brought  abou 
by  cold,  when  the  fingers  are  benumbed ;  gentle  contact  excite 
tactile  impressions,  while  the  ordinary  sensations  of  pain  ca 
only  be  aroused  by  severe  pressure. 


SENSE    OF    LOCALITY.  541 

However,  most  of  the  nerves  we  call  sensory  nerves  convey 
actile  impressions,  and,  speaking  generally,  those  parts  of  the 
>uter  skin  which  have  the  keenest  tactile  sense  are  also  those 
nost  ready  to  excite  feelings  of  pain. 

The  intensity  of  the  stimulation  for  the  sense  of  touch  must 
>e  kept  within  certain  limits  in  order  to  be  adequate,  i.  e.,  capable 
)f  exciting  the  specific  mental  perceptions.  If  the  stimulus 
jxceed  these  limits,  only  a  general  impression,  approaching  that 
f  pain,  is  produced.  . 

The  power  of  forming  judgments  by  touch  differs  very  much 
n  different  parts  of  the  body,  being  generally  most  keen  where 
he  surface  is  richest  in  touch  corpuscles,  namely,  the  palmar 
ispect  of  the  hands  and  feet,  and  especially  the  finger  tips,  tongue, 
ips,  and  face. 

When  we  feel  a  thing  in  order  to  learn  its  properties,  we  make 
use  of  all  the  qualities  of  which  our  sense  of  touch  is  made  up. 
We  estimate  the  number  of  points  at  which  it  impinges  on  our 
inger  tips,  rub  it  to  judge  of  smoothness,  press  it  to  find  out  its 
lardness,  and  at  the  same  time  gain  some  knowledge  of  its  tem- 
perature and  power  of  absorbing  heat. 

To  get  a  clear  idea  of  our  complex  sense  of  touch,  we  must 
consider  each  kind  of  impression  separately. 

SENSE  OF  LOCALITY. 

By  this  is  meant  our  power  of  judging  the  exact  position  of  any 
point  or  points  of  contact  which  may  be  applied  to  the  skin. 
Thus,  if  the  point  of  a  pin  be  gently  laid  on  a  sensitive  part  of  the 
kin  we  know  at  once  when  we  are  touched,  and  if  a  second  pin 
>e  applied  in  the  same  neighborhood,  we  feel  the  two  points  of 
contact  and  can  judge  of  their  relative  position.  When  we  feel 
mything,  we  receive  impulses  from  many  points  of  contact  bear- 
ng  varied  relationships  to  each  other,  and  thus  become  conscious 
)f  a  rough  or  smooth  surface. 

The  delicacy  of  the  sense  of  locality  differs  very  much  in  dif- 
ferent parts  of  the  skin.  It  is  most  accurate  in  those  parts  which 
have  been  used  as  touch  organs  during  the  slow  evolution  of  the 
animal  kingdom. 


542  MANUAL   OF   PHYSIOLOGY. 

The  method  of  testing  the  delicacy  of  the  sense  of  locality  is 
that  of  applying  the  two  points  of  a  compass  to  different  parts  of 
the  skin,  and  by  varying  their  position,  experimentally  determine 
the  nearest  distance  at  which  the  two  points  give  rise  to  distinct 
sensations.  The  following  precautions  must  be  attended  to  in 
carrying  out  this  experiment :  1.  The  points  must  be  simultane- 
ously applied,  or  two  distinct  sensations  will  be  produced  at 
abnormally  small  distances.  2.  The  force  with  which  the  points 
are -applied  must  be  equal  and  minimal,  because  excessive  pres 
sure  causes  a  diffusion  of  the  stimulus  and  a  blurring  of  the  tac 
tile  sense.  3.  Commencing  with  greater  and  gradually  reducing 
the  distance  of  the  points  enables  a  person  to  appreciate  a  less 
separation  than  if  the  smaller  distances  were  used  at  first.  4 
The  duration  of  the  stimulus;  two  points  of  contact  being  dis- 
tinguished at  a  much  nearer  distance  if  the  points  be  allowed  to 
rest  on  the  part,  than  when  they  are  only  applied  for  a  moment 
5.  The  temperature  and  material  of  the  points  should  be  the 
same.  6.  Moisture  of  the  surface  makes  it  more  sensitive.  7 
Previous  or  neighboring  stimulation  takes  from  the  accuracy  o 
the  sensations  produced.  8.  The  temperature  of  the  differen 
parts  of  the  skin  should  be  equal,  as  cold  impairs  its  sensibility 

The  following  table  gives  approximately  the  nearest  distances 
at  which  some  parts,  which  may  be  taken  as  examples  of  th( 
more  or  less  sensitive  regions  of  the  skin,  can  recognize  th( 
points  of  contact  by  their  giving  rise  to  two  distinct  sensations:— 

Tip  of  the  tongue 1  mm. 

Palmar  aspect  of  the  middle  finger  tip 2 

Tip  of  the  nose 4 

Back  of  the  hand 15 

Plantar  surface  of  great  toe 18 

Fore  arm,  anterior  surface 40 

Front  of  thigh 55 

Over  ensiform  cartilage 50     " 

Between  scapulae 70     u 

If  one  point  of  the  compass  be  applied  to  the  same  spot,  an< 
the  other  moved  round  so  as  to  mark  out  in  different  direction 
the  limits  at  which  the  points  can  be  distinguished  as  separate 
we  get  an  area  of  a  somewhat  circular  form,  for  which  the  nam 
sensory  circle  has  been  proposed.  It  would  be  convenient 


SENSE   OF    PRESSURE.          .  543 

>xplain  this  on  the  simple  anatomical  basis  that  the  impressions 
of  this  area  were  carried  by  one  nerve  fibre  to  the  brain,  and 
thus  but  one  sensation  could  be  produced  in  the  sensorium.  We 
know  this  cannot  be  the  true  explanation,  from  the  following 
facts:  1.  No  such  anatomical  relationship  is  known  to  exist.  2. 
By  practice  we  can  reduce  the  area  of  our  sensory  circles  in  a 
manner  that  could  not  be  explained  by  the  development  of  new 
nerve  fibres.  3.  If  the  two  points  of  the  compass  be  placed  near 
the  edges  of  two  well-determined  neighboring  sensory  circles, 
and  so  in  relation  with  the  terminals  of  two  nerve  fibres,  they 
will  not  give  distinct  impressions ;  they  require  to  be  separated 
as  much  as  if  they  were  applied  within  the  boundary  of  one  of 
the  circles  where  they  also  give  rise  to  the  double  perception. 

To  explain  better  the  sense  of  locality,  it  has  been  supposed 
that  sensory  circles  are  made  up  of  numerous  small  areas,  form- 
ing a  fine  mosaic  of  touch  fields,  each  of  which  is  supplied  by 
one  nerve  fibre,  and  that  a  certain  number  of  these  little  fields 
must  intervene  between  the  stimulating  points  of  the  compass  in 
order  that  the  sensorium  be  able  to  recognize  the  two  impulses  as 
distinct.  For,  although  every  touch  field  is  supplied  by  a  sepa- 
rate nerve  fibril  which  carries  its  impulses  to  the  brain,  and 
is  therefore  quite  sensitive,  the  arrangement  of  the  cells  in  the 
sensorium  is  such  that  the  stimuli  carried  from  two  adjoining 
touch  fields  are  confused  into  one  sensation.  Thus,  when  an 
edge  is  placed  on  our  skin,  we  do  not  feel  a  series  of  points  cor- 
responding to  the  individual  fields  with  which  it  comes  in  con- 
tact, but  the  confusion  of  the  stimuli  gives  rise  to  an  uninter- 
rupted sensation,  and  we  have  a  right  perception  of  the  object 
touched. 

THE  SENSE  OF  PRESSURE. 

There  seems  to  be  a  reason  for  separating  the  perception  of 
differences  in  the  degree  of  pressure  exercised  by  a  body  from  the 
simple  tactile  or  local  impression.  If  we  support  a  part  of  the 
body  so  that  no  muscular  effort  be  called  into  play  in  the  sup- 
port of  an  increasing  series  of  weights  placed  upon  the  same  area 
of  skin,  we  can  distinguish  tolerably  accurately  between  the 
different  weights.  It  has  been  found  that  if  a  weight  of  about 


544  MANUAL   OF    PHYSIOLOGY. 

30  grammes  be  placed  on  the  skin  a  difference  of  about  1 
gramme  can  be  recognized — that  is,  we  can  distinguish  between 
29  and  30  grammes,  if  they  are  applied  soon  after  one  another. 
If  the  weights  employed  are  smaller,  a  less  difference  can  be 
detected ;  if  larger  weights  are  used,  the  difference  must  be 
greater,  and  it  appears  that  the  weight  difference  always  bears 
the  same  proportion  to  the  absolute  weight  used.  We  can  per- 
ceive a  difference  between  7i  and  7i,  14J  and  15,  29  and  30,  58 
and  60,  etc.,  the  discriminating  power  decreasing  in  proportion 
as  the  absolute  degree  of  stimulation  increases. 

One  of  the  reasons  why  the  sense  of  pressure  is  regarded  as 
distinct  from  that  of  locality  is  that  the  former  is  found  not  to  be 
most  keenly  developed  in  the  parts  where  the  impressions  of 
locality  are  most  acute.  Thus,  judgment  of  pressure  can  be  more 
accurately  made  with  the  skin  of  the  fore  arm  than  the  finger 
tip,  which  is  nine  times  more  sensitive  than  the  former  to  ordi- 
nary tactile  impressions,  while  the  skin  of  the  abdomen  has. 
an  accurate  sense  of  pressure^  though  dull  to  ordinary  tactile 
sensation. 

It  has  been  said  above  that  the  weights  by  which  pressure 
sense  is  to  be  tested  should  be  applied  rapidly  one  after  the  other. 
This  fact  depends  upon  the  share  taken  in  the  mental  judgment 
by  the  function  we  call  memory.  In  a  short  time  the  recollec- 
tion of  the  impression  passes  away,  and  there  no  longer  exists  any 
sensation  with  which  the  new  stimulation  can  be  compared. 

At  best  we  can  form  but  imperfect  judgments  of  pressure  by 
the  skin  impressions  alone.  When  we  want  to  judge  the  weight 
of  a  body  we  poise  it  in  the  free  hand,  which  is  moved  up  and 
down  so  as  to  bring  the  muscles  which  elevate  it  into  repeated 
action.  Hereby  we  call  into  action  a  totally  different  evidence, 
namely,  the  amount  of  muscle  power  required  to  raise  the  weight 
in  question,  and  we  find  we  can  arrive  at  much  more  accurate 
conclusions  by  this  means.  The  peculiar  recognition  of  how  much 
muscular  effort  is  expended  is  commonly  spoken  of  as  muscle 
sense,  which  may  arise  from  a  knowledge  of  how  much  voluntary 
impulse  is  expended  in  exciting  the  muscles  to  action,  but 
more  probably  it  depends  upon  afferent  impulses  arriving  at  the 


SENSE    OF   TEMPERATURE.  545 

sensorium  from  the  muscles.  By  its  means  we  aid  the  pressure 
sense  in  arriving  at  accurate  conclusions  of  the  weight  of  bodies, 
so  that  in  the  free  hand  we  can  distinguish  between  39  grin,  and 
40  grm. 

TEMPERATUKE  SENSE. 

We  are  able  to  judge  of  the  difference  in  temperature  of  bodies 
which  come  in  contact  with  our  skin.  Since  our  sensations  have 
no  accurate  standard  for  comparison,  we  are  unable  to  form  any 
exact  conception  of  the  absolute  temperature  of  the  substances 
we  feel.  The  sensation  of  heat  or  cold,  derived  from  the  skin 
itself,  without  its  coming  into  contact  with  anything  but  air  of 
moderate  temperature,  varies  with  many  circumstances,  and 
because  of  these  variations  the  powers  of  judgment  of  high  or 
low  temperature  must  be  imperfect.  The  skin  feels  hot  when  its 
blood  vessels  are  full ;  it  feels  cold  when  they  are  comparatively 
empty.  An  object  of  constant  temperature  can  thus  give  the 
impression  of  being  hot  or  cold  according  as  the  skin  itself  is 
full  or  empty  of  warm  blood.  But,  independent  of  any  very 
material  change  in  the  blood  supply  of  the  cutaneous  surface  of 
a  part,  any  change  in  the  temperature  of  its  surroundings  causes 
a  sensation  of  change  of  temperature,  which  is,  however,  a 
purely  relative  judgment.  Thus,  if  the  hand  be  placed  in  cold 
water,  we  have  at  first  the  sensation  of  cold  ;  to  which,  however, 
the  skin  of  the  hand  soon  becomes  accustomed  and  no  longer 
feels  cold ;  if,  now,  the  hand  be  placed  in  water  somewhat  warmer 
— but  not  higher  in  temperature  than  the  atmosphere — we  have 
a  feeling  of  warmth.  If  the  hand  be  placed  in  as  hot  water  as 
the  skin  can  bear,  it  feels  at  first  unpleasantly  hot,  but  this  feel- 
ing soon  passes  away  and  the  sensation  is  comfortable.  If  from 
this  hot  water  it  be  placed  again  in  the  water  of  the  air  tempera- 
ture, this — which  before  felt  warm — feels  very  cold. 

An  important  item  in  the  estimation  of  the  temperature  of 
an  object  by  the  sensations  derived  from  the  skin  depends  upon 
whether  it  be  a  good  or  a  bad  conductor  of  heat.  Those  sub- 
stances which  are  good  conductors,  and  therefore,  when  colder 
than  the  body,  quickly  rob  the  skin  of  its  heat,  are  said  to  feel 
cold,  while  badly-conducting  bodies,  of  exactly  the  same  tempera- 
46 


546  MANUAL   OF   PHYSIOLOGY. 

ture,  do  not  feel  cold.  It  is,  then,  the  rapid  loss  of  heat  that 
gives  rise  to  the  sensation  of  cold. 

The  power  of  the  skin  in  recognizing  changes  of  temperature 
is  very  accurate,  although  the  power  of  judging  of  the  absolute 
degree  of  temperature  is  very  slight. 

By  dipping  the  finger  rapidly  into  water  of  varying  tempera- 
ture, it  has  been  found  that  the  skin  can  distinguish  between 
temperatures  which  differ,  by  only  i°  Cent,  or  £°  Fahr.  The 
time  required  for  the  arrival  of  temperature  impressions  at  the 
brain  is  remarkably  long  when  compared  with  the  rate  at  which 
ordinary  tactile  impulses  travel.  To  judge  satisfactorily  of  the 
temperature  of  an  object,  we  must  feel  it  for  some  time. 

There  must  be  special  nerve  endings  which  are  capable  of 
receiving  heat  impressions,  because  warmth  applied  to  the  nerve 
fibres  themselves  is  not  capable  of  giving  rise  to  the  sensation  of 
heat.  Thermic  stimuli,  no  doubt,  do  affect  nerve  fibres,  but  only 
cause  the  sensation  of  pain  when  applied  to  them. 

These  nerve  endings  are  not  the  same  as  those  that  receive 
touch  and  pressure  impressions,  because  the  appreciation  of  dif- 
ferences of  temperature  is  not  very  delicately  developed  in  the 
parts  where  the  tactile  sensations  are  most  acute.  Thus,  the 
cheeks  and  the  eyelids  are  especially  sensitive  to  changes  of 
temperature,  a  fact  known  by  people  who  want  a  ready  gauge  of 
the  heat  of  a  body,  e.  </.,  a  barber  approaches  the  curling  tongs 
to  his  cheek  to  measure  its  temperature  before  applying  it  to  the 
hair  of  his  client.  The  middle  of  the  chest  is  very  sensitive  to 
heat,  while  it  is  dull  in  feeling  tactile  impressions.  The  hand  is 
far  from  being  the  best  gauge  of  temperature,  for  heat  apprecia- 
tion is  not  developed  in  a  due  proportion  to  the  keenness  of  its 
tactile  sensibility.  The  larger  the  surface  exposed  to  changes  of 
temperature  the  more  accurate  the  judgment  at  which  we  can 
arrive — the  slightest  changes  being  at  once  recognized  when  the 
entire  surface  of  the  body  is  exposed  to  them.  The  foregoing 
facts  are  well  known  to  persons  in  the  habit  of  testing  the  tem- 
perature of  a  warm  bath  without  the  aid  of  a  thermometer ; 
they  do  not  use  the  limited  surface  of  a  sensitive  tactile  finger 
tip,  but  plunge  the  entire  arm  into  the  water.  The  elbow,  indeed, 


GENERAL   SENSATIONS.  547 

is  the  common  test  used  by  nurses  in  ascertaining  that  the  water 
in  which  they  are  about  to  wash  an  infant  is  not  too  warm  for 
that  purpose, 

Great  extremes  of  heat  or  cold,  such  in  fact  as  would  act  as 
stimuli  to  a  nerve  fibre,  do  not  give  rise  to  sensations  of  different 
temperatures,  but  simply  excite  feelings  of  pain.  Thus,  if  we 
plunge  our  hand  into  a  freezing  mixture  or  into  extremely  hot 
water,  it  is  difficult  to  say  at  once  whether  they  are  hot  or  cold 
— in  both  cases  pain  being  the  only  sensation  produced. 

GENERAL  SENSATIONS, 

We  call  general  sensations  those  feelings,  pleasurable  or  other- 
wise, which  can  be  excited  without  our  being  able  to  refer  them 
to  external  objects,  compare  their  sensation  with  those  of  the 
special  senses,  or  even  describe  their  exact  mode  of  perception. 
Under  this  head  are  enumerated  Pain,  Hunger,  Thirst,  Nausea, 
Giddiness,  Shivering,  Titillation,  Fatigue,  etc. 

Of  these  only  pain  is  commonly  referred  to  any  given  part, 
and  the  attempt  to  localize  pain  with  exactness  soon  shows  how 
very  different  is  our  power  in  this  respect,  in  the  case  of  pain 
and  in  the  case  of  tactile  impressions.  Thus,  when  we  strike  our 
"  funny  bone  "  (the  ulnar  nerve  passing  over  the  condyle  of  the 
humerus),  by  the  tactile  impressions  of  the  skin  we  know  the 
elbow  is  the  injured  part,  but  the  locality  of  the  pain  is  not  so 
exactly  to  be  determined,  for  it  shoots  down  the  arm  to  the  little 
finger,  and  is  indefinitely  spread  over  the  region  to  which  the 
nerve  is  distributed. 

In  studying  the  laws  which  govern  the  perception  of  painful 
impressions,  we  must  make  the  experiments  upon  ourselves,  since 
we  alone  can  form  conclusions  from  the  sensations  produced. 

The  best  way  to  carry  out  experiments  upon  pain  is  to  use 
extremes  of  temperature,  as  we  can  thus  graduate  the  stimula- 
tion. The  application  of  a  liquid  over  50°  C.,  or  below  2°  C., 
causes  pain.  The  suddenness  of  application  to  the  part,  and  its 
duration,  and  the  extent  of  surface,  as  well  as  the  previous  tem- 
perature, have  important  influence  in  the  amount  of  pain  pro- 
duced. 


548  MANUAL    OF    PHYSIOLOGY. 

The  various  kinds  of  pain  with  which  we  are  all  more  or  less 
familiar  seem  to  be  related  in  some  way  to  their  mode  of  pro- 
duction, but  we  are  unable  to  assign  any,  definite  cause  for  these 
differences  of  character.  Thus,  though  such  terms  as  shooting, 
stabbing,  burning,  throbbing,  boring,  racking,  dragging  pain, 
are  frequently  used,  and  may  be  of  diagnostic  value,  we  have 
only  an  indistinct  knowledge  that  throbbing  depends  on  exces- 
sive vascular  distention  in  a  part,  that  sharp  pains' are  produced 
by  sudden  excitation  of  a  sensitive  part,  and  the  dull  pains  by. 
the  more  permanent  stimulation  of  a  part  less  well  supplied  with 
nerves. 

Further,  pain,  as  we  think  of  it,  is  a  complex  mental  process, 
made  up  of  many  items,  such  as  real  sensory  impressions,  fear, 
disgust,  etc.  When  a  finger  is  to  be  lanced,  patients  often  cry 
out  most  loudly  before  they  are  touched  with  the  knife,  and  show 
intense  feeling  when  they  look  at  the  blood  flowing  from  the 
wound. 

Hunger  and  thirst  are  peculiar  and  indefinite  sensations  which 
are  experienced  when  some  time  has  elapsed  since  food  or  drink 
has  been  taken.  The  exact  part  of  the  nervous  system  in  which 
these  impressions  arise  has  not  been  determined.  They  are, 
however,  said  to  be  associated  with  peculiar  sensations  in  the 
stomach  and  throat  respectively.  In  the  same  way  the  venereal 
appetite,  though  associated  with  local  sensations,  cannot  be 
referred  to  any  one  part  of  the  nervous  system. 

Nausea  is  also  a  sensation  which  cannot  be  attributed  to  any 
part  of  the  nervous  centres.  It  commonly  arises  in  response 
to  afferent  impulses,  such  as  smells,  sights;  tastes,  pharyngeal, 
gastric  or  other  visceral  irritation,  and  is  antagonistic  to  the 
appetites  just  named.  All  the  sensations  that  give  rise  to  or 
precede  nausea  are  associated  in  our  minds  with  disagreeable 
impressions,  and  no  doubt  mental  operations  have  much  to  do 
with  its  production.  A  child,  free  from  affection,  may  be  heard 
to  say  of  a  castor-oil  bottle  which  in  itself  is  not  ugly,  "1  can't 
bear  to  look  at  it;  the  very  thought  of  it  makes  me  feel  sick." 

Without  any  participation  on  the  part  of  the  mental  functions, 
unavoidable  nausea  may  come  on  from  irregular  movement,  as 


GENERAL   SENSATIONS.  549 

that  of  a  ship,  which  often  causes  nausea  in  those  unaccustomed 
to  the  sea.  Certain  conditions  of  the  blood  flowing  through  the 
nerve  centres  also  causes  nausea,  as  when  emetics  are  injected 
into  the  blood. 

Giddiness,  which  consists  of  a  feeling  of  inability  to  keep  the 
normal  balance,  is  often  produced  in  connection  with  the  last  by 
irregular<  movements,  but  more  surely  by  a  rotatory  motion  of 
the  body.  Other  afferent  influences  may  give  rise  to  it,  viz.,  from 
the  stomach,  in  some  cases  of  irritation  ;  from  the  eye,  when  we 
look  from  a  height ;  from  the  semicircular  canals  of  the  ear  by 
rotation  of  the  body  ;  and  also  from  conditions  of  the  blood,  as 
in  alcoholic  toxaemia. 

Skivering  is  the  result  of  a  peculiar  nervous  effect  produced  by 
afferent  influences  of  an  unpleasant  kind,  the  sudden  application 
of  cold  to  the  skin,  a  revolting  sight,  a  shrill  noise,  or  an 
intensely  nasty  taste — all  excite  a  nervous  condition  which 
makes  us  shiver. 

Titillation  follows  tight  stimulation  of  certain  parts  of  the 
cutaneous  surfaces.  It  is  a  peculiar  general  sensation,  in  modera- 
tion not  disagreeable,  and  usually  accompanied  by  a  tendency  to 
meaningless  laughter  and  other  reflex  movements. 


550  MANUAL   OF   PHYSIOLOGY. 


CHAPTER  XXXI. 

TASTE  AND  SMELL. 
SENSE  OF  TASTE. 

Next  to  the  sense  of  touch,  which  is  unevenly  distributed  over 
the  whole  cutaneous  surface,  taste  is  anatomically  the  least  accu  - 
rately  localized.  Though  confined  to  the  mouth,  its  more 
accurate  limitations  are  not  easily  fixed.  The  point,  sides  and 
posterior  part  of  the  dorsum  of  the  tongue  can  appreciate  tastes  ; 
and  probably  parts  of  the  palate  also  have  the  power,  but  in  a 
much  less  degree.  Indeed,  though  "  the  palate"  is  often  spoken 
of  as  if  it  were  the  seat  of  taste,  it  really  enjoys  an  insignificant 
share  of  this  function  compared  with  the  tongue. 

The  power  of  being  stimulated  by  various  tastes  is  not 
restricted  to  the  terminals  of  any  one  nerve,  but  is  shared  by 
some  of  those  of  at  least  three  trunks,  which  also  transmit  im- 
pulses arising  from  other  forms  of  stimulation.  The  glosso- 
pharyngeal  division  of  the  8th  pair  sends  branches  to  the  posterior 
part  of  the  tongue,  which  are  no  doubt  connected  with  the 
special  taste  organs.  The  lingual  branches  of  the  5th — com- 
monly called  the  gustatory  nerve — have  also  terminals  capable 
of  being  excited  by  taste,  and  probably  some  fibres  of  the  chorda 
tympani  are  employed  in  this  function. 

In  the  furrows  around  the  circumvallate  papillae,  and  also,  but 
more  sparsely,  on  the  sides  of  the  fungiform  papillae  of  the 
tongue,  are  found  peculiar  organs  called  "  taste  buds  "  or  "  taste 
goblets."  They  are  imbedded  in  the  stratified  epithelium,  with 
the  cells  of  which  their  outer  layers  are  intimately  connected. 
They  are  flask-shaped  bodies,  composed  of  concentric  series  of 
modified  epithelium  cells  arranged  like  the  staves  of  a  barrel, 
pinched  together  at  the  base  and  at  the  free  surface,  where  they 
closely  surround  the  projecting  points  of  the  central  elements. 
These  consist  of  nucleated  bars,  supposed  to  be  the  nerve  ter- 
minals. The  whole  arrangement  reminds  one  somewhat  of  the 
construction  of  the  head  of  a  ripe  artichoke. 


SENSE   OF   TASTE. 


551 


Nerves  can  be  seen  entering  these  bodies,  and  are  in  all  proba- 
bility directly  connected  with  the  modified  epithelial  cells  of 


FIG.  213. 


Drawing  of  upper  surface  of  the  tongue,  showing  the  position  of  the  papillae.    1  and  2. 
Circumvallate  papillae.    3.  Fungiform  papillae.    4.  Filiform  papillae.     (Sappey.) 

which  they  are  made  up.     The  relation  of  the  glosso-pharyngeal 
nerves  to  these  taste  buds  has  been  shown  by  the  fact  that  in  the 


552 


MANUAL   OF   PHYSIOLOGY. 


rabbit  (in  which  animal  they  are  crowded  together  in  a  special 
organ  so  as  to  be  easily  found)  they  degenerate,  and  in  a  few 
months  disappear,  after  one  of  these  nerves  has  been  cut. 

The  genuine  taste  sensations  are  very  few.  Much  of  what  we 
commonly  call  taste  depends  almost  exclusively  upon  the  smell 
of  the  substance,  and  we  habitually  confuse  the  impressions 
derived  from  these  two  senses.*  The  different  tastes  have  been 
divided  into  four,  viz.,  sweet,  sour,  bitter  and  salty,  under  some 

FIG.  214. 


Section  through  depression  between  two  circumvallate  papilla?,  showing  taste  buds. 

(Cadiat.) 

a,  fibrous  tissue  of  papilla;   d  and  c,  epithelial  covering  of  papilla;   6,  taste  buds. 
On  the  right,  a,  6,  show  the  separate  cells  of  a  taste  bud. 

one  or  other  of  which  headings  all  our  tastes,  properly  so  called, 
would  naturally  fall.  Though  this  classification  has  no  just  claim 
to  being  a  chemical  one,  it  is  interesting  to  know  that  each  taste 
pretty  well  corresponds  to  a  distinct  group  of  substances  chemi- 
cally allied  one  to  the  other.  Thus,  acids  are  sour,  alkaloids  are 


*Many  of  the  comestibles,  the  taste  of  which  we  most  prize,  have  really  no  taste,  but 
only  a  smell  which  we  habitually  confound  with  taste,  having  mingled  the  experience 
obtained  from  the  two  senses.  Thus,  if  the  draft  of  air  be  carefully  excluded  from  the 
nose,  wine,  onion,  etc.,  may  easily  be  proved  to  have  no  taste.  Hence  the  familiar  rule 
of  holding  the  nose  adopted  in  taking  medicine  with  a  nasty  "  taste." 


SENSE   OF    SMELL.  553 

bitter,  the  soluble  neutral  salts  of  the  alkalies  are  salty,  and 
polyatomic  alcohols,  as  glycerine,  grape  sugar,  etc.,  are  sweet. 

These  substances  probably  act  on  the  nerve  terminals  as 
chemical  stimuli,  because  they  must  be  in  solution  to  be  appre- 
ciated. If  solid  particles  be  placed  on  the  tongue  they  must  be 
dissolved  in  the  mouth  fluid  before  they  can  excite  the  taste 
organs. 

In  order  to  explain  the  appreciation  of  the  different  tastes,  we 
may  imagine  that  there  are  different  kinds  of  terminals,  each  of 
which  is  or  is  not  influenced  by  various  substances  as  they  possess 
a  special  sweet,  sour,  bitter  or  salt  energy.  From  these  different 
terminals  pass  fibres  bearing  impulses  to  certain  central  cells, 
each  of  which  is  capable  of  exciting  a  sweet,  sour,  bitter  or 
salty  sensation,  as  the  case  may  be. 

SENSE  OF  SMELL. 

The  numerous  delicate  nerves  which  pass  from  the  olfactory 
bulb  to  the  muoous  membrane  of  the  upper  and  part  of  the 
middle  meatus  of  the  nose  form  the  special  nerves  of  smell. 
When  certain  subtle  particles  we  call  odors  come  in  contact  with 
the  terminals  of  these  nerves  they  excite  impulses  which,  on 
arriving  in  the  special  centres  of  the  brain,  give  rise  to  the 
impressions  of  smell. 

Anatomically,  the  relations  of  the  olfactory  region  are  well 
defined.  Its  mucous  membrane  is  not  covered  with  motile  cilia, 
as  is  that  of  the  rest  of  the  nasal  cavity,  and  it  is  less  vascular 
and  peculiarly  pigmented,  looking  yellow  to  the  naked  eye  when 
compared  with  the  neighboring  membrane.  The  epithelial  cells 
are  elongated  into  peculiar  cylinders,  between  which  lie  long 
thin  rods,  ending  on  the  surface  in  free  hair-like  processes.  The 
deeper  extremities  of  these  rod-shaped  filaments  expand  to  sur- 
round a  nucleus,  and  are  then  continued  into  a  network  of 
filaments,  into  which  prolongations  of  the  epithelial  cells  also 
seem  to  pass,  and  in  which  the  delicate  fibrils  of  the  olfactory 
nerve  can  be  traced.  The  existence  of  direct  communication 
between  the  nerves  and  the  rod-shaped  filaments  and  the  epithe- 
lial cells  is  satisfactorily  established  in  some  animals. 
47 


554 


MANUAL   OF   PHYSIOLOGY. 


The  odorous  particles  must  be  in  the  form  of  gases,  in  order  to 
be  carried  by  the  air  into  the  olfactory  region,  and  the  air  must 
be  kept  in  motion,  by  sniffing  it  in  and  out  of  the  nasal  cavity, 
in  order  to  excite  the  nerve  terminals,  which  are  not  influenced 
by  the  odors  of  air  absolutely  at  rest,  though  it  be  in  contact 
with  the  mucous  membrane  of  the  olfactory  tract. 

The  extreme  delicacy  of  appreciation  of  odors  by  the  olfactory 
terminals  is  very  remarkable.  Even  in  human  beings, 


nerve 


FIG.  215. 


Section  through  the  mucous  membrane  of  the  nasal  fossa  in  the  level  of  the  olfactory 

region. 

a,  Epithelial  cells  and  bundles  of  nerves;  b,  Glands  separated  from  each  other  by 
bundles  of  nerves, c.    (Cadiat.) 


whose  sense  of  smell  is  but  poorly  developed  when  compared 
with  that  of  animals,  an  amount  of  odorous  substance  can  be 
perceived  which  the  finest  chemical  tests  fail  to  appreciate.  Thus, 
Valentin  has  estimated  that  the  two-millionth  of  a  milligram  of 
musk  is  sufficient  to  excite  the  specific  energy  of  a  man's  olfac- 
tory apparatus. 

No  satisfactory  classification  of  odors  has   been    made  out. 
The  common  division  into  agreeable  and  disagreeable  smells,  or 


SENSE   OF   SMELL.  555 

scents  and  stinks,  is  dissimilar  in  different  individuals,  and  there- 
fore cannot  have  a  physiological  basis. 

With  smell,  as  with  taste,  no  degree  of  intensity  of  stimula- 
tion can  be  said  to  produce  pain,  though  disgust,  nausea,  vomit- 
ing, and  many  other  nervous  operations,  may  be  induced  by 
various  smells.  The  appetites  are  either  excited  or  annulled  by 
different  excitations  of  the  olfactory  nerves. 


556  MANUAL   OF   PHYSIOLOGY. 


CHAPTER  XXXII. 

VISION. 

Next  in  importance  to  those  impulses  which  we  receive  from 
the  skin  are  those  conveyed  to  the  brain  from  the  outer  world  by 
the  second  pair  of  cranial,  or  the  optic  nerves. 

The  ending  of  the  optic  nerve  differs  from  any  of  those  met 
with  in  the  skin,  by  being  enclosed  in  a  very  specially  arranged 
organ — the  eyeball — an  apparatus  for  bending  the  rays  of  light, 
so  that  they  exactly  reach  the  delicate  sheet  of  complicated 
nerve  ending  which  is  here  spread  out.  Only  the  blood  and 
other  tissues  of  the  eye  come  in  contact  with  the  endings  of  the 
optic  nerve,  which  are  thus  placed  out  of  the  way  of  ordinary 
nerve  stimulation. 

Further,  the  light,  of  which  the  optic  nerves  convey  intelli- 
gence to  the  brain,  is  not  properly  a  nerve  stimulus,  being  merely 
the  waving  of  an  imponderable  medium,  the  existence  of  which 
is  assumed.  Besides  the  special  arrangements  in  the  eyeball  for 
bringing  the  rays  of  light  to  bear  on  the  nerve  endings,  there 
must  here  be  some  extremely  sensitive  arrangement  by  which 
the  ether  waves,  which  we  call  light,  can  be  converted  into  a 
nerve  stimulus,  or  in  some  way  made  to  affect  the  nerve  terminals 
in  the  retina. 

By  means  of  the  sense  of  sight  we  obtain  knowledge  of  objects 
at  a  distance  from  us,  because  all  these  objects  reflect  more  or 
less  light,  and  thus  make  different  impressions  upon  the  terminals 
of  the  optic  nerve  forming  the  outer  layer  of  the  retina. 

Light,  then,  is  the  adequate  stimulus  for  the  retinal  nerve  end- 
ings, and  the  impulse  caused  by  light  is  the  only  impression  the 
optic  nerve  is  in  the  habit  of  carrying  to  our  sensoria,  where  the 
sensation  of  light  is  formed  and  distributed  among  the  cells  of 
the  brain,  so  as  to  enable  us  to  come  to  visual  judgments  and 
conclusions.  As  already  mentioned,  no  matter  what  stimulus, 
electric,  mechanical,  or  other,  be  applied  to  the  fibres  of  the  optic 


THE   TUNICS    OF   THE    EYEBALL.  557 

nerve,  the  sensation  produced  is  simply  light,  and  this  is  thought 
of  as  if  it  came  through  the  eye  from  the  outer  world. 

The  study  of  the  function  of  vision  may  be  divided  into  : — 

1.  The  path  the  light  takes  on  its  way  through  the  eye  to  reach 
the  retina. 

2.  The  molecular  changes  in  the  retina  which  give  rise  to 
stimulation  of  the  optic  nerves. 

3.  The  sensations  arising  in  the  sensorium  as  the  result  of  the 
molecular  changes  set  up  in  the  cerebral  nerve  cells  by  the  im- 
pulses from  the  optic  nerve. 

4.  The  visual  perceptions  and  judgments  which  our  conscious- 
ness is  capable  of  elaborating  from  the  visual  sensations. 

THE  TUNICS  OF  THE  EYEBALL. 

The  organ  of  vision  of  vertebrate  animals  is  enclosed  in  a  firm 
case  of  fibrous  tissues  called  the  sclerotic  coat,  which  is  continuous 
with  the  sheath  of  the  optic  nerve.  It  is  seen  between  the  eye- 
lids under  the  transparent  conjunctiva,  and  known  as  the  white 
of  the  eye.  It  gives  shape  and  protection  to  the  eye,  and  though 
translucent,  is  not  transparent.  In  front,  a  round,  window-like 
portion,  called  the  cornea,  forms  the  anterior  segment  of  this 
protecting  covering  of  the  eyeball.  The  cornea  is  distinguished 
from  the  sclerotic  not  only  by  its  glass-like  transparency,  but 
also  by  being  part  of  a  lesser  sphere  than  the  sclerotic,  and  thus 
projecting  a  little  more  than  the  rest  of  the  bulb. 

Closely  attached  to  the  inside  surface  of  the  sclerotic  is  a  soft, 
thin,  black,  vascular  sheet  of  tissue  called  the  choroid  coat,  which 
supplies  the  eyeball  with  blood.  It  is  made  up  chiefly  of  blood 
vessels  and  stellate,  pigmented,  connective  tissue  cells.  Its  outer 
layer  is  traversed  by  arteries  and  veins  of  relatively  large  size, 
and  its  inner  layer  is  composed  of  a  dense  network  of  close- 
meshed  capillary  vessels.  As  the  cornea  region  is  approached, 
the  choroid  is  peculiarly  modified  and  thrown  into  folds,  called 
ciliary  processes,  forming  a  series  of  vascular  folds,  radiating  from 
the  margin  of  the  cornea.  At  the  edge  of  the  cornea  the 
choroid  is  more  firmly  attached  to  the  sclerotic  by  a  circular 
muscle  (ciliary  muscle),  and  also  by  bands  of  tissue  from  the 


558 


MANUAL   OF   PHYSIOLOGY. 


posterior  surface  of  the  cornea,  which  hold  it  in  position  ;  the 
fibres  of  the  ciliary  muscle,  running  under  the  ciliary  processes, 
radiate  from  the  margin  of  the  cornea  toward  the  choroid,  to 
which  they  are  attached.  In  a  modified  form,  known  as  the  iris, 
this  vascular  and  pigmented  coat  of  the  eye  leaves  the  sclerotic, 
and  hangs  freely  in  a  fluid,  being  recognized  through  the  clear 


Diagram  of  a  horizontal  section  through  the  human  eye. 

1.  Cornea;  1'.  Conjunctiva;  2.  Sclerotic;  3.  Choroid;  4.  Ciliary  processes;  4'.  Ciliary 
muscle;  5.  Suspensory  ligament  of  lens ;  6.  So-calkd  posterior  chamber,  between  the 
iris  and  the  lens;  7.  Iris;  V.  Anterior  chamber  in  front  of  the  iris;  8.  Optic  nerve; 
8'.  Entrance  of  central  artery  of  the  retina;  8".  Central  depresssion  of  retina  or 
yellow  spot;  9.  Anterior  limit  of  the  retiua ;  10.  Canal  of  Petit  in  front  of  the  hyaloid 
membrane;  11.  Aqueous  chamber;  12.  Crystalline  lens;  13.  Vitreous  humor;  14.  Cir- 
cular venous  sinus  which  lies  around  the  cornea;  a — a,  anterior-posterior,  and  b— b, 
transverse  axis  of  bulb. 

cornea  as  a  colored  circular  curtain,  attached  to  the  inside  of 
the  periphery  of  the  cornea,  having  a  central  aperture,  which 
looks  black,  and  is  familiarly  known  as  the  pupil.  The  pupil  is 
merely  an  opening  in  the  iris,  which  allows  the  rays  of  light 
to  pass  into  the  interior  of  the  eyeball. 


THE   TUNICS   OF   THE    EYEBALL. 


559 


Besides  supplying  nutrition  to  the  non-vascular  central  parts 
of  the  eyeball,  the  choroid  is  useful  in  vision  by  preventing  the 
reflection  of  the  light  from  the  background  of  the  eye  in  such  a 
way  as  would  cause  irregularity  of  its  distribution,  and  thus 
dazzle  and  interfere  with  the  distinctness  of  the  image.  The 
choroid  is  elastic,  and  can  move  over  the  neighboring  sclerotic ; 
it  can  be  drawn  forward  by  the  contraction  of  the  radiating  cili- 
ary muscle,  which  acts  as  a  tensor  of  the  choroid  membrane. 

FIG.  217. 


Showing  the  ciurse  of  the  fibres  of  the  optic  nerve,  N,  as  they  pass  along  the  inner  sur- 
face of  retina,  R,  to  meet  the  ganglion  cells,  g,  whence  special  communications  pass  out- 
ward to  the  layer  of  rods  and  cones  in  the  pigment  layer, p,  next  the  choroid,  c,  within 
the  sclerotic,  s. 

The  iris  has  a  special  power  of  motion,  by  means  of  which  the 
opening  in  it  can  be  made  smaller,  so  as  to  regulate  the  amount 
of  light  admitted  to  the  eye,  and  cut  off  more  or  less  of  the  rays 
which  would  pass  through  the  margin  of  the  dioptric  media.  The 
importance  of  this  will  be  better  understood  further  on. 

Within  the  choroid  coat,  and  in  immediate  contact  with  it,  is 


560  MANUAL   OF   PHYSIOLOGY. 

the  nervous  coat,  or  retina,  formed  by  the  expansion  of  the  optic 
nerve,  which  passes  toward  the  sclerotic  obliquely,  and  enters  it 
somewhat  to  the  nasal  side  of  the  axis  of  the  eye.  The  retina 
lines  all  the  back  part  of  the  eyeball,  and  stretching  forward, 
becomes  fused  with  the  ciliary  processes,  where,  however,  the 
nervous  elements  of  the  coat  are  wanting.  The  fibrils  of  the 
optic  nerve  reach  the  inner  surface  of  the  coats  of  the  eye,  and 
lie  in  immediate  relation  to  the  transparent  medium,  which  occu- 
pies the  greater  part  of  the  bulb.  The  fibres  then  lie  internally 
to  their  terminals,  which  turn  outward  and  are  set  against  the 
choroid  coat.  The  ultimate  nerve  endings  are  situated  in  pig- 
mented  protoplasmic  cells,  which  form  the  outer  layer  of  the 
retina. 

THE  DIOPTRIC  MEDIA  OF  THE  EYEBALL. 

The  transparent  substances  which  fill  the  eyeball  are,  together 
with  the  cornea,  called  the  dioptric  media.  The  aqueous  humor 
lies  in  contact  with  the  posterior  surface  of  the  cornea,  and  just 
fills  the  prominence  which  is  formed  by  this  part  of  the  eye.  It 
is  in  this  fluid  that  the  movable  iris  is  stretched  and  separates 
the  aqueous  department  of  the  eye  into  anterior  and  posterior 
chambers.  The  vitreous  humor  occupies  much  the  larger  share 
of  the  eyeball.  It  lies  in  apposition  to  the  retina,  being  separated 
from  it  only  by  a  thin,  transparent  structure  called  the  hyaloid 
membrane,  which  encloses  the  clear,  gelatinous  vitreous  humor, 
and  is  fused  with  the  ciliary  part  of  the  retina  and  choroid.  The 
vitreous  humor  is  developed  from  the  young  connective  tissue  of 
the  mesoblast,  and  we  find  in  the  adult  that  mucus  is  the  most 
abundant  chemical  substance  in  its  texture,  though  the  branched 
cells  of  the  original  mucous  tissue  have  nearly  all  disappeared. 

The  most  important  of  the  dioptric  media  is  the  crystalline  lens. 
It  is  placed  between  the  aqueous  and  the  vitreous  humors,  just 
behind  the  iris,  which  lies  in  contact  with  its  anterior  surface.  It 
is  like  a  strong  magnifying  glass,  biconvex  in  shape,  the  poste- 
rior surface  being  more  convex  than  the  anterior.  The  lens  is 
much  harder  than  the  vitreous  humor,  but  its  outer  layers  are 
little  denser  than  a  stiff  jelly.  It  is  enclosed  by  a  firm,  elastic 
capsule,  which  is  drawn  tightly  over  the  anterior  surface,  and  influ- 


THE    DIOPTEIC    MEDIA    OF    THE    EYEBALL. 


561 


ences  its  shape.  The  lens  is  held  in  its  position  by  the  suspensory 
ligament,  a  thickened  part  of  the  hyaloid  membrane,  which  is 
continued  forward  and  attached  to  the  anterior  surface  of  the 
capsule,  near  its  margin.  The  lens  and  its  capsule,  together 


FIG.  218. 


o  ft  0 


Diagram  of  lens  viewed  from  the  side  at  different  periods  of  life,    a,  At  birth  ;  6,  Adult ; 
c,  Old  age.    (Allen  Thomson.) 

with  the  vitreous  humor,  may  be  said  to  be  enclosed  in  the  hya- 
loid membrane,  which,  in  front,  is  thickened  and  attached  to  the 
ciliary  part  of  the  choroid  and  the  capsule.  Thus,  any  tension 
exercised  by  the  suspensory  ligament  tends  to  tighten  the  ante- 

FIG.  219. 


Showing  early  stages  of  the  development  of  the  lens,    c,  Epithelial  tissue  about  to  form 
the  lens;  o,  Optic  cup;  a,  Epidermis.     (Cadiat.) 

rior  part  of  the  capsule  and  flatten  the  anterior  surface  of  the 
lens. 

The  shape  of  the  lens  varies  at  different  times  of  life,  being 
nearly  spherical  in  the  infant  and  tending  to  become  less  convex 
in  old  age  (Fig.  218;.  The  lens  is  developed  from  the  outer 


562 


MANUAL   OF   PHYSIOLOGY. 


layer  of  the  embryo  by  the  gradual  thickening  and  growing 
inward  of  the  epithelium,  which  meets  the  optic  cup,  and  after  a 
time  is  cutoff  from  the  parent  tissue.  The  stages  of  its  develop- 
ment may  be  followed  in  the  preceding  woodcuts  (Fig.  219). 

The  lens  is  composed  of  a  number  of  peculiar  band -like  cells, 
derived  from  the  epithelium.     These  are  cemented  together  in 


FIG.  220. 


A  further  stage  of  the  development  of  the  lens.    (Cadiat.) 

a,  Elongating  epithelial  cells  forming  lens;  b,  Capsule;  c,  Cutaneous  tissue  becoming 
conjunctiva;  d,  e,  Two  layers  of  optic  cup  forming  retina  ;  /,  Cell  of  mucous  tissue  of 
the  vitreous  humor;  g,  Intercellular  substance;  h,  Developing  optic  nerve. 

parallel  rows,  eccentrically  arranged  in  layers.  These  bands  are 
hexagonal  in  transverse  section,  and  in  the  younger  periods  of 
life  may  be  seen  to  contain  nuclei. 

In  the  living  state  the  lens  is  perfectly  transparent,  but  after 
death  it  becomes  slightly  opaque.  The  nutriment  for  the  adult 
lens  is  derived  from  the  vessels  of  the  choroid,  which,  however, 


THE    DIOPTRIC    MEDIA    OF    THE    EYEBALL. 


563 


do  not  come  into  direct  communication  with  its  texture.  On 
this  account  the  nutrition  of  the  lens  is  not  so  perfect  as  that  of 
many  other  tissues,  and  is  but  imperfectly  repaired  after  injury, 


Fragment  of  lens  teazed  out  to  show  the  separate  fibres.    (Cadial.)—a,  b,  and  c,  show 
fibres  .with  different  sized  nuclei. 

which  always  leaves  more  or  less  opacity.     Even  without  injury, 
opacity,  giving  rise  to  cataract,  sometimes  occurs  during  life. 


564 


MANUAL   OF   PHYSIOLOGY. 


Chemically,  the  lens  is  made  up  of  globulin,  and  furnishes  a 
ready  source  for  obtaining  this  form  of  albumin  for  examination. 

DIOPTRICS  OF  THE  EYE. 

Light  travels  through  any  even  transparent  body,  such  as  the 
atmosphere,  in  a  straight  line.  But  when  it  meets  any  change 
in  density,  particularly  when  it  has  to  pass  obliquely  into  a 

FlG.  222. 


Diagram  showing  the  course  of  parallel  rays  of  light  from  A  in  their  passage  through  a 
biconvex  lensL,  in  which  they  are  so  refracted  as  to  bend  toward  and  come  to  a  focus 
at  a  point  F. 

denser  medium,  the  ray  is  bent  so  as  to  run  in  a  direction  more 
perpendicular  to  the  surface  of  the  denser  body.  The  degree  of 
bending  or  refraction  of  the  rays  depends  on  the  difference  in 
optical  density  of  the  two  media  and  the  angle  at  which  the  ray 
strikes  the  surface  of  the  more  dense. 

Ou  its  way  to  the  sensitive  retina,   the   light   has   to 

FlG.  223. 


Diagram  showing  the  course  of  diverging  rays,  which  are  bent  to  a  point  further  from 
the  lens  than  the  parallel  rays  in  last  figure. 

through  the  various  transparent  media  just  named,  viz.,  the 
cornea,  the  aqueous  humor,  the  crystalline  lens,  ^nd  the  vitreous 
humor.  On  entering  these  media,  which  have  different  densities, 
the  rays  of  light  emitted  by  any  luminous  body  become  bent  or 
refracted,  so  that  they  are  brought  to  a  focus  on  the  retina,  just 


MEDIA    AND    REFRACTING    SURFACES.  565 

in  the  same  way  as  parallel  rays  of  light  from  the  sun  may  be 
focused  on  a  near  object  by  means  of  an  ordinary  convex  lens. 

Only  so  much  light  reaches  the  fundus  of  the  eye  as  can  pass 
through  the  opening  in  the  iris,  so  that  a  comparatively  narrow 
and  varying  beam  is  admitted  to  the  chamber  in  which  the 
nerve  endings  are  spread  out  for  its  reception. 

If  we  hold  a  biconvex  lens  at  a  certain  distance  from  the  eye 
and  look  out  of  the  window  through  it,  we  see  an  inverted  image 
of  the  landscape.  If  we  place  a  piece  of  transparent  paper 
behind  the  lens,  we  can  throw  a  representation  of  the  picture  on 
it,  which  will  be  seen  to  be  inverted.  This  power  of  convex 
lenses  is  employed  in  the  instrument  used  for  taking  photographs, 
called  a  camera,  which  consists  of  a  box  or  chamber  into  which 
the  light  is  allowed  to  pass  through  a  convex  lens,  so  that  an 
inverted  image  of  the  objects  before  it  is  thrown  upon  a  screen 
of  ground  glass  within  the  box.  When  the  sensitive  plate  re- 
places the  screen,  the  light  coming  through  the  lens  makes  a 
photograph. 

Just  in  the  same  way  an  inverted  image  of  the  things  we  look 
at  is  thrown  on  the  retina  by  the  refracting  media  of  the  eye. 
This  may  be  seen  in  a  dark  room,  if  a  candle  be  placed  at  a 
suitable  distance  in  front  of  the  cornea  of  an  eye  taken  from  a 
recently  killed  white  rabbit.  When  cleared  of  fat  and  other 
opaque  tissues,  the  sclerotic  is  transparent  enough  to  act  as  a 
screen  upon  which  the  inverted  candle  flame  can  be  recognized. 

Though  our  organ  of  vision  is  often  compared  to  a  camera 
obscura,  the  refractions  of  light  which  occur  in  it  are  far  more 
complex  than  those  taking  place  in  that  simple  instrument.  In 
the  latter  we  have  only  two  media — the  glass  lens  and  the  air  ; 
in  the  eye,  on  the  other  hand,  we  have  several,  which  are  known 
to  have  a  distinct  refractive  influence  on  the  rays  which  pass 
through  the  pupil. 

THREE  MEDIA  AND  REFRACTING  SURFACES. 
Since  the  surfaces  of  the  cornea,  however,  are  practically 
parallel,  we  may  neglect  the   difference    between   it  and   the 
aqueous  humor,  and  look  upon  the  two  as  one  medium,  having 


566  MANUAL   OF   PHYSIOLOGY. 

in  front  the  shape  of  the  anterior  surface  of  the  cornea,  and  be- 
hind, the  anterior  surface  of  the  lens,  so  as  to  form  a  concavo- 
convex  lens.  We  thus  have  only  three  media  to  consider,  viz., 

(1)  the  aqueous  humor  and  cornea ;  (2)  the  lens  and  its  capsule ; 
and  (3)  the  vitreous  humor.     And  only  three  refracting  surfaces 
need  be  enumerated,  viz.,  (1)  the  anterior  surface  of  the  cornea; 

(2)  the  anterior  surface  of  the  lens ;    and  (3)  the  posterior  sur- 
face of  the  lens. 

These  refracting  surfaces  may  all  be  looked  upon  as  portions 
of  spheres  whose  centres  lie  in  the  same  right  line,  and  hence 
may  be  said  to  have  a  common  axis.  The  eye  may  be  regarded 
as  an  optical  system,  centred  round  an  axis  which  passes  through 


FIG.  224. 


Showing  the  course  of  the  rays  of  light  from  two  luminous  points  to  the  retina.  The 
rays  from  the  point  a  on  passing  through  the  cornea,  lens,  etc.,  are  collected  on  the 
retina  at  b.  Those  from  a'  meet  at  &',  and  thus  the  lower  point  becomes  the  upper. 

the  middle  point  of  the  cornea  in  front,  and  the  central  depres- 
sion (fovea  centralis)  of  the  retina  behind.  This  is  spoken  of  as 
the  optic  axis  of  the  eye. 

The  rays  of  light  entering  the  eye  are  most  strongly  refracted 
at  the  surface  of  the  cornea,  because  they  have  to  pass  from  the 
rare  medium,  the  air,  to  the  denser  cornea  and  aqueous  humor. 
So  also  more  bending  of  the  rays  occurs  between  the  aqueous 
humor  and  the  anterior  surface  of  the  lens  than  between  the  pos- 
terior surface  of  the  lens  and  the  vitreous  humor. 

The  lens  is  not  of  the  same  density  throughout,  but  denser  in 
the  centre,  and  being  made  up  of  layers,  the  central  part  refracts 
more  than  the  outer  layers. 


MEDIA   AND   REFRACTING    SURFACES.  5(57 

The  manner  in  which  the  inversion  of  the  image  is  produced  by 
a  convex  lens  is  shown  in  the  preceding  figure,  in  which  the  lines 
correspond  to  the  rays  passing  from  two  points  through  the  lens. 
If  the  arrow  a  a'  be  taken  for  the  object,  from  either  extremity 
of  it  rays  pass  through,  and  are  more  or  less  bent  by  the  lens.  It 
will  be  sufficient  to  follow  the  course  of  three  rays  from  the  head 
of  the  arrow.  One  of  these  passes  through  the  centre  of  the  lens, 
and  leaves  it  in  the  same  direction  which  it  entered,  because  the 
two  surfaces  at  the  points  where  it  entered  and  left  may  be  re- 
garded as  parallel,  and  so  cause  no  refraction.  The  rays  which  do 
not  pass  through  the  centre  are  bent  on  entering  and  on  leaving 
the  lens,  so  that  they  all  meet  at  the  same  point  and  there  produce 
an  image  of  the  head  of  the  arrow,  at  bf.  In  the  same  way  the 
feather  end  of  the  arrow  is  produced  at  b  ;  the  position  of  the 
image  of  the  object  is  thus  reversed  by  the  light  rays  passing 
through  the  lens. 

In  a  biconvex  lens,  with  two  surfaces  of  the  same  degree  of 
convexity,  the  central  point  through  which  the  rays  pass  without 
being  refracted  is  easily  made  out,  as  it  is  the  geometrical  centre 
of  the  lens.  This  central  point  is  spoken  of  as  the  optical  centre. 
With  systems  of  lenses  of  varying  convexity,  and  of  more  than 
one  in  number,  as  we  have  in  the  eye,  where  the  rays  of  light  are 
bent  at  different  surfaces,  it  is  much  more  difficult  to  determine 
the  optical  centre.  However,  by  means  of  the  measurements 
made  by  Listing,  two  points  close  together  are  known,  which 
ma^  be  said  to  correspond  practically  with  the  optical  centres  of 
the  eye ;  they  lie  in  the  lens,  between  its  centre  and  posterior 
surface.  The  path  of  the  various  rays  may  thus  be  exactly  made 
out.* 

The  rays  which  come  from  a  distant  luminous  point  and  fall 
upon  the  eye,  are  refracted  by  the  cornea  and  aqueous  humor,  so 
as  to  be  made  convergent  on  their  way  to  the  lens  ;  they  are  then 


*  The  impossibility  of  making  clear  the  important  relationships,  such  as  nodal  points, 
and  other  constants  of  the  eye  in  a  short  text-book,  and  the  deterrent  effect  exerted  upon 
the  mind  of  a  junior  student  by  brief  incomprehensible  statements,  have  induced  the 
author  to  omit  this  part  of  the  subject.  He  must  refer  those  who  are  anxious  to  learn 
the  cardinal  points  of  the  eye,  to  the  more  advanced  text-books. 


568  MANUAL   OF    PHYSIOLOGY. 

further  bent  at  the  surfaces  of  the  lens,  so  that  they  are  brought 
exactly  to  a  point  on  the  retina.  That  is  to  say,  for  distant 
luminous  points,  the  retina  lies  exactly  in  the  plane  of  focus  of 
the  dioptric  media  of  the  normal  eye. 

This  convergence  of  the  rays  to  a  point  on  the  retina,  is  the 
first  essential  for  seeing  clear  and  distinct  images ;  for  if  the  rays 
from  each  point  of  a  luminous  body  were  not  united  on  the  retina 
as  points,  the  effects  of  the  different  rays  from  the  various  points 
of  a  body  would  become  mixed,  and  there  would  be  loss  of  defi- 
nition of  its  image. 

The  rays  from  any  bright  point  which  enter  the  eye  through 
the  pupil  may  be  imagined  to  form  a  luminous  cone,  the  point  of 
which  lies  at  the  retina,  and  its  base  at  the  pupil.  After  their 
union  at  the  point  of  the  cone,  the  rays  would  diverge  again  if 
the  retina  were  not  there  to  receive  them. 

SCHEINER'S  EXPERIMENT. 

It  may  be  seen  from  the  foregoing  figure  that  if  the  retina, 
which  normally  would  lie  at  2,  were  placed  nearer  the  dioptric 
apparatus,  say  at  1,  or  further  from  it,  at  3,  it  would  not  meet 
the  exact  point  of  the  luminous  cone,  but  would  receive  the  rays 
either  before  they  came  to  a  point,  or  after  they  had  diverged 
from  it.  Thus  indistinct  rings  of  light  would  be  seen  instead  of 
one  luminous  point,  and  an  image  would  be  blurred  and 
indefinite. 

From  this  it  follows  that  the  eye,  when  quite  passive,  can  only 
get  an  exact  image  of  bodies  which  are  placed  at  a  certain  dis- 
tance from  it,  just  as,  for  any  given  state  of  a  camera,  only  those 
bodies  in  one  plane  come  into  focus  and  give  a  clear  picture  on 
the  screen.  If  the  dioptric  apparatus  of  the  eye  were  rigid  and 
unalterable,  since  the  relation  of  the  retina  to  it  is  permanently 
the  same,  we  could  only  see  those  objects  clearly  which  are  at  a 
given  distance  from  the  eye.  We  know,  however,  that  we  see  as 
distinct  an  image  of  distant  as  of  near  objects,  and  we  can  look 
through  the  window  at  a  distant  tree,  or  can  adjust  our  eyes  so 
as  to  see  a  fly  walking  on  the  window  pane.  We  cannot  see  both 
distinctly  at  the  same  moment.  This  power  of  focusing  may 


SCHEINER'S  EXPERIMENT. 


569 


experiment, 


FIG.  225. 


To    illustrate    Schemer's    experiment; 
explanation,  see  text. 


for 


be  demonstrated  by  what  is  known  as  Schemer's 
which  is  carried  out  in  the  following  way. 

Two  pin  holes  are  made  in 
a  card  at  a  distance  from  each 
other  not  wider  than  the  diam- 
eter of  the  pupil.  The  card  is 
then  brought  close  to  the  eye, 
so  that  a  small  object — such 
as  the  head  of  a  bright  pin — 
can  be  seen  through  the  holes. 
The  dioptric  media  being  fixed, 
moving  the  object  nearer  to  or 
further  from  the  eye  would  have 
the  same  effect  as  changing  the 

relation  of  the  retina  to  m  n  or  p  q  in  Fig.  225,  by  means  of  which 
we  may  explain  the  following  observations:  (1)  The  eye  being 
fixed  upon  the  object  (of  which  only  one  image  is  seen),  move 
the  pin  rapidly  away ;  two  objects  now  appear,  showing  that  the 
rays  coming  through  the  holes  have  met  before  they  reach  the 
retina  as  at  p  q.  (2)  Move  the  pin  near  the  eye ;  again  two  very 
blurred  objects  are  seen,  for  the  rays  have  not  met  when  they 
strike  the  retina,  as  at  m  n.  (3)  Keeping  the  object  in  the  same 
position,  alter  the  gaze,  as  if  to  look  first  at  distant  and  then  at 
near  objects  ;  in  both  extremes  two  images  are  seen.  (4)  When 
the  object  is  in  exact  focus,  as  at  c,  the  closure  of  one  of  the  holes 
does  not  affect  the  single  image.  (5)  When  two  images  are 
seen,  closing  the  right-hand  hole  at  g  causes  the  right  or  left 
image  to  disappear,  according  as  the  focus  c  falls  short  ofmn 
or  is  beyond  p  q,  the  retina.  (6)  By  moving  the  pin's  head 
nearer  the  eye,  a  point  is  reached  at  which  the  object  cannot  be 
brought  to  a  focus  as  a  single  image.  This  limit  of  near  accom- 
modation marks  the  near  point.  A  little  attention  teaches  us 
that  looking  at  the  near  object  requires  an  effort  which  looking 
at  the  distant  one  does  not ;  in  fact,  we  have  to  do  something  to 
see  things  near  us  distinctly.  This  act  is  the  voluntary  adjust- 
ment of  the  eye  which  we  call  its  accommodation  for  near 
vision. 

48 


570  MANUAL   OF    PHYSIOLOGY. 

ACCOMMODATION. 

The  difference  of  distance  for  which  we  can  adjust  our  eyes  is 
great,  so  that  our  range  of  distinct  vision  is  very  extensive.  As 
already  stated,  the  normal  eye  is  considered  to  be  constructed  so 
that  parallel  rays  of  light,  i.  e.,  those  coming  from  practically 
infinite  distance,  are  brought  to  a  focus  on  the  retina.  This  is 
why  we  see  the  stars — which  are  practically  infinitely  remote 
from  us— as  mere  luminous  points.  It  is,  therefore,  impossible 
to  fix  a  far  limit  to  our  power  of  distant  vision.  The  nearer  an 
object  is  brought  to  our  eyes,  the  more  effort  is  required  to  see  it 
distinctly,  until  at  last  a  point  is  reached  where  we  cannot  get  a 
clear  outline,  no  matter  how  we  "strain  our  eyes."  For  a  nor- 
mal eye,  called  the  emmetropic  eye,  this  near  limit  is  about  12  cm. 
or  5  inches,  but  it  varies  in  different  individuals. 

For  objects  that  are  over  10  metres  distance,  very  little  change 
in  the  eye  is  required  to  see  each  distinctly,  and  the  nearer  the 
object  approaches,  the  more  frequently  the  adjustment  of  the  eye 
has  to  be  altered  to  see  it  clearly.  When  the  eye  is  focused  for 
any  point  within  the  limits  of  distinct  vision,  a  certain  range  of 
objects  at  different  distances  from  the  eye  can  be  recognized 
without  moving  the  adjustment.  The  range  of  this  power  is 
measured  on  the  line  of  vision,  and  called  the  focal  depth.  In 
the  distance  we  can  take  in  a  great  depth  of  landscape,  without 
effort  or  fatigue ;  but  when  looking  at  near  objects  the  focal 
depth  is  less,  and  we  must  constantly  accommodate  onr  eyes 
afresh  in  order  to  see  clearly  objects  at  slightly  different  distances 
because  of  the  shallowness  of  the  focal  depth  in  the  nearer  parts 
of  visual  distance. 

The  method  by  which  the  accommodation  of  the  eye  is  effected 
differs  from  anything  that  can  be  applied  to  an  artificial  optical 
instrument,  and  is  more  perfect. 

The  following  alterations  are  observed  to  occur  in  the  eye 
during  active  accommodation,  i.  e.,  when  looking  at  near  objects : 
(1)  The  iris  contracts  so  that  the  pupil  becomes  smaller ;  (2)  the 
central  part  of  the  anterior  surface  of  the  crystalline  lens  moves 
slightly  forward,  pushing  before  it  the  pupillary  margin  of  the 
iris,  so  that  the  lens  becomes  more  convex;  (3)  the  posterior 


MECHANISM   OF    ACCOMMODATION, 


571 


surface  of  the  lens  also  becomes  more  convex,  owing  to  the  general 
change  of  shape.of  the  lens,  but  the  centre  of  this  surface  does 
not  change  its  position  ;  (4)  both  eyes  converge. 

These  changes  can  be  seen  in  the  accompanying  diagram, 
showing  a  section  of  the  lens,  cornea  and  ciliary  region  (Fig. 
226),  in  the  left-hand  side  of  which  the  lens  is  drawn  in  the 
position  it  assumes  when  accommodated  for  near  objects.  These 
movements  can  be  seen  in  life  by  observing  the  changes  in  rela- 
tive positions,  etc.,  of  the  reflections  of  a  candle  flame  thrown 
from  the  cornea  and  the  two  surfaces  of  the  lens.  On  the  cor- 
nea is  seen  a  bright  upright  flame ;  next  comes  a  large  diffused 
reflection  from  the  anterior  surface  of  the  lens,  and  at  the  other 

FlG.  226. 


Diagram  showing  the  changes  in  the  lens  during  accommodation.  The  muscle  on  the 
right  in  supposed  to  be  passive  as  in  looking  at  distant  objects,  the  ligament  (L)  is, 
therefore,  tight,  anl  compresses  the  anterior  surface  of  the  lens  (A)  so  as  to  flatten  it. 
On  the  left,  the  ciliary  muscle  (M)  is  contracting  so  as  to  relax  the  ligament,  which 
allows  the  lens  to  become  more  convex.  This  contraction  occurs  when  looking  at  near 
objects. 

side  of  this  a  small,  inverted  image  of  the  flame  reflected  from 
the  posterior  surface  of  the  lens.  When  the  adjustment  is 
changed  by  looking  from  a  far  to  a  near  object,  the  image  on 
the  front  of  the  lens  becomes  smaller  and  moves  toward  the 
centre  of  the  pupil.  The  image  on  the  back  of  the  lens  also 
becomes  smaller,  but  does  not  change  its  position.  The  amount 
of  movement  has  been  accurately  measured  by  a  special  instru- 
ment called  an  ophtfialmometer.  The  motions  can  be  more  exactly 
studied  by  means  of  the  phakoscope,  a  dark  box,  in  which  prisms 
are  placed  before  the  observed  eye,  and  each  image  is  made 
double.  The  change  in  relative  position  of  the  two  is  more 
readily  recognized  than  a  mere  change  of  size  of  the  one. 


572  MANUAL   OF   PHYSIOLOGY. 

Muscular  Mechanism  of  Accommodation. — The  alteration  in  the 
shape  of  the  lens  is  accomplished  by  the  action.of  the  muscular 
layer,  already  named,  which  radiates  from  the  edge  of  the  cornea 
to  the  ciliary  region  of  the  choroid  coat,  where  it  is  attached. 
When  the  ciliary  muscle  contracts,  it  draws  the  choroid  coat  and 
the  connections  of  the  suspensory  ligament  of  the  lens  slightly 
forward,  the  junction  of  the  cornea  and  sclerotic  being  its  fixed 
point.  Under  ordinary  circumstances,  the  eye  being  at  rest,  the 
suspensory  ligament  is  tense  and  exerts  a  radial  traction  on  the 
anterior  part  of  the  capsule  of  the  lens,  tending  to  stretch  it  flat; 
this  affects  the  shape  of  the  soft  lens  and  reduces  its  convexity. 
When  the  ciliary  muscle  shortens,  it  draws  forward  the  attach- 
ment of  the  suspensory  ligament,  relaxes  it,  and  removes  the  ten- 
sion of  the  capsule,  so  that  the  unconstrained  elastic  lens  bulges 
into  its  natural  form.  The  posterior  surface  cannot  extend  back- 
ward, because  there  it  is  in  contact  with  the  vitreous  humor, 
which  is  held  more  firmly  against  it  by  the  increased  tension 
of  the  hyaloid  membrane  during  the  contraction  of  the  ciliary 
muscle. 

Some  circular  muscular  fibres  help  to  relax  the  ligament  and 
relieve  it  from  the  increased  pressure  which  the  contraction  of 
the  radiating  fibres  must  indirectly  cause  on  the  vitreous  humor. 

The  act  of  accommodation  is  a  voluntary  one,  the  nerve  bear- 
ing the  impulse  to  the  ciliary  and  iris  muscles,  coming  from  the 
3d  nerve  by  the  ciliary  branches  of  the  lenticular  ganglion.  The 
local  application  of  the  alkaloid  of  the  belladonna  plant  (atropin) 
causes  paralysis  of  the  ciliary  muscle  and  wide  dilatation  of  the 
pupil ;  and  the  alkaloid  of  the  Calabar  bean  (physostigmin) 
produces  contraction  of  the  muscle  of  accommodation  and 
extreme  contraction  of  the  pupil. 

DEFECTS   OF  ACCOMMODATION. 

Myopia. — It  has  been  said  that  the  "near  limit"  of  distinct 
vision  differs  in  many  persons  from  the  twelve  centimeters  of  the 
normal  emmetropic  eye,  and  it  is  found  that  the  power  of  accom- 
modation varies  very  much  in  different  individuals.  Thus,  in 
"short-sighted"  people,  who  have  myopic  eyes,  i.e.,  in  which 


DEFECTS    OF   THE    EYE.  573 

parallel  rays  are  focused  short  of  the  retina,  the  near  limit  may 
only  be  half  the  normal,  i.e.,  five  centimeters,  and  the  far  limit, 
which  is  normally  indefinite,  is  found  to  be  within  a  compara- 
tively short  distance  of  the  eye.  They,  therefore,  cannot  see  dis- 
tant objects  clearly,  since  the  rays  are  focused  before  the  retina 
is  reached,  and  then  diverging,  cause  diffusion  circles  and  a 
blurred  picture.  The  work  of  their  accommodation  is  also  much 
more  laborious,  since  they  can  only  see  in  that  part  of  the  range 
of  accommodation  where  the  adjustment  has  to  be  altered  for 
slight  variations  of  distance.  The  defect  can  be  made  much  less 
distressing  by  the  use  of  concave  glasses,  which  make  parallel 
rays  strike  the  cornea  as  divergent  ones,  and  thus  allow  them  to 
be  focused  on  the  retina. 

Hypermetropia. — Another  abnormality  is  "  long  sight."    In  the 


FIG.  227. 


Showing  the  course  of  the  rays  ofliglit  from  two  luminous  points  to  the  retina.  The 
rays  from  the  point  a  on  passing  through  the  cornea,  lens,  etc.,  are  collected  on  the 
retina  at  b.  Those  from  a'  meet  &',  and  thus  the  lower  point  becomes  the  upper. 

hypermetropic  eye,  parallel  rays  of  light  are  brought  to  a  focus 
at  a  point  beyond  the  retina,  so  that  divergent  or  parallel  rays 
cause  diffusion  circles  and  a  blurred  image.  This  may  be  cor- 
rected by  means  of  convex  glasses,  which  make  the  rays  conver- 
gent before  they  strike  the  corneal  surface,  and  thus  enable  them 
to  be  sooner  brought  to  a  focus  by  the  dioptric  media  of  the  eye. 
Presbyopia  is  the  name  given  to  a  change  in  the  perfectness 
of  accommodation  frequently  accompanying  old  age.  The  lens 
probably  becomes  less  elastic  and  the  ciliary  muscle  weaker,  so 
that  the  change  in  form  required  to  see  near  objects  is  difficult 
or  impossible  to  attain.  Biconvex  lenses  help  to  overcome  the 
difficulty. 


574  MANUAL  OF  PHYSIOLOGY. 

DEFECTS  OF  DIOPTRIC  APPARATUS. 

Ill  common  with  all  dioptric  instruments  the  eye  has  certain 
optical  defects  which  tend  to  interfere  with  the  distinctness  of 
the  image. 

Chromatic  aberration  is  due  to  the  breaking  up  of  white  light 
into  several  colors,  owing  to  the  different  colored  rays  of  which 
ordinary  light  is  composed,  possessing  different  degrees  of  re- 
frangibility.  We  see  this  in  the  spectrum  and  in  the  colored 
rings  of  the  marginal  part  of  a  biconvex  lens  made  of  a  single 
kind  of  glass.  This  form  of  aberration  can  be  corrected  by  mak- 
ing lenses  of  two  kinds  of  glass,  one  of  which  counteracts  the 
dispersion  caused  by  the  other.  Optical  instruments  may  thus 
be  made  achromatic.  This  defect  is  minimized  by  the  iris,  which 
cuts  off  the  marginal  rays  in  which  it  is  most  apt  to  occur.  Pos- 
sibly the  different  density  of  the  various  parts  of  the  dioptric 
media  may  have  a  correcting  effect  on  the  chromatism  of  the 
eye.  Further  correction  takes  place  in  the  nerve  centres  which 
receive  the  sensation,  for  just  as  we  mentally  rein  vert  the  image, 
we  are  unconscious  of  the  color.  At  any  rate,  the  chromatic 
aberration  is  so  slight  that  it  needs  certain  artifices  to  make  it 
observable. 

Spherical  aberration  depends  upon  the  fact  that  luminous 
rays,  on  passing  through  a  convex  lens,  strike  the  various  parts 
of  its  surface  at  different  angles,  and  hence  are  differently  re- 
fracted. The  rays  striking  the  margin  of  the  lens  are  more  bent 
than  those  passing  through  the  centre,  and  hence  the  former 
come  sooner  to  a  focus.  Thus,  a  luminous  point  gives  rise  to  a 
diffused  figure,  which  is  circular  in  perfectly  centred  dioptric 
systems,  but  is  stellate  in  our  eyes  where  the  centering  of  the 
lenses  is  not  absolutely  accurate.  Spherical  aberration  causes  us 
no  inconvenience,  as  the  iris  only  allows  the  more  central  rays 
to  pass,  in  which  its  influence  is  not  noticed. 

Another  optical  defect  in  our  eyes  is  astigmatism,  depending 
upon  some  irregularity  of  the  curvature  of  the  cornea,  which 
may  be  bent  more  horizontally  than  vertically,  or  vice  versa.  In 
either  of  these  cases  the  light  in  the  vertical  and  horizontal 
planes  will  be  differently  refracted,  so  that  lines  drawn  in  the 


MUSCLES   OF   THE   IRIS.  575 

two  directions  will  require  different  adjustments  to  see  them  dis- 
tinctly. This  may  be  recognized  if  we  gaze  with  one  eye  at  a 
centre  from  which  many  sharply-defined  lines  radiate ;  some  of 
the  lines  cannot  be  seen  distinctly,  unless  we  move  the  eye  or 
change  its  accommodation.  When  the  greater  curvature  extends 
evenly  over  the  whole  diameter  of  the  cornea  it  gives  rise  to 
what  is  called  regular  astigmatism,  and  when  the  unevenness  is 
localized  to  one  part  of  the  cornea  surface  it  is  called  irregular 
astigmatism. 

The  astigmatism  which  may  be  called  physiological  is  not 
noticed  by  the  individual,  but  pathological  astigmatism  often 
occurs  and  requires  cylindrical  glasses  to  correct  it. 

Entoptic  images  are  those  which  depend  on  the  presence  of 
some  opacity  or  difference  in  density  in  the  transparent  media 
of  the  eye  itself.  They  look  like  variously-shaped  specks  mov- 
ing over  the  field  of  vision.  They  are  only  remarkable  when 
we  look  at  an  evenly-colored  object  or  through  a  pin  hole  in  a 
black  card.  In  using  the  microscope  they  often  annoy  the 
unpracticed  observer. 

THE  IRIS. 

Functions. — It  has  already  been  mentioned  that  the  motions 
of  the  iris  alter  the  size  of  the  pupillary  aperture  through  which 
the  rays  of  light  must  pass,  and  while  it  regulates  the  amount  of 
light  admitted,  it  also  acts  like  the  diaphragm  of  an  optical 
instrument,  and  always  cuts  off  a  large  amount  of  the  marginal 
rays.  The  great  importance  of  not  allowing  the  rays  which 
would  traverse  the  margin  of  the  lens  to  enter  the  eyeball  can  be 
understood  after  what  has  been  said  of  spherical  and  chromatic 
aberration.  The  iris  also  contracts  when  the  eye  is  adjusted  for 
near  vision,  independent  of  the  amount  of  light  by  which  the  ob- 
ject is  illuminated.  This  action  is  of  advantage,  because  the 
more  convex  the  lens  becomes  in  viewing  near  objects,  the  greater 
is  the  aberration  of  the  marginal  rays.  If  the  iris  did  not  con- 
tract in  near  vision,  the  closer  an  object  was  brought  to  the  eye 
the  greater  would  be  the  tendency  to  indistinctness  caused  by 
spherical  aberration. 

In  structure,  the  iris  consists  of  a  framework  of  delicate  connec- 


576 


MANUAL   OF   PHYSIOLOGY. 


tive  tissue,  like  that  of  the  choroid  coat,  containing  many  blood 
vessels.  On  its  posterior  surface  is  a  dense  layer  of  pigment  cells 
called  the  uvea,  which  gives  the  eye  its  color.  The  act  of  contract- 
ing the  pupil  is  performed  by  a  very  definite  set  of  instriated  mus- 
cular fibres,  forming  the  sphincter  which  surrounds  the  margin  of 
the  pupil.  The  sphincter  muscle  seems  always  to  be  more  or  less  in 
action,  because  if  it  be  paralyzed,  the  pupil  dilates.  The  muscu- 
lar character  of  the  dilator  mechanism  has  been  doubted  from 
the  fact  that  radiating  muscular  fibres  have  not  been  satisfac- 
torily demonstrated  under  the  microscope.  Certainly  the  sphincter 


FIG. 228. 


Section  through  the  ciliary  region,  showing  the  relation  of  the  iris  (/)  to  the  choroid  (g) 
and  the  ciliary  muscle  (a),  which  arises  from  the  margin  of  the  cornea  at  (e),  and 
passes  toward  the  choroid  to  the  right,  where  it  separates  the  latter  from  the  sclerotic. 
Some  bundles  of  circular  fibres  are  shown,  one  marked  (d). 

is  the  most  distinctly  contractile,  for  strong  electric  stimulation 
always  causes  contraction  of  the  pupil,  and  shortly  after  death 
the  pupils  dilate  from  the  relaxation  of  the  sphincter.  If  we 
admit  the  active  dilatation  of  the  pupil,  we  must  assume  that 
the  power  of  the  sphincter  dies  more  quickly  than  that  of  the 
dilator,  or  that  it  at  once  relaxes  when  it  has  lost  the  stimulus 
reflected  from  the  retina.  There  appears  to  be  no  difficulty  in 
explaining  the  dilatation  of  the  pupil  as  the  effect  of  the  elas- 
ticity of  the  tissue  and  the  changes  in  vasomotor  influences. 

NERVOUS  MECHANISMS  CONTROLLING  THE  IRIS. 
When  the  sympathetic  in  the  neck  is  cut,  the  pupil  becomes 
considerably  contracted.     Hence,  it  has  been  argued  that  the 
nerves  supplying  the  dilator  are  derived  from  the  sympathetic. 


MUSCLES    OF   THE    IRIS.  577 

These  fibres  are  supposed  to  take  origin  in  the  gray  matter  of  the 
cervical  spinal  cord.  The  sympathetic  also  supplies  the  walls 
of  the  vessels,  and  thus  controls  the  amount  of  blood  going  to 
the  iris,  and  this  contraction  of  the  pupil  has  been  explained  as 
due  to  vascular  engorgement.  It  is  argued  that  though  the  vaso- 
motor  mechanisms  may  cooperate  in  dilatation,  they  cannot  be 
its  only  cause,  as  the  widening  of  the  pupil  may  occur  in  a  blood- 
less eye.  Since  the  other  tissues  are  elastic  and  antagonize  the 
sphincter,  paralysis  of  that  muscle  would  give  rise  to  dilatation 
even  in  anaemic  mydriasis. 

The  constricting  nerve  mechanism  of  the  sphincter  muscle  is 
distinct,  and  more  definitely  understood.  Its  common  action  is 
reflex ;  the  stimulus  starts  in  the  retina,  and  travels  along  the 
optic  nerve  as  an  afferent  channel  to  the  corpora  quadrigemina, 
where  there  is  a  centre  governing  the  contractions  of  both  irides. 
The  efferent  impulses  are  sent  by  the  third  nerve  to  the  lenticular 
ganglion,  and  thence  by  the  short  ciliary  nerves  to  the  eyeball. 

In  accommodation  for  near  objects  three  muscles  act  together, 
their  movements  being  "  associated "  by  the  central  nerve 
mechanisms.  The  same  voluntary  effort  that  causes  the  ciliary 
muscle  to  contract,  makes  the  sphincter  of  the  iris  contract,  and 
also  causes  the  internal  rectus  to  move  the  eye  inward.  The 
voluntary  nerve  centre  must  be  in  intimate  relation  with  the 
reflex  centre,  which  keeps  up  the  tonic  action  of  the  sphincter 
iridis. 

We  have  then  central  governors  for  the  ciliary  and  iris  move- 
ments. The  ciliary  muscle  and  sphincter  of  the  pupil  are  both 
caused  to  act  by  the  will,  and  the  sphincter  alone  is  excited  by 
means  of  a  centre,  which  reflects  the  stimulus  arriving  from  the 
retina  by  the  optic  nerve  to  the  branches  of  the  third  nerve. 
The  dilator  of  the  pupil,  if  a  muscle,  is  also  kept  in  gentle  tonic 
action  by  the  impulses  sent  from  the  spinal  cord  with  the  vaso- 
motor  impulses,  via  the  sympathetic;  but  some  think  that  the 
blood  supply  and  tissue  elasticity  explain  the  dilatation. 

Further,  from  the  undoubted  facts  (1)  that  some  reflex  con- 
traction of  the  pupil  may  be  produced  by  stimulating  the  retina 
even  when  the  eye  is  cut  off  from  the  brain  centres,  and  (2)  that 
49 


578  MANUAL    OF   PHYSIOLOGY. 

the  local  effect  of  atropia  in  dilating,  and  calabar  bean  in  nar- 
rowing the  pupil,  seem  in  a  measure  independent  of  the  central 
nerve  organs,  it  has  been  concluded  that  there  must  also  be  some 
local  nerve  mechanism  in  the  eye  which  is  capable  of  reflecting 
nerve  impulses,  and  is  affected  by  these  poisons. 

The  student  must  carefully  bear  in  mind  all  the  circumstances 
under  which  the  pupils  contract,  namely : — • 

1.  The  application  of  strong  light  to  either  retina  causes  reflex 
stimulation  of  the  ciliary  nerves  of  both  eyes. 

2.  Stimulation  of  the  nasal  or  ophthalmic  branches  of  the  fifth 
afferent  nerve  reflexly  excites  the  sphincter. 

3.  Contraction  of  the  pupil  is  "  associated  "  with  accommoda- 
tion for  near  objects. 

4.  Similar  "associated"  contraction  always  accompanies  in- 
ward movement  of  the  eyeball. 

5.  During  sleep,  or  as  the  result  of  vasomotor  disturbances  in 
the  brain  (anaemia),  the  pupil  contracts. 

6.  Under  the  influence  of  physostigmin,  nicotin  and  morphia. 

7.  From  any  stimulation  of  the  optic  or  third  nerves  or  the 
corpora  quadrigemina. 

The  circumstances  in  which  the  pupils  are  found  to  be  dilated 
are  equally  important  from  a  practical  point  of  view,  namely  : — 

1.  In  the  dark  or  with  insensitive  retinse. 

2.  Irritation  of  the  cervical  sympathetic. 

3.  Under  the  influence  of  atropin,  daturin,  etc. 

4.  In  asphyxia  or  dyspnoea  from  venosity  of  the  blood. 

5.  Painful  sensations  from  the  skin,  etc. 

THE   OPHTHALMOSCOPE. 

When  we  look  into  the  eye  the  pupil  appears  quite  black,  no 
matter  in  what  position  we  place  the  light.  The  reason  of  this 
is  that  the  retina  can  only  be  made  visible  by  the  light  reflected 
outward  from  it,  and  that  the  portion  of  the  rays  which  is 
reflected  by  the  retina  is  so  refracted  in  passing  out  of  the  eye 
that  it  occupies  exactly  the  same  path  as  that  traveled  by  the 
light  on  its  way  from  the  point  of  illumination  to  the  eye.  Con- 
sequently, unless  the  eye  of  the  observer  be  placed  directly  in 


THE   OPHTHALMOSCOPE.  579 

this  path,  none  of  these  reflected  rays  can  reach  it  to  enable  him 
to  see  the  fundus. 

That  is  to  say,  the  lens  and  other  refractive  media  that  bend 
the  rays  of  the  ingoing  cone  of  light  to  a  focus  on  the  retina  also 
bend  those  of  the  outgoing  cone  reflected  from  the  retina  to  a 
focus  at  the  point  of  illumination. 

The  fact  that  the  blackness  of  the  interior  of  the  eye  is  caused 
by  the  lens,  etc.,  can  be  shown  by  a  simple  experiment. 

FIG.  229. 


Diagram  showing  the  effect  of  a  lens  on  the  rays  of  light  reflected  from  the  paper  (retina) 
in  the  experiment  given  in  the  text.  E.  Observer's  eye.  c.  Point  of  illumination. 
On  the  left  the  reflected  rays  diverge,  and  some  pass  to  E.  On  the  right  they  are 
refracted  by  the  lens  to  form  a  cone.* 

Blacken  the  inside  of  a  pill  box  (about  an  inch  deep),  paste 
printed  paper  on  the  bottom,  and  cut  a  round  hole  half  an  inch 
in  diameter  in  the  lid.  By  illuminating  the  interior  of  the  box 
obliquely  the  print  can  be  easily  recognized.  If  a  convex  lens 
of  one  inch  focus  be  placed  behind  the  opening,  the  paper  can- 
not even  be  seen,  and  the  opening  looks  black  like  the  pupil  with 

*  "  How  to  Use  the  Ophthalmoscope,"  by  Edgar  A.  Brown,  p.  32. 


580  MANUAL   OF    PHYSIOLOGY. 

any  position  of  the  light.     The  paths  traversed  by  the  rays  in 
this  experiment  may  be  seen  in  Fig.  229. 

In  the  left-hand  figure  of  the  above  woodcut  the  first  case  is 
illustrated.  Here  the  divergent  rays  passing  from  the  candle  C 
to  the  surface  P  P'  are  reflected  in  various  directions  from  it ; 
those  which  strike  the  blackened  interior  of  the  box  are  ab- 
sorbed ;  others  emerge  through  the  hole  in  the  lid,  and  reaching 
the  eye  placed  at  E,  enable  the  print  at  P  to  be  seen. 

The  second  case  is  shown  in  the  right-hand  figure.  Here, 
instead  of  diverging  till  they  reach  the  bottom  of  the  box,  the 
rays  are  refracted  by  the  lens  to  a  focus  at  the  point  F,  from 
which  they  pass  back  through  the  lens,  and  are  thereby  bent  to 
a  cone  converging  to  the  source  of  light.  No  rays  pass  in  the 
direction  E,  so  the  interior  of  the  box  looks  quite  black. 

In  attempting,  then,  to  view  the  fundus,  the  observer  must 
either  place  his  head  in  the  line  of  light,  or  the  light  in  the  line 
between  the  observed  eye  and  his  own  ;  in  short,  his  eye  must  lie 
in  the  line  of  reflection,  in  order  to  see  the  fundus.  If  we  could 
see  through  the  source  of  light,  the  above  object  would  be  ac- 
complished. Helmholtz,  by  reflecting  light  into  the  eye  by 
means  of  transparent  glass  plates,  originally  succeeded  in  seeing 
through  the  plates  some  of  the  rays  reflected  from  the  fundus. 
In  this  method,  however,  the  power  which  enables  the  glass  plates 
to  reflect  the  luminous  rays  toward  the  eye  also  robs  the 
observer  of  much  of  the  light  sent  back  from  the  retina  by  re- 
flecting it  toward  the  source  of  light,  and  the  remaining  rays 
which  penetrate  the  glass  cannot  give  a  clear  image  of  the 
retina. 

A  simple  instrument,  the  ophthalmoscope,  is  now  in  general  use 
for  examining  the  retina.  This  consists  of  a  concave  mirror  of 
short  focal  distance,  which  is  substituted  for  the  transparent 
reflecting  plates.  The  rays  converging  from  the  mirror  to  the 
eye  are  brought  to  a  focus  on  the  retina,  and  thence  some  are 
reflected  outward,  and  converged  by  the  dioptric  media  to  the 
hole  in  the  centre  of  the  mirror,  behind  which  the  observer's  eye 
is  placed  to  receive  the  cone  of  converging  rays. 

If  the  observer  place  his  eye  and  the  mirror  at  a  distance  of 


THE    OPHTHALMOSCOPE. 


581 


about  3  cm.  from  the  observed  eye,  and  the  refraction  of  both 
eyes  be  normal,  he  can  see  an  enlarged  virtual  image  of  the 
fuudus.  If  the  refraction  of  either  eye  be  abnormal,  it  must  be 
corrected  by  a  suitable  lens  placed  behind  the  aperture  in  the 
mirror.  This  is  called  the  direct  method  of  examination. 

To  overcome  the  inconvenience  and  difficulty  of  this  mode  of 
examination  the  indirect  method  is  usually  employed.  In  it  a 
convex  lens  of  20  or  40  diopters  is  used  in  addition,  enabling  the 
observation  to  be  made  at  a  more  convenient  distance.  When 


Ophthalmoscopic  view  of  fund  us  of  eye,  in  which  the  central  artery  (gr  and  c)  and  the 
corresponding  veins  (h  and  d)  are  seen  coursing  through  the  retina  from  the  optic  disc 

(A). 

the  eye  has  been  illuminated,  the  lens  is  placed  at  its  proper  focal 
distance  (2  or  1  inches  respectively)  in  front  of  the  eye.  By  the 
converging  power  of  the  lens  a  real  inverted  image  of  the  fundus 
is  formed  in  the  air  a  couple  of  inches  to  the  observer's  side  of 
the  lens,  and  can  be  seen  by  him  through  the  aperture  in  the 
mirror,  if  he  hold  his  head  at  a  distance  to  suit  his  refraction. 

With  this  instrument  a  round  whitish  part  is  seen  a  little  to 
the  nasal  side  of  the  axis  of  the  eye,  where  the  nerve  pierces  the 


582  MANUAL   OF   PHYSIOLOGY. 

dark  choroid  coat.  This  is  called  the  optic  disc.  The  fundus 
now,  when  lighted  up,  does  not  look  black,  but  is  of  a  lurid  red 
color,  owing  to  the  great  vascularity  of  the  choroid  coat.  Over 
this  red  field  are  seen  a  number  of  blood  vessels,  which  start  from 
the  centre  of  the  optic  disc,  and  radiating  over  the  fundus  send 
branches  to  the  most  anterior  parts  that  can  be  seen.  These  are 
the  branches  of  the  vessel  which  runs  in  the  centre  of  the  nerve. 
In  the  very  axis  of  the  eye  a  peculiar  depression,  free  from 
branches  of  the  blood  vessels,  can  be  seen.  This  central  depres- 
sion (fovea  centralis)  differs  a  little  in  color  from  the  neighbor- 
ing parts  during  life,  and  turns  yellow  at  death,  and  hence  has 
been  called  the  "  yellow  spot."  The  retina  is  so  transparent  that 
we  cannot  see  it  with  the  ophthalmoscope,  but  the  radiating  ves- 
sels (central  arteries  and  veins  of  the  retina)  lie  in  it  and  belong 
to  the  nervous  structure  only. 

The  ophthalmoscope  has  proved  of  inestimable  value  not  only 
to  the  ophthalmologist,  but  also  to  the  physician,  as  a  means  of 
arriving  at  an  accurate  knowledge  of  disease.  Hence,  it  has 
become  more  a  pathological  than  a  physiological  instrument. 

LIGHT   IMPRESSIONS. 

The  retina  is  that  part  of  the  eye  by  which  the  physical  mo- 
tions called  light  are  changed  into  what  are  known  physiologi- 
cally as  nerve  impulses,  by  means  of  which  the  impression  of 
light  is  excited  in  the  brain.  In  reaching  the  retina  the  light  is 
not  altered  from  the  light  with  which  physicists  experiment,  but 
at  the  retina  this  physical  motion  is  stopped.  The  optic  nerves 
no  more  convey  the  light  waves  from  the  eye  to  the  brain  than 
the  tactile  nerves  carry  the  objects  that  stimulate  their  endings. 
They  only  send  a  nerve  impulse  which  the  retina,  on  its  exposure 
to  the  light,  excites  in  the  terminals  of  the  optic  nerve.  Any 
form  of  stimulation,  if  applied  to  the  optic  nerve,  will  cause  an 
impulse  to  pass  to  the  brain,  which  there  sets  up  the  sensation  of 
light.  Thus,  we  are  told,  by  persons  who  have  had  their  optic 
nerves  cut  that  the  section  was  accompanied  by  the  sensation  of 
a  flash  of  light  but  not  pain.  Any  violent  injury  of  the  eyeball 
causes  a  flash  of  light  to  be  experienced.  This  fact  has  long 


THE   FUNCTION   OF   THE   RETINA.  583 

since  been  recognized  in  a  practical  manner,  for  a  blow  implicat- 
ing the  eyeball  is  vulgarly  said  to  "  make  one  see  stars."  Also, 
without  violent  injury,  if  we  close  the  eyes  and  turn  them  to  the 
one  side  and  then  press  through  the  lid  with  the  point  of  a  pencil 
on  the  other  side  of  the  eyeball,  we  have  a  sensation  of  a  point  or 
ring  of  light  from  the  retinal  stimulation.  Thus  we  say  that  the 
specific  energy  of  the  optic  nerves  excites  a  sensation  of  light, 
and  the  adequate  stimulus  of  the  nerve  terminals  of  the  organ  of 
vision  is  light.  The  first  question  that  arises  is,  What  part  of  the 
retina  does  this  important  work  of  stimulating  the  optic  nerve 
when  light  impinges  on  its  terminals  ? 

THE  FUNCTION  OP  THE  RETINA. 

The  retina  is  a  complex  peripheral  nervous  mechanism  com- 
posed of  many  elements,  the  special  functions  of  which  are  not 
adequately  known.  It  spreads  over  the  fundus  of  the  eye,  but 
where  the  nerve  pierces  the  coats  of  the  eyeball  there  is  nothing 
but  nerve  fibres,  and  hence  no  retina,  properly  so  called,  exists 
at  the  optic  disc. 

The  structure  of  the  retina  varies  in  different  parts,  but  the 
following  layers  can  be  recognized  in  most  regions  (Fig.  231). 
The  exceptions  will  be  mentioned  afterward. 

Lying  next  to  the  hyaloid  membrane  is  the  layer  of  nerve 
fibres  which  radiate  from  the  optic  disc  to  the  ora  serrata  near 
the  ciliary  region.  The  fibres  spread  evenly  over  the  fundus 
except  at  the  central  point  (fovea  centralis),  which  they  avoid 
by  passing  above  and  below  it.  These  fibres  form  the  inner 
layer  of  the  retina. 

Next  to  the  fibres  is  a  layer  of  nerve  cells,  which  seem  to  have 
one  pole  connected  with  a  fibre  from  the  optic  nerve,  while  from 
the  other  side  two  or  three  poles  send  processes  into  the  adjacent 
layers  of  the  retina.  The  cells  are  numerous  near  the  yellow 
spot. 

Outside  the  foregoing  are  four  less  distinctive  layers.  The 
first  is  brOad  and  granular ;  next,  two  layers  of  peculiar  nuclear 
bodies  are  found,  with  a  thin,  dense  one  of  granular  material 
between  them. 


584 


MANUAL   OF   PHYSIOLOGY. 


Outside  these,  and  separated  by  a  fine  limiting  membrane,  is 
the  terminal  layer  of  the  retina.  It  consists  of  rods  and  cones 
which  are  connected  with  those  parts  of  the  retina  already 
named,  and  are  embedded  in  the  protoplasm  of  pigrnented  epi- 


FTG.  231. 


Pigniented  epithelium  lying  next  to 
the  clioroid  coat. 


Rods  and  cones  whh  their  extremi- 
ties embedded  in  the  epithelial 
cells. 


External  nuclear  layer. 


External  granular  layer. 


Internal  nuclear  layer. 


Internal  granular  layer. 


Layer  of  nerve  cell?. 


Nerve  fibre  layer  in  which  the  reti- 
nal vessels  run  next  to  the  vitreous 
humor. 

Diagrammatic  section  of  retina  showing  the  relation  of  the  different  layers  in  the  pos- 
terior part  of  the  fundus  (not  the  macula  lulea).    (SchuUze.) 

thelial  cells,  which,  on  their  outer  face,  show  a  striking  hexagonal 
outline  (Fig.  234).  The  rods  and  cones  are  easily  torn  away  in 
histological  sections  from  the  pigmented  epithelium,  but  the 


THE    FUNCTION   OP   THE    RETINA. 


585 


epithelium  and  rods  and  cones  are  so  intimately  connected  in 
their  development  and  function  that  they  ought  to  be  regarded 
as  a  single  layer. 

A  retinal  nerve  fibril  may  be  said  to  have  the  following  course  : 
entering  the  eyeball  from  the  optic  nerve  at  the  porus  options,  it 
reaches  the  immediate  vicinity  of  the  hyaloid  membrane,  and 
runs  a  certain  distance  in  contact  with  that  membrane  ;  it  then 


FIG,  232. 


Showing  (he  course  of  the  fibres  of  the  optic  nerve,  N,  as  (hey  pass  along  the  inner  surface 
of  retina,  R,  to  meet  the  ganglion  cells  g,  whence  special  communications  pass  outward 
to  (he  layer  of  rods  and  cones  in  (he  pigment  layer p,  next  the  choroid  c,  within  the 
sclerotic  s. 

turns  outward  toward  the  choroid  and  enters  a  nerve  cell. 
From  the  nerve  cell  pass  a  couple  of  filaments  which  traverse 
the  various  granular  and  nuclear  layers — where  they  probably 
inosculate  with  the  filaments  from  other  cells — and  finally  ter- 
minate in  a  rod  or  a  cone.  The  rods  and  cones  are  the  ultimate 
terminals  of  the  nerves,  and  they  lie  in  the  active  protoplasm  of 
the  peculiar,  pigrnented  epithelial  cells. 


586  MANUAL   OF   PHYSIOLOGY. 

This  outer  layer,  consisting  of  rods  and  cones  lodged  in  epi- 
thelial protoplasm,  is  the  effective  part  of  the  retina.  Of  this 
we  have  the  following  evidence  : — 

1.  The  anatomical  fact  that  the  rods  and  cones  must  be  regarded 
as  the  nerve  terminals  of  the  optic  nerve. 

2.  That  the  macula  lutea,  where  the  retina  is  chiefly  made  up 
of  the  cone  layer,  is  very  much  the  most  sensitive  part,  and  near 
the  ora  serrata,  where  the  rods  and  cones  are  less  developed,  sight 
is  least  acute. 

3.  The  Blind  Spot. — From  the  facts  that  where  the  optic  nerve 
enters  the  eyeball  there  are  no  rods  and  cones,  and  though  the 
nerve  fibres  are  fully  exposed  to  the  light,  they  cannot  appreciate 
it,  this  part,  the  optic  disc,  is  called  the  "blind  spot."     This 
shows  that  the  nerve  fibres  are  quite  insensitive  to  light,  and  that 
we  must  look  to  the  terminals  for  its  appreciation.     The  exist- 
ence of  the  blind  spot  can  be  demonstrated  as  follows:     Shut  the 
left  eye,  and  hold  the  left  thumb,  at  ordinary  reading  distance, 
in  front  of  the  other  eye.     While  the  right  eye  is  fixed  on  the 
left  thumb,  bring  the  right  thumb  to  within  about  four  inches, 
and  move  it  slowly  an  inch  or  so,  from  side  to  side.     A  little 
practice  soon  enables  one  to  find  a  place  when  the  right  thumb 
nail  disappears.     It  also  can  be  demonstrated  by  keeping  the 
right  eye,  the  left  being  closed,  fixed  on  the  small  letter  "  a  "  and 
moving  the  page  to  or  from  the  eye  very  slowly ;  a  distance 


-     -    x 


A 


(about  10  inches)  may  thus  be  reached  when  the  large  letter 
"A"  is  quite  lost.  On  approaching  the  page  when  "A"  is 
invisible,  the  letter  reappears  from  the  inner  side  and  "x"  is 
first  seen ;  on  withdrawing  the  page  it  comes  into  view  from  the 
outer  side  and  "  o  "  is  first  seen.  By  varying  the  direction  and 
noting  the  near  and  far  limits  of  "A's"  being  invisible,  one  can 


RETINAL   STIMULATION.  587 

mark  out  the  extent  of  the  fundus  which  is  blind.  This  blind 
spot  is  not  noticed  in  ordinary  vision,  as  we  have  habitually  over- 
come the  deficiency  by  the  experience  derived  from  the  use  of 
both  eyes  since  infancy.  By  rapid  movements  one  eye  hides  the 
deficiency,  as  is  found  when  attempting  the  experiment  just 
described. 

4.  Purkinjes  Figures. — The  fact  that  when  the  eye  is  illumi- 
nated in  a  peculiar  way  we  can  see  the  shadow  of  the  blood 
vessels  which  lie  in  the  inner  layers  of  the  retina  thrown  upon 
the  outer  layer  of  rods  and  cones,  also  shows  the  latter  to  be  the 
sensitive  part.  This  phenomenon,  known  as  "  Purkinje's  figures," 
can  be  demonstrated  as  follows :  Close  the  left  eye  in  a  dark 
room,  with  an  evenly  dull-colored  wall,  and  while  you  stare  fix- 
edly at  the  wall  with  the  right  eye  turned  inward,  hold  a  candle 
to  its  outer  side  so  that  the  light  can  reach  the  fundus  of  the  eye 
from  the  side.  With  a  little  practice  the  least  motion  of  the 
candle  will  bring  out  an  arborescent  figure  on  the  wall,  which 
exactly  corresponds  to  the  retinal  vessels.  It  may  also  be  seen 
by  arranging  a  microscope  so  as  to  show  a  bright  light,  on  look- 
ing into  the  instrument  and  either  moving  it  or  the  head  slightly 
but  constantly,  the  shadows  of  the  retinal  vessels  will  be  clearly 
seen,  as  though  they  were  under  the  instrument. 

RETINAL  STIMULATION. 

Point  of  Greatest  Sensitiveness. — As  in  the  perception  of  two 
points  of  contact  with  the  skin,  so  we  find  the  retina  ceases  to  be 
able  to  distinguish  the  difference  between  two  luminous  points, 
if  they  be  brought  to  a  focus  at  a  distance  of  less  than  .002 
mm.  from  one  another.  This  .distance  nearly  corresponds  to  the 
diameter  of  the  cones,  and  it  is  supposed  that  the  rays  from  two 
luminous  points  must  come  upon  two  different  cones  in  order  to 
be  visible  as  two  distinct  objects.  The  cones  are,  however,  very 
irregularly  distributed  over  the  retina,  being  packed  closely 
together  at  the  yellow  spot,  and  scattered  more  and  more  widely 
apart  as  one  passes  to  the  peripheral  parts  of  the  retina.  It  is 
only  at  the  yellow  spot  that- the  cones,  which  are  here  very  thin, 
are  so  close  together  as  .002  mm.,  so  that  this  estimation  of  the 


588 


MANUAL   OF   PHYSIOLOGY. 


size  of  visual  areas  could  only  hold  good  of  the  yellow  spot,  and 
toward  the  peripheral  parts  the  power  of  discrimination  must  be 
much  less  keen.  This  is  found  to  be  the  case,  for  in  ordinary 
vision  everything  seen  clearly  with  a  sharp  outline  must  be 
brought  upon  the  yellow  spot.  This  is  spoken  of  as  "direct 
vision."  The  images  falling  on  the  other  parts  of  the  retina  are 
but  dim  and  indistinct  outlines,  and  these  are  spoken  of  as 
"indirect  vision." 

Variations  in  Stimulation. — The  stimulus  need  only  be  applied 
for  a  very  short  time  to  cause  a  distinct  sensation,  for  we  can 
readily  see  a  single  electric  spark;  and  it  need  only  affect  an 

FIG.  233. 


Section  of  the  retina  at  the  yellow  spot,  showing  the  great  number  of  cones  (a)  at  this 
point,  and  the  thinness  of  the  other  layers.    (Cadiat.) 

extremely  small  part  of  the  retina,  as  a  minute  speck  of  light 
can  be  seen  by  direct  vision,  and.  a  very  feeble  ray  suffices  to 
stimulate  the  retina.  The  amount  of  stimulation  produced 
depends  upon  (1)  the  intensity  of  the  light,  i.e.,  the  amount  of 
light  received  in  a  given  area ;  (2)  the  duration  of  its  applica- 
tion ;  (3)  the  extent  of  retina  to  which  it  is  applied  ;  (4)  the  part 
of  the  retina  stimulated;  (5)  the  darker  the  background  the 
weaker  the  illumination  we  can  distinguish,  i.  e.,  the  greater  the 
stimulating  effect  of  a  weak  light ;  (6)  by  fatigue  the  retina  loses 
its  power  of  appreciating  light,  and  more  stimulus  is  required  to 


EXCITATION    OF    NERVE    IMPULSE.  589 

produce  a  given  effect.  On  waking,  the  daylight  is  at  first  daz- 
zling, but  soon  the  retina  can  bear  the  stimulus.  An  increase  of 
intensity  does  not  cause  an  exactly  proportional  increase  of  stimu- 
lation, for  we  find  the  more  the  light  is  intensified  the  less  we  notice 
a  fresh  increment  of  light  until  a  degree  of  intensity  is  arrived 
at,  when  no  further  addition  can  be  detected,  and  the  light 
becomes  blinding.  The  less  the  absolute  intensity  of  two  lights 
the  better  we  distinguish  any  difference  that  may  exist  between 
them. 

Duration. — The  effect  lasts  for  an  appreciable  time  after  the 
stimulus  has  been  removed,  particularly  if  the  light  be  very 
intense.  This  can  be  observed  when  a  brilliant  point  is  in  rapid 
motion  ;  instead  of  a  point  a  streak  of  light  is  seen.  Thus,  part 
at  least  of  the  trail  of  falling  stars  is  caused  by  the  persistence 
of  the  stimulation,  and  a  luminous  body  rapidly  rotated  gives 
the  impression  of  a  circle  of  fire. 

When  the  stimulus  is  very  intense,  such  as  an  electric  light,  or 
when  we  look  at  a  bright  object-  like  the  globe  of  a  lamp  steadily 
for  some  time,  the  effect  persists,  and  after  the  eyes  are  shut  we 
see  a  faint  image  of  the  object.  This  is  called  the  positive  after 
imaue.  If  the  retina  be  exposed  to  a  bright  light  until  it  be 
fatigued,  and  then  suddenly  turning  we  gaze  at  a  white  wall,  the 
bright  part  of  the  positive  after  image  is  replaced  by  a  dark 
figure  which  is  termed  the  negative  after  image. 

A  strong  stimulus  applied  to  the  retina  spreads  from  the  part 
upon  which  the  bright  image  falls  to  those  in  its  immediate 
neighborhood,  so  that  the  bright  object  looks  larger.  This  phe- 
nomenon is  called  irradiation.  It  helps  to  explain  many  of  the 
peculiarities  of  vision. 

EXCITATION  OF  NERVE  IMPULSE. 

The  question  now  arises,  How  do  the  retinae,  or  rather  their 
outer  layers,  convert  light  into  a  nerve  stimulus?  It  would  appear 
quite  out  of  the  question  that  the  394  to  760  billions  of  waves  of 
light  per  second  could  mechanically  excite  the  nerve  terminals 
as  the  waves  of  sound  are  believed  to  excite  the  endings  of  the 
auditory  nerve.  We  know  that  light  has  a  very  distinct  action 


590  MANUAL   OF   PHYSIOLOGY. 

on  many  chemical  combinations,  such  as  reducing  salts  of  silver 
and  gold,  etc.  We  therefore  imagine  that  the  light  waves  may 
set  up,  in  the  outer  layer  of  the  retina,  certain  interrnolecular 
motions  or  chemical  changes,  the  result  of  which  is  that  the  nerve 
fibres  are  stimulated  to  activity  and  transmit  an  impulse  to  the 
brain.  The  light  possibly  produces  a  change  in  the  outer  layer 
of  the  retina  which  in  some  respects  may  be  compared  to  that 
which  occurs  on  a  sensitive  photographic  plate.  In  some  respects 
only,  because,  while  the  chemical  change  on  the  sensitive  plate 
persists  so  as  to  give  rise  to  a  permanent  photograph,  in  the  eye 
it  only  lasts  for  the  brief  moment  during  which  we  can  recognize 
the  positive  after  image.  The  chemical  change  in  muscle  may 
be  compared  to  the  explosion  of  gunpowder,  in  giving  rise  to 
force,  but  not  in  the  result  produced  in  the  materials.  For  in 
muscle  the  chemical  change  causing  the  contraction  is  rapidly 
repaired,  while  in  the  powder  permanent  alteration  of  the  sub- 
stance is  produced.  In  the  retina  a  new  sensitive  plate  is  at  once 
produced  by  the  restoration  of  the  normal  condition  of  the  mole- 
cules, and  similarly  its  explosive  qualities  are  at  once  restored  to 
the  muscle. 

The  view  that  the  layer  of  rods  and  cones  undergoes  a  chemi- 
cal change  on  exposure  to  light  which  suffices  to  excite  the  optic 
nerve,  receives  support  from  the  observation  that  a  color  of  a  red 
or  purplish  hue  exists  in  the  outer  part  of  the  rods,  and  that  this 
color  changes  when  exposed  to  the  light.  But  this  so-called 
visual  purple  has  not  an  inseparable  connection  with  vision,  since 
it  is  absent  where  the  retina  is  most  sensitive,  i.e.,  the  fovea  cen- 
tralis,  where  there  are  no  rods,  and  further,  frogs  with  blanched 
eyes  seem  to  see  quite  well.  Certain  rays  of  light  have  a  distinct 
thermic  influence,  and  hence  the  possibility  exists  that  the  nerve 
impulse  is  started  in  the  retina  by  some  delicate  thermic  stimulus. 

Against  the  chemical  and  thermic  origin  of  the  retinal  stimu- 
lation may  be  urged  the  fact  that  the  rays  of  the  spectrum  which 
are  most  efficient  in  exciting  chemical  and  thermic  variations 
(ultra  violet  and  ultra  red  respectively)  do  not  excite  any  nerve 
impulse  in  the  retina. 

The  pigmented  epithelial  cells  of  the  retina  have  been  observed 


COLOR   PERCEPTIONS. 


591 


to  change  their  shape  slightly,  and  definitely  to  alter  the  position 
of  the  pigment  granules  they  contain  when  exposed  to  light. 
When  we  remember  how  sensitive  to  light  the  protoplasm  of 
many  unicellular  infusoria  is,  we  cannot  be  surprised  that  the 
protoplasm  of  the  retinal  epithelium  is  affected  by  it.  In  the 
pigment  cells  of  the  frog's  skin  we  are  familiar  with  a  change  in 
shape  and  in  the  arrangement  of  their  pigment  granules  in 
response  to  different  light  stimuli.  We  know  further  that  in  the 
nervous  centres  nerve  impulses  often  originate  in  protoplasm 
under  the  influence  of  slight  changes  in  temperature  or  nutri- 
tion. It  would  hardly  be  too  much  to  assume,  then,  that  the 
retinal  epithelium  has  some  important  share  in  the  transforma- 
tion of  light  into  a  nerve  stimulus.  The  arguments  pointing  to  the 

FIG.  234. 


Epithelial  cells  of  the  retina,    a,  Seen  from  the  outer  surface;    6,  seen  from  the  side  as 
in  a  section  of  the  retina;  c,  shows  some  rods  projecting  into  t  he  pigmented  protoplasm. 

rods  and  cones  as  the  essential  part  of  the  retina  apply  equally 
well  to  the  pigmented  epithelium,  for  they  are  so  dove-tailed  one 
into  the  other  that  practically  they  form  but  one  layer.  They 
are  not  known  to  be  connected  with  the  nerve  fibres,  but  they 
may  still  be  influenced  by  the  light,  and  communicate  the  effect 
to  the  contiguous  nerve  terminals,  which  appear  to  be  elaborately 
adapted  to  the  appreciation  of  subtle  forms  of  stimulation. 

COLOR  PERCEPTIONS. 

If  a  beam  of  white  sunlight  be  allowed  to  pass  through  an 
angular  piece  of  glass  it  is  decomposed  into  a  number  of  colors 
which  may  be  seen  by  looking  through  the  prism,  or  may  be 
thrown  on  a  screen,  like  that  of  a  camera.  These  colors,  which 


592  MANUAL   OF    PHYSIOLOGY. 

look  like  a  thin  slice  of  a  rainbow,  are  together  called  the  spec- 
trum. The  white  solar  light  is  thus  shown  to  be  a  compound  of 
rays  of  several  colors  which  possess  different  degrees  of  refrangi-, 
bility,  and  hence  are  separated  on  their  way  through  the  prism. 
The  violet  rays  are  the  most  bent,  and  the  red  the  least,  so  that 
these  form  the  two  extremes  of  the  visible  spectrum.  The  differ- 
ence of  color  depends  upon  the  different  lengths  of  the  waves, 
the  vibrations  of  violet  (762  billions  per  sec.)  being  much  more 
rapid  than  those  of  red  (394  billions  per  sec.).  Beyond  the  vis- 
ible spectrum  at  the  red  end  there  are  other  rays  which,  though 
they  look  black  to  the  eye,  are  capable  of  transmitting  heat. 
This  thermic  power  is  best  developed  in  these  ultra-red  rays  and 
fades  gradually  toward  the  middle  of  the  spectrum.  Outside  the 
violet  are  ultra-violet  rays,  which,  though  non-exciting  to  the 
retina,  are  very  active  in  inducing  many  chemical  changes.  Only 
those  ether  vibrations  which  have  a  medium  length  can  stimu- 
late the  retina. 

If  two  different  colors  be  mixed  before  reaching  the  retina,  or 
be  applied  to  it  in  very  rapid  succession  one  after  the  other,  an 
impression  is  produced  which  differs  from  both  the  colors  when 
looked  at  separately  ;  thus,  violet  and  red  give  the  impression  of 
purple,  a  color  not  in  the  spectrum.  If  all  the  colors  of  the 
spectrum  in  the  same  proportion  and  with  the  same  brightness 
fall  upon  the  retina,  the  result  is  white  light.  This  we  know 
from  the  common  experience  of  ordinary  white  light,  which  is 
really  a  mixture  of  all  the  colors  of  the  spectrum,  and  we  can  see 
it  with  a  "  color  top  "  painted  to  imitate  the  colors  of  the  spec- 
trum. When  the  top  is  spinning,  the  colors  meet  the  eye  in  such 
rapid  succession  that  the  stimulus  of  each  falls  on  the  retina 
before  that  of  the  others  has  faded  away,  and  thus  many  colors 
are  practically  applied  to  the  retina  at  the  same  time,  and  the 
top  looks  nearly  white. 

It  has  been  found  that  certain  pairs  of  colors  taken  from  the 
spectrum  when  mixed  in  a  certain  proportion  produce  white. 
These  are  complementary  to  one  another.  The  complementary 
colors  are  : — 

Red  and  peacock-blue.         Yellow  and  indigo. 
Orange  and  deep  blue.        Greenish-yellow  and  violet. 


COLOR    PERCEPTIONS. 


593 


If  colors  which  lie  nearer  to  each  other  in  the  spectrum  than 
these  complementary  colors  be  mixed,  the  result  is  some  color 
which  is  to  be  found  in  the  spectrum  between  the  two  mixed. 

The  perception  of  the  vast  variety  of  shades  of  color  that  we 
can  distinguish  can  only  be  explained  by  means  of  this  color 
mixing.  We  may  suppose  (with  Hering)  that  there  are  three 
varieties  of  material  in  the  retina,  each  of  which  gives  rise  to 
antagonistic  or  complementary  color  sensations  according  as  they 
undergo  increased  or  decreased  molecular  activity,  these  antago- 
nistic states  being  produced  by  the  complementary  colors.  Thus, 
one  substance  gives  the  sensation  of  black  or  white,  another  red 
or  green,  another  yellow  or  blue,  according  as  they  are  in  exalted 


FIG.  235. 


Diagram  of  the  three  Primary  Sensations  :  1  =  red  ;  2  =  green  ;  3  =  violet. 
The  letters  below  are  the  initials  of  the  colors  of  the  spectrum. 

The  height  of  the  shaded  part  gives  extent  to  which  the  several  primary  sensations  are 
excited  by  different  kinds  of  light  in  the  spectrum. 

or  diminished  activity.  A  varying  degree  of  these  stimulations 
can  be  easily  shown  to  give  many  differences  of  shade. 

Or  we  may  assume  that  there  are  three  primary  colors  which 
overlap  one  another  in  the  spectrum  so  as  to  produce  all  the 
various  tints.  These  are  red,  green,  and  violet ;  the  arrangement 
of  which  may  be  diagrammatically  explained  (Fig.  235). 

We  must  in  this  case  further  assume  (Young,  Helmholtz)  that 
there  are  in  the  retina  three  special  sets  of  nerve  terminals,  each 
of  which  can  only  be  stimulated  by  red,  green,  or  violet  respect- 
ively, and  the  innumerable  shades  of  color  we  see  depend  upon 
50 


594  MANUAL   OF    PHYSIOLOGY. 

mixtures  of  different  strengths  of  these  primary  colors,  producing 
different  degrees  of  stimulation  of  each  set  of  nerve  terminals. 

The  view  that  such  special  nerve  apparatus  exists  for  red, 
green  and  violet  is  supported  by  -the  fact  that  the  most  anterior 
or  marginal  part  of  the  retina  is  incapable  of  being  stimulated 
by  red  objects,  which  look  black  when  only  seen  by  this  part  of 
the  retina.  This  inability  to  see  red  may  extend  over  the  whole 
retina,  as  is  found  in  some  persons  who  may  be  said  to  be  "  red 
blind."  If  we  investigate  our  negative  after  images,  after  look- 
ing for  a  long  time  at  a  red  object,  we  find  them  to  be  greenish 
blue.  That  is  to  say,  the  nervous  mechanism  for  receiving  red 
impressions  is  fatigued,  and  that  of  its  complementary  color  is 
easily  stimulated. 

MENTAL  OPERATIONS  IN  VISION. 

Our  visual  sensations  enable  us  to  perceive  the  existence,  posi- 
tion and  correct  form  of  the  various  objects  around  us.  For 
visual  perception  much  more  is  necessary  than  the  mere  perfec- 
tion of  the  dioptric  media  of  the  eye,  and  of  the  retinal  nerve 
mechanisms.  Besides  the  changes  produced  in  the  retina  by  light 
and  the  excitations  in  the  nerve  cells  of  the  visual  centre,  there 
must  be  psychical  action  in  other  cells  of  the  cortex  of  the  brain. 
This  psychical  action  of  the  brain  consists  of  a  series  of  conclu- 
sions drawn  from  the  experiences  gained  by  our  visual  and  other 
sensations, 

Our  ideas  of  external  objects  are  not  in  exact  accord  with  the 
image  produced  on  the  retina  and  transmitted  to  the  brain,  but 
are  the  result  of  a  kind  of  argument  carried  on  unconsciously  in 
our  minds.  Thus,  when  no  light  reaches  the  retina,  we  say  (with- 
out what  we  call  thought)  that  it  is  dark  ;  our  retina  being  un- 
stimulated,  no  impulse  is  communicated,  and  the  sensation  of 
blackness  arises  in  our  sensorium.  When  luminous  rays  are 
reflected  to  the  retina  from  various  objects  around  us,  the  physio- 
logical impulse  starts  from  the  eye,  but  in  the  brain,  by  uncon- 
scious psychical  activity,  it  is  referred  in  our  minds  to  the  objects 
around  us,  so  that  mentally  we  project  into  the  outer  world  what 
really  occurs  in  the  eye.  So  also,  from  habit,  we  re-invert  in  our 


MOVEMENTS  OF  THE  EYEBALLS. 


595 


FIG.  236. 


offi.svp. 


o&Z.  sup. 


minds  the  image  which  is  thrown  on  the  retina  upside  down,  by 
the  lens,  and  so  unconscious  are  we  of  the  psychical  act  that  we 
find  it  hard  to  believe  that  our  eyes  really  receive  the  image  of 
everything  inverted,  and  our  minds  have  to  reinstate  it  to  the 
upright  position. 

One  of  the  most  important  means  employed  to  enable  us  to 
form  accurate  visual  perceptions  is  the  varied  motion  which  the 
eyeballs  are  capable  of  performing. 

MOVEMENTS  OF  THE  EYEBALLS. 

The  eyeballs  may  be  regarded  as  spherical  bodies,  lying  in 
loosely  fitted  sockets  of  connective  tissue  padded  with  fat,  in 
which  they  can  move  or  revolve  freely  in  all  directions,  in  a  lim- 
ited degree.  The  muscles  which 
act  directly  on  the  eyeball  are 
six  in  number.  Four  recti  pass- 
ing from  the  back  of  the  orbit  are 
attached  to  the  eyeball,  one  at 
each  side  and  one  above  and 
below,  not  far  from  the  cornea. 
These  move  the  front  of  the 
eye  to  the  right  or  left,  up  or 
down  respectively.  Two  oblique 
passing  nearly  horizontally  out- 
ward, and  a  little  backward,  are 
attached  to  the  upper  and  under 
surface  of  the  eyeball  respect- 
ively. These  muscles  can  slightly 
rotate  the  eye  on  its  antero-pos- 
terior  axis,  the  upper  one  draw- 
ing the  upper  part  of  the  eyeball 
inward,  and  its  antagonist,  the 
lower,  drawing  the  lower  part 
inward,  so  as  to  rotate  the  eye- 
ball in  an  opposite  direction 
round  the  same  axis. 

The  internal  and  external  recti  draw  the  centre  of  the  cornea 


r.  ihf 


r.ext.    r.snp.  r.tnf 
r.inf* 

Diagram  of  the  direction  of  action  of  the 
muscles  of  the  eyeball,  which  is  shown 
by  the  dark  lines.  The  axes  of  the  ro- 
tation caused  by  the  oblique  and  upper 
and  lower  recti  are  shown  by  the  dotted 
lines.  The  inner  and  outer  recti  rotate 
the  ball  on  its  vertical  axis,  which  is  cut 
across.  The  abbreviated  names  of  the 
muscles  are  affixed  to  the  lines. 


596  MANUAL   OF   PHYSIOLOGY. 

to  or  from  the  median  line  respectively,  directly  opposing  one 
another. 

As  the  direction  of  the  superior  and  inferior  recti  is  different 
from  that  of  the  axis  of  the  eyeball,  they  draw  the  outer  edge  of 
the  cornea,  not  its  centre,  up  and  down  respectively,  and  at  the 
same  time  tend  to  give  the  eyeball  a  slight  rotation  in  the  same 
direction  as  the  corresponding  oblique  muscles.  The  tendency 
ito  rotation  is  counteracted  by  the  antagonistic  oblique  muscle 
when  simple  elevation  or  depression  is  formed. 

Thus,  pure  abduction  or  adduction  only  requires  the  unaided 
action  of  the  internal  or  external  recti,  while  direct  depression  of 
the  eye  requires  the  combined  action  of  the  inferior  rectus  and 
superior  oblique,  and  direct  elevation  requires  the  superior  rectus 
and  inferior  oblique  to  act  together.  The  oblique  movements 
are  accomplished  by  various  combined  coordinations  of  move- 
ment of  the  different  muscles. 

From  the  foregoing  it  is  obvious  that  the  simplest  movements 
of  the  eye  require  the  cooperation  of  different  muscles. 

The  diagram  shows  the  directions  toward  which  the  different 
muscles  tend  to  draw  the  eyeball. 

In  the  ordinary  movements  of  both  eyes  more  than  this  is 
necessary.  Both  eyes  must  move  in  the  same  direction  at  the 
same  time,  now  to  the  right,  now  to  the  left,  so  that  while  the 
external  rectus  moves  the  right  eye  to  the  right  side,  the  internal 
rectus  moves  the  other  eye  in  the  same  direction.  The  coordi- 
nation of  the  movements  of  the  eyeball  is  so  arranged  that  the 
contractions  of  the  external  and  internal  recti  of  opposite  sides 
must  occur  together,  and  are  called  "associated  movements." 
This  associated  movement  has  been  acquired  by  the  habit  of  vol- 
untarily directing  both  eyes  at  the  same  object,  and  has  gradu- 
ally become  involuntary,  for  few  persons  have  the  power  of 
exerting  control  over  the  muscles  of  one  eye  alone. 

BINOCULAR  VISION. 

When  we  look  at  an  object  with  both  eyes  we  have  a  separate 
image  thrown  upon  each  retina,  and  therefore  two  sets  of  im- 
pulses are  sent  to  the  sensoriura,  one  from  the  right  and  one  from 


BINOCULAR   VISION.  597 

the  left  eye.  Yet  we  are  only  conscious  of  the  occurrence  of 
one  stimulation.  The  reason  of  this  is,  that  experience  has 
taught  us  that  similar  images  thrown  upon  certain  parts  of  the 
two  retinae  correspond  to  the  same  object,  and  in  our  minds  we 
fuse  the  sensations  caused  by  the  two  images  so  that  they  produce 
but  one  idea. 

These  points  of  the  retina  which  are  thus  habitually  stimu- 
lated by  the  same  objects  are  called  "corresponding  points." 

Besides  being  of  great  use  in  making  up  for  such  deficiencies 
as  the  blind  spots  (which  are  not  corresponding  points),  binocular 
vision  is  useful  for  the  following  purposes  : — 

To  judge  of  distance.  When  using  one  eye  only,  some  knowl- 
edge of  distance  may  be  gathered  by  the  force  employed  to 
accommodate,  but  a  much  more  accurate  judgment  can  be  formed 
when  both  eyes  are  used  and  the  muscular  sense  of  the  ocular 
muscles,  employed  in  converging  the  eyeballs  for  near  objects, 
gives  further  evidence  of  their  distance. 

In  judging  of  size,  in  the  same  way,  with  one  eye,  we  can  only 
have  an  idea  of  the  apparent  size  of  an  object,  which  will  vary 
with  its  distance.  With  a  knowledge  of  apparent  size  and  dis- 
tance such  as  is  gained  by  binocular  vision,  we  can  come  to  a 
fairly  accurate  conclusion  as  to  the  size  of  an  object. 

To  judge  of  the  relative  distances  of  objects  so  as  to  see  depth 
in  the  picture  before  our  eyes,  binocular  vision  is  necessary.  If 
one  eye  alone  is  used  we  see  a  flat  picture,  without  having  an 
accurate  idea  of  the  relative  distances  of  the  different  objects. 
With  each  eye,  however,  we  get  a  slightly  different  view  of  each 
object,  and  thus  we  are  helped  to  a  conclusion  as  to  their  exact 
distances  and«shapes,  and  arrive  at  fairly  correct  judgments  as  to 
their  form,  etc. 


593  MANUAL   OF   PHYSIOLOGY. 


CHAPTER  XXXIII. 

HEARING. 

Just  as  impulses  traveling  along  the  optic  nerves  can  only  give 
rise,  in  the  sensorium,  to  impressions  of  light,  so  impulses  passing 
to  the  sensorium  via  the  auditory  part  of  the  portio  mollis  of  the 
seventh  pair  of  cranial  nerves  can  only  excite  impressions  of 
sound,  and  any  stimulation  of  that  nerve  gives  rise  to  sound  sen- 
sations. 

The  peripheral  end  of  the  special  nerve  of  hearing  is  distrib- 
uted to  an  organ  of  very  peculiar  construction  situated  in  the 
internal  ear,  which,  from  its  complexity,  has  been  called  the 
labyrinth.  The  nerve  endings  are  spread  out  between  layers  of 
fluid,  so  that  they  must  be  stimulated  by  very  gentle  forms  of 
movement ;  and  when  we  consider  their  delicacy,  we  cannot  be 
surprised  that  even  sound  vibrations  suffice  to  stimulate  these 
terminals  and  transmit  nerve  impulses  to  the  brain.  The  organs 
of  hearing  of  mammalia  are  so  deeply  placed  in  the  petrous  part 
of  the  temporal  bone,  that  special  mechanisms  have  to  be  adopted 
to  convey  the  sound  with  sufficient  intensity  from  the  air  to  the 
fine  nerve  terminals.  These  make  up  a  complex  piece  of  anat- 
omy which  will  be  briefly  referred  to  presently. 

SOUND. 

Before  attempting  to  describe  the  complex  mechanisms  by 
which  sound  is  conveyed  from  the  air  to  the  nerve  endings,  some 
notion  must  be  formed  of  what  sound  is  from  a  merely  physical 
standpoint.  By  means  of  the  sense  of  hearing  we  form  an  idea 
of  sound,  and  here  the  knowledge  of  sound  ends  with  many 
people,  since  they  only  think  of  it  as  something  they  can  hear. 
A  physicist,  however,  regards  sound  in  a  different  way.  He 
knows  that  it  is  produced  by  the  vibrations  of  elastic  bodies, 
such  as  a  tense  string,  a  metal  rod,  or  an  elastic  membrane. 
These  vibrations,  being  communicated  to  the  air,  are  conveyed 


SOUND.  599 

by  it  to  our  nerve  endings,  where  they  set  up  a  nerve  impulse. 
The  impulse  is  transmitted  along  the  nerve  to  the  brain,. and 
there  gives  rise  to  the  sensation  with  which  we  are  familiar  as 
sound. 

The  vibrations  of  the  air  are  wave-like  movements  depending 
upon  a  series  of  changes  of  density  in  the  gases,  the  particles  of 
which  move  toward  or  from  one  another,  and  transmit  the 
motion  to  their  neighbors,  so  as  to  propagate  the  sound  wave. 
To  demonstrate  these  vibrations  a  special  apparatus  must  be  used. 

When  a  tuning  fork  is  struck  it  is  thrown  into  vibration,  and 
a  sound  is  given  forth.  But  the  vibrations  are  often  so  rapid  and 
so  small  that  the  motion  of  the  tuning  fork  cannot  be  appreci- 
ated by  the  eye.  But  if  a  fine  point  be  attached  to  one  prong  of 
the  tuning  fork — or,  indeed,  any  elastic  body,  such  as  a  bar  of 
metal — and  this  point  be  brought  into  contact  with  a  moving 
smoked  surface,  such  as  has  been  already  described  for  similar 
records,  a  little  wavy  line  is  drawn,  showing  that  the  vibrating 
fork  moves  up  and  down  at  an  even  and  regular  rate.  Each  up 
and  down  stroke  indicates  a  vibration.  The  length  of  the  wave, 
as  drawn  on  the  evenly-moving  surface  of  the  recorder,  shows  the 
amount  of  time  occupied  by  each  vibration.  This  is  always 
found  to  be  the  same  for  a  tuning  fork  of  a  given  pitch,  and  thus 
the  recording  fork  is  in  constant  use  by  the  physiologist  as  ail 
exact  measure  of  small  intervals  of  time.  The  pitch  of  the  note 
depends  upon  the  rate  or  period  of  vibration,  a  tone  of  a  certain 
pitch  being  simply  a  sound  caused  by  so  many  vibrations  per 
second.  The  quicker  the  vibration  the  higher  the  note,  and  the 
slower  the  deeper,  until,  at  the  rate  of  about  thirty  per  second, 
no  sound  is  audible.  Whether  a  note  be  produced  by  a  metal 
fork,  a  tense  string,  or  any  other  vibrating  body,  if  the  number 
of  vibrations  per  second  be  the  same,  the  note  must  have  the 
same  pitch. 

The  elevation  of  each  vibration  as  seen  in  the  tracing  made  by 
a  recording  fork  is  different  at  different  times.  When  the  fork 
is  first  struck,  the  waves  are  high  and  well  marked  ;  the  excur- 
sions of  the  recording  prong  become  less  and  less  extensive  as 
the  fork  gradually  ceases  to  vibrate  and  the  sound  diminishes ; 


600  MANUAL   OF    PHYSIOLOGY. 

or  in  other  words,  as  the  sound  produced  becomes  fainter,  the 
vibrations  become  smaller.  The  amount  of  excursion  made  by 
the  vibrating  body  is  spoken  of  as  the  amplitude  of  the  vibration, 
and  upon  it  depends  the  loudness  or  intensity  of  the  sound.  The 
pitch  of  a  tone  bears  no  relation  to  the  amplitude  of  the  waves 
of  vibration,  but  depends  upon  their  rate;  while  its  loudness  is 
quite  independent  of  the  period  occupied  by  the  vibrations,  but 
is  in  proportion  to  the  square  of  the  amplitude  of  the  waves. 

So  far  only  tones  or  musical  notes  have  been  mentioned.  They 
are  produced  by  vibrations  occurring  at  perfectly  regular 
periods.  The  simpler  and  more  regular  the  vibrations,  the 
purer  the  tone.  The  great  majority  of  the  sounds  we  are  accus- 
tomed to  hear  are  not  pure  tones,  but  are  the  result  of  an  associa- 
tion of  vibrations  bearing  some  relation  to  one  another.  When 
the  variety  of  vibrations  is  very  great,  their  intervals  irregular 
and  out  of  proportion,  they  give  rise  to  a  discordant  sound 
called  a  noise.  So  long  as  such  commensurability  exists  in  the 
rate  of  the  vibrations  as  to  produce  a  sound  not  disagreeable 
to  the  sense  of  hearing,  it  may  be  called  a  note. 

By  the  use  of  a  series  of  different  resonators,  each  of  which  is 
capable  of  magnifying  a  certain  tone,  it  can  be  shown  that  the 
clearest  and  purest  notes  of  our  musical  instruments  are  far  from 
being  simple  tones,  but  are  really  compounds  of  one  prominent 
note  or  fundamental  tone,  modified  by  the  addition  of  numerous 
over-tones  or  harmonics.  If  one  blows  forcibly  across  an  orifice 
leading  to  a  space  in  which  a  small  amount  of  air  is  confined, 
such  as  the  barrel  of  a  key  or  the  mouth  of  a  short-necked  flask 
or  bottle,  either  a  clear  shrill  or  dull  booming  sound  is  heard, 
which  varies  in  pitch  according  to  the  proportions  of  the  air- 
containing  cavity.  This  dull  note  is  a  simple  tone.  It  is  devoid 
of  character,  and  in  this  respect  differs  greatly  from  the  notes 
produced  by  a  musical  instrument.  The  notes  of  every  instru- 
ment have  certain  characters  or  qualities  which  enable  even  an 
unpracticed  ear  to  distinguish  them. 

This  quality,  which  is  independent  of  the  pitch  (i.  e.,  rate  of 
vibration),  or  the  intensity  (i.  e.,  amplitude  of  wave),  is  called 
the  color  or  timbre  of  the  note.  It  depends  on  the  number, 


SOUND.  601 

variety  and  relative  intensity  of  the  over-tones  or  harmonics, 
which  accompany  the  notes.  So  that  really  the  timbre  or  quality 
of  a  note,  and  therefore  the  special  characters  of  the  different 
musical  instruments,  is  produced  by  their  impurity,  or  the  com- 
plexity of  the  over-tones  which  aid  in  producing  them. 

All  elastic  bodies  can  vibrate,  and  therefore  are  capable  of 
conducting  sounds.  Sound  vibrations  can  be  transmitted  from 
one  body  to  another  placed  in  contact  with  it.  From  a  hard 
material  the  waves  are  readily  communicated  to  the  air,  and  this 
is  the  ordinary  medium  by  means  of  which  sound  is  transmitted 
to  our  organs  of  hearing.  In  the  old  experiment  of  placing  a 
small  bell  under  the  glass  of  an  air  pump,  and  making  the  tongue 
strike  after  the  air  has  been  removed,  the  fact  that  no  sound  is 
produced  shows  that  the  medium  of  the  air  is  essential  for  the 
transmission  of  sound  vibrations. 

The  transmission  of  waves  of  sound  from  the  air  to  more  dense 
materials,  such  as  those  which  surround  our  auditory  nerve  termi- 
nals, takes  place  with  much  greater  difficulty  than  that  from  a 
solid  to  the  air,  and  we  find  a  variety  of  contrivances  by  which 
the  gentle  air  waves,  arriving  at  the  ear,  are  collected  and  inten- 
sified on  their  way  to  the  labyrinth. 

The  medium  of  the  air  is  not  necessary  in  order  that  sound 
may  reach  the  internal  ear.  Nor  is  the  route  through  the  outer 
canal,  and  the  drum  and  its  membrane,  the  only  one  by  which 
the  vibrations  can  arrive  at  the  cochlea.  The  solid  bone  which 
surrounds  the  labyrinth  is  in  direct  communication  with  all  the 
bones  of  the  head,  and  sound  can  travel  along  these  bones  and 
reach  the  nerve  endings.  This  can  easily  be  proved  by  placing 
the  handle  of  a  vibrating  tuning  fork  against  the  forehead,  or 
better  still,  against  the  incisor  teeth.  The  sound,  although  pre- 
viously hardly  audible,  at  once  becomes  quite  distinct,  or  even 
appears  loud. 

This  direct  conduction  through  the  bones  of  the  head  is,  under 
normal  conditions,  of  little  use  to  man ;  but  attempts  have  been 
made,  in  cases  where  the  ordinary  auditory  passages  were 
rendered  inefficient  by  disease,  to  gather  the  vibrations  on  an 
elastic  plate,  and  apply  this  to  the  teeth.  This  direct  conduction 
51 


602  MANUAL   OF    PHYSIOLOGY. 

of  sound  is  very  valuable  in  determining  the  seat  of  disease  in 
cases  of  deafness.  So  long  as  a  clear  sensation  of  sound  reaches 
the  brain  through  the  bones  of  the  head,  we  know  that  the 
important  nerve  endings  and  their  central  connections  are 
unimpaired,  and  conclude  that  the  disease  lies  in  the  mechanical 
conducting  parts  of  the  hearing  organ. 

In  fishes,  where  the  labyrinth  is  the  only  existing  part  of  the 
auditory  apparatus,  it  is  embedded  in  the  cranium,  and  the  sound 
waves,  arriving  through  the  medium  of  water,  are  directly  con- 
veyed to  the  nerve  endings  by  the  bones  of  the  head.  An  air- 
containing  tympanum  would  rather  impede  the  hearing  of  these 
animals. 

The  parts  of  the  ear  through  which  sound  passes  before  it 
reaches  the  nerve  are  separated  into  three  departments,  viz., 
(1)  the  auditory  canal  and  external  ear;  (2)  the  middle  ear, 
tympanum  or  drum,  which  is  shut  off  from  the  latter  by  the 
tympanic  membrane;  and  (3)  the  labyrinth. 

CONDUCTION  OF  SOUND  VIBRATIONS  THROUGH  THE  EXTERNAL 

EAR. 

External  Ear. — In  man,  the  muscles  are  so  poorly  developed 
that  he  can  hardly  move  the  external  ear  or  pinna  perceptibly, 
and  the  part  commonly  called  the  ear  is  of  little  use.  We  know 
this,  because  the  outer  ear  may  be  quite  removed  without 
materially  affecting  the  power  of  hearing.  The  sound  reflected 
from  the  pinna  may  be  excluded,  without  reducing  the  intensity 
of  that  heard,  by  placing  a  little  tube  in  the  auditory  canal. 
Birds  hear  well  without  any  outer  ear.  But  the  movable  ears 
of  many  animals  are,  no  doubt,  useful  in  helping  them  to  ascer- 
tain the  direction  of  a  sound  by  catching  more  of  the  vibrations 
coming  toward  their  pinna.  That  the  external  ear  may  be  of 
some  use,  even  to  man,  one  is  led  to  believe  by  the  natural  readi- 
ness with  which  a  person  with  dull  hearing  supplements  it  by 
means  of  his  hand.  In  this  act  the  ear  is  pushed  away  from  the 
head  to  an  angle  of  about  forty-five  degrees,  and  its  projection  is 
considerably  increased. 

External  Auditory  Meatus. — The  auditory  canal  is  a  crooked 


CONDUCTION   THROUGH    THE   TYMPANUM.  603 

and  irregular  passage,  getting  rather  wider  as  it  approaches  the 
tympanic  cavity.  It  is  the  seat  of  some  short,  stiff  hairs,  which 
help  to  prevent  the  entrance  of  foreign  matters.  It  is  supplied 
with  a  peculiar  modification  of  sweat  glands,  which  secrete  a 
waxy  material  that  helps  to  keep  the  walls  of  the  canal  and  the 
outside  of  the  membrane  moist  and  soft. 

The  elastic  column  of  air  in  any  circumscribed  space  resounds 
more  readily  to  some  one  tone,  varying  according  to  the  capacity 
of  the  space;  thus  resonators  of  different  pitch  are  formed. 
Different  tubes  have  different  notes  when  blown  into,  so  the  audi- 
tory canal  has  a  note  of  its  own,  and  if  the  canal  be  short,  the 
note  is  one  of  a  very  high  pitch.  When  a  tone  of  the  same  pitch 
as  that  to  which  the  canal  is  tuned  strikes  the  ear,  it  is  unpleas- 
antly magnified,  and  such  sounds  are  called  shrill  and  disagree- 
able. Upon  the  more  ordinary  sound  vibrations,  however,  the 
auditory  canal  has  little  or  no  effect. 

CONDUCTION  OF  SOUND  VIBRATIONS  THROUGH  THE  TYMPANUM. 

The  end  of  the  auditory  canal  is  closed  by  the  membrana  tym- 
pani,  which  slopes  obliquely  from  above  downward  and  inward, 
in  which  direction  its  size  is  greater  than  if  it  were  straight 
across  the  canal.  This  membrane  is  not  flat,  for  the  central 
point  is  drawn  in  by  the  handle  of  the  malleus,  which  is  firmly 
attached  to  it.  The  membrane  is  thus  held  in  the  shape  of  a  very 
blunt  cone,  somewhat  like  a  Japanese  umbrella,  the  apex  of  which 
points  inward  toward  the  cavity  of  the  drum.  The  peculiar  form 
of  the  membrane  of  the  drum  is  of  great  importance  for  distinct 
hearing. 

As  every  confined  volume  of  air  has  a  certain  proper  tone  to 
which  it  resonates  readily,  so  a  membrane  of  a  given  size  and 
tension  has  a  proper  tone  (self-tone),  the  vibration  period  of  which 
it  follows  naturally.  This  tone  varies  with  the  tension,  as  may 
be  seen  in  a  common  drum,  the  note  of  which  can  be  changed 
with  the  tension  of  its  parchment;  the  tenser  the  membrane, 
the  higher  the  pitch.  If  the  membrane  of  the  drum  of  our  ears 
were  set  to  one  tone,  our  hearing  would  be  imperfect  and 
unpleasant,  for  we  should  be  wearied  by  the  reiteration  and 


604 


MANUAL   OF    PHYSIOLOGY. 


persistence  of  the  one  note.  This  does  not  occur  ;  the  tympanic 
membrane  has  no  marked  self-tone,  and  no  succession  of  vibra- 
tions follows  the  first  effect  of  the  sound  waves. 

Any  self-tone  is  prevented  by  the  conical  shape  of  the  mem- 
brane, which  is  partly  due  to  the  traction  of  the  handle  of  the 
malleus.  If  a  stretched  membrane,  such  as  that  of  a  drum,  be 


FIG.  237. 


Diagram  of  the  tympanum,  showing  the  relation  of  the  ossicles  to  the  tympanic  mem- 
brane and  the  internal  ear.  The  tympanum  is  cut  through  nearly  transversely,  and 
the  cavity  viewed  from  the  front  (left  ear).  (Scliitfer.) 

Membrane,  m.t,  of  the  drum  to  which  the  handle  of  the  malleus,  m,  is  attached  at  v. 
Head  of  malleus,  m,  which  is  held  in  position  by  its  suspensory  ligament,  s.l.m.,  and 
external  ligament,  l.e.m ;  long  process  of  incus,  i.,  connecting  malleus  and  stapps,,s.<., 
the  base  of  which  closes  the  oval  opening  of  the  vestibule  p.  External  auditory 
meatus  e.au.m.  Internal  auditory  meatus  i.au  m.,  where  the  two  parts  of  the  auditory 
nerve  enter,  a  and  b. 

drawn  out  at  its  centre,  so  that  it  is  no  longer  a  flat  surface,  its 
tension  is  different  at  the  centre  and  the  periphery,  being  greatest 
at  that  point  at  which  it  is  drawn,  and  gradually  decreasing 
toward  the  margin.  Since  the  existence  of  a  tone  of  a  definite 
pitch  depends  upon  a  certain  degree  of  tension,  if  no  two  parts 
of  the  membrane  are  similarly  tense,  no  one  tone  can  be  more 


CONDUCTION   THROUGH   THE   TYMPANUM.  605 

conspicuous  than  another.     This  is  the  case  with  the  tympanic 
membrane. 

The  independent  vibrations  of  the  membrane  are  further  pre- 
vented by  the  tympanic  ossicles.  These  little  bones  do  not 
vibrate  molecularly,  but  move  en  masse  in  time  with  the  sound 
vibrations  which  they  deaden.  If  a  substance  incapable  of 
vibrating  be  attached  to  the  membrane  of  a  common  drum,  it 
ceases  to  vibrate.  A  touch  of  the  finger  to  the  membrane  suf- 
fices to  check  the  sound  produced  by  a  drum.  The  handle  of  the 
malleus,  which  is  joined  to  the  other  bones,  being  fixed  to  the 
membrane,  acts  in  this  way  as  a  damper,  and  checks  the  continu- 
ance of  any  vibration  in  the  membrana  tympani. 

A  small  muscle,  called  the  tensor  tympani,  is  attached  to  the 
malleus,  so  as  to  draw  it  toward  the  cavity  of  the  tympanum. 

The  motions  occurring  in  the  membrane  of  the  drum  are  con- 
veyed across  the  tympanic  cavity  by  means  of  the  three  small 
bones  known  as  the  malleus,  the  incus,  and  the  stapes.  These 
ossicles  form  an  angular  lever,  one  arm  of  which  (the  handle  of 
the  malleus)  is  attached  to  the  centre  of  the  tympanic  membrane, 
and  the  other  shorter  arm  (the  long  limb  of  the  incus)  unites 
with  the  stapes,  the  base  of  which  is  held  by  the  secondary  tym- 
panic membrane  in  the  oval  opening  leading  into  the  vestibule. 
The  stapes  is  attached  at  right  angles  to  the  extremity  of  the 
inner  arm  of  the  lever,  being  jointed  to  the  long  arm  of  the 
incus.  This  little  angular  lever  works  round  an  axis  which 
passes  from  before  backward  through  the  head  of  the  malleus, 
and  lies  above  the  membrane  of  the  drum,  the  two  points  which 
act  as  the  bearings  or  pivots  of  the  motion  being  the  slender  pro- 
cess of  the  malleus  in  front,  and  the  short  limb  of  the  incus 
behind. 

When  the  tympanic  membrane  vibrates  in  response  to  the 
sound  waves  of  the  air,  it  moves,  and  the  handle  of  the  malleus 
moves  in  and  out  with  it.  The  body  of  the  incus,  being  fixed 
by  a  firm  joint  to  the  head  of  the  malleus,  must  follow  these 
movements,  and  cause  the  oval  base  of  the  stapes  to  press  in  or 
draw  out  the  membrane  which  separates  the  tympanum  from  the 
vestibule.  Thus,  the  vibrations  of  the  air  communicated  to  the 


606  MANUAL    OF    PHYSIOLOGY. 

tympanic  membrane  are  conveyed  across  the  tympanic  cavity  to 
the  liquid  in  the  labyrinth. 

A  small  muscle — the  stapedius — is  attached  to  the  stapes  near 
its  junction  with  the  incus,  and  pulls  upon  it  in  such  a  direction 
that  the  bone  is  drawn  out  of  the  direct  line  of  motion.  This 
action,  possibly,  reduces  the  more  ample  vibrations  of  the  tym- 
panic membrane,  which  might  injure  the  delicate  mechanism  of 

the  labyrinth. 

EUSTACHIAN  TUBE. 

The  tympanum  is  connected  with  the  pharynx  by  means  of 
the  Eustachian  tube,  which,  though  habitually  closed,  is  opened 
for  a  moment  by  swallowing  and  other  motions  of  the  pharynx. 
On  these  occasions  air  can  pass  in  or  out  of  the  tympanum,  so 
that  the  pressure  on  both  sides  of  the  membrane  of  the  drum  is 
equalized.  When  there  is  too  much  or  too  little  air  in  the  tym- 
panic cavity,  the  tympanic  movements  are  impeded.  This  diffi- 
culty is  felt  during  a  cold  in  the  head,  when  the  tube  is  occluded, 
and  the  oxygen  being  absorbed,  the  pressure  in  the  tympanic 
cavity  is  reduced.  Or  in  performing  what  is  known  as  Valsalva's 
experiment,  i.  e.,  holding  the  nose  and  blowing  air  into  it,  where- 
by the  Eustachian  tubes  are  opened,  arid  too  much  air  is  often 
retained  in  the  tympanum,  so  that  the  pressure  from  within  is 
higher  than  that  from  without,  and  hearing  becomes  dull.  If 
the  act  of  swallowing  be  then  performed,  the  feeling  of  tension 
leaves  the  ears  as  the  excess  of  air  escapes,  and  hearing  becomes 
as  acute  as  before. 

The  Eustachian  tube  also  acts  as  a  way  of  escape  for  any  fluid 
that  may  be  secreted  by  the  epithelial  lining  of  the  tympanic 
cavity.  The  amount  of  fluid  is  so  small,  that  the  occasional 
opening  of  the  tube  suffices,  under  ordinary  circumstances,  for 
its  complete  escape.  When  increased  by  disease,  it  may  collect 
in  the  tympanum,  and  require  catheterization. 

If  the  tubes  were  permanently  open,  we  should  suffer  from 
two  great  disadvantages.  At  every  breath,  during  ordinary 
respiration,  some  change  in  tension  of  the  air  contained  in  the 
cavity  of  the  drum  would  occur  and  impair  hearing ;  the  vibra- 
tions of  the  air  in  the  pharynx,  produced  by  the  voice,  would 


TERMINALS   OF   THE   AUDITORY   NERVE.  607 

enter  the  drum  directly,  and  give  rise  to  an  exaggerated  shout- 
ing noise. 

CONDUCTION  THROUGH  THE  LABYRINTH. 

Every  motion  of  the  oval  base  of  the  stapes  causes  a  wave  to 
pass  along  the  liquid  in  the  labyrinth.  The  bony  case  of  the 
internal  ear  being  firm,  the  wave  travels  through  all  parts  of  the 
internal  ear.  Through  the  cochlea  it  arrives  at  the  inner  tym- 
panic membrane  which  closes  the  fenestra  rotunda,  and  separates 
the  cavity  of  the  tympanum  from  the  scala  tympani  of  the 
cochlea.  The  waves  have  a  very  complex  route  in  passing  from 
the  fenestra  ovalis  closed  by  the  stapes  to  the  membrane  closing 
the  cochlea.  By  means  of  the  liquid  lying  around  the.  rnem- 


Diagram  of  the  membranous  labyrinth,  all  of  which  is  filled  with  endolymph  and  sur- 
rounded by  perilymph.  a,  ft,  c,  semicircular  canals  opsnin'*  into  the.  ventricle  d  ;  e, 
the  saccule  from  which  the  uniting  canal,  /,  leads  into  the  membranous  canal  of  the 
cochlea,  g.  (Cleland.) 

branous  labyrinth — perilymph — the  waves  pass  up  the  vestibular 
spiral  of  the  cochlea,  and  arriving  at  its  summit,  they  descend  by 
the  tympanic  spiral  to  the  fenestra  rotunda.  In  this  course  they 
pass  over  and  under  the  fluid — endolymph — contained  in  the 
membranous  canal  of  the  cochlea  in  which  the  special  nerve 
terminations  are  placed. 

For  the  construction  of  the  labyrinth  the  student  is  referred 
to  the  text-books  of  anatomy,  as  space  only  admits  of  a  brief 
account  of  the  special  arrangements  of  the  nerve  ending. 

TERMINALS  OF   THE   AUDITORY  NERVE. 

The  nervous  mechanisms  which  are  most  important  for  the 
appreciation  of  tones  are  those  situated  in  the  cochlea. 


608  MANUAL   OF   PHYSIOLOGY. 

The  nerve  endings  found  in  the  membranous  sacs  in  the  vesti- 
bule are  connected  with  peculiar  epitheloid  cells,  to  which  are 
attached  fine  bristle-like  processes.  These  processes  lie  in  the 
endolymph,  and  are  related  to  calcareous  masses  called  otoliths. 
Waves  in  this  endolymph  possibly  bring  the  otoliths  into  collision 
with  the  hairs,  and  thus  give  a  stimulus  to  the  nerve  endings. 
Noises  may  be  heard  from  this,  but  no  impressions  of  tone  can 
be  appreciated.  The  use  of  the  nerves  going  to  the  other  parts 
of  the  labyrinth — ampullae  of  the  semicircular  canals — is-  doubt- 
ful, and  probably  not  immediately  connected  with  hearing.* 
The  coils  of  the  cochlea  are,  throughout  their  entire  length, 
partially  divided  by  a  bony  shelf  projecting  from  the  central  axis 
into  the  spiral  cavity.  This  is  called  the  osseous  spiral  lamina. 
In  the  fresh  state  the  separation  of  the  spiral  canal  into  an  upper 
(vestibular)  and  a  lower  (tympanic)  coil  is  completed  by  a  mem- 
branous partition,  which  stretches  from  the  bony  spiral  lamina 
to  the  opposite  side  of  the  spiral  canal.  This  is  called  the  mem- 
branous spiral  lamina,  and  forms  the  base  upon  which  the  special 
nerve  endings  of  the  organ  of  hearing  are  placed.  An  extremely 
delicate  membrane  called  the  membrane  of  Reissner  stretches 
from  the  upper  side  of  the  spiral  partition  obliquely  upward  to 
the  outer  wall  of  the  spiral  cavity,  so  as  to  form  a  canal  and 
cover  the  special  organ,  shutting  off  a  portion  of  the  vestibular 
coil  which  lie's  over  the  membranous  spiral  lamina.  The  canal 
of  the  cochlea  thus  formed  is  triangular  in  section.  Its  floor  is 
made  up  chiefly  of  the  membranous  spiral  lamina,  particularly 
the  part  called  the  basilar  membrane,  while  the  oblique  roof  is 
composed  of  only  the  thin  membrane  of  Reissner.  The  canal 
follows  the  turns  of  the  cochlea,  lying  between  the  vestibular 
coil  and  that  leading  to  the  tympanum,  and  is  filled  with  a  fluid 
(endolymph)  which  is  quite  separate  and  distinct  from  that  in 
the  vestibular  or  tympanic  coils  (perilymph). 

The  cochlear  division  of  the  auditory  nerve  passes  into  little 
tunnels  in  the  central  bony  column  around  which  the  coils  of  the 
cochlea  turn,  and  gives  off  a  series  of  spiral  branches  which  run 

*  Compare  equilibration,  in  connection  with  which  they  will  be  described. 


TERMINALS   OF    THE   AUDITORY   NERVE. 


609 


through  the  osseous  spiral  lamina  to  reach  the  membranous  por- 
tion. A  collection  of  ganglion  cells  connected  with  the  radiating 
nerve  fibres  is  found  lying  in  the  spiral  canal  of  the  osseous 


Transverse  section  through  the  membranous  canal  of  the  cochlea.  Striated  zone  of 
basilar  meinbraue,  a.  Pectinate  zone  of  the  basilar  membrane,  b.  Perforated  zone 
of  basilar  membrane  through  which  the  nerves  pass,  c.  Nerve  fibres  from  spiral  gan- 
glion, d.  Spiral  ganglion,  e.  Limbus,/.  Reissner's  membrane,  g.  Tectorial  membrane, 
h.  Internal  rod  of  Corti,  i.  External  rod  of  Corti,  m.  Special  cells  receiving  nerve 
terminals,  o,  p,  p'.  Epithelial  cells  covering  the  basilar  membrane,  q.  Nerve  fibres,  s. 
Spiral  ligament,  t.  (Cadiaf.) 


610  MANUAL   OF   PHYSIOLOGY. 

lamina.  Passing  through  the  bony  spiral  the  nerves  reach  the 
basilar  membrane,  which,  as  before  mentioned,  forms  a  great  part 
of  the  membranous  spiral  lamina,  and  upon  which  the  organ  of 
Corti  is  placed. 

The  organ  of  Corti,  placed  upon  the  basilar  membrane  within 
the  membranous  canal  of  the  cochlea,  is  made  up  of  a  series  of 
peculiarly  curved  bars  or  fibres,  called  the  rods  of  Corti,  and 
some  epitheloid  cells  provided  with  short,  bristle-like  processes. 
The  rods  of  Corti  are  fixed  by  their  broad  bases  upon  the  basilar 
membrane,  and  unite  above  in  such  a  way  that  the  outer  and 
inner  rods  form  a  bow  or  arch.  The  spiral  series  of  rods  thus 
propped  up  against  each  other  leave  a  small  space  or  tunnel 
under  them,  which  runs  the  entire  length  of  the  basilar  mem- 
brane. Beside  these  rods  of  Corti  are  placed  rows  of  cells  of  an 
epithelial  type  into  which  the  nerve  endings  pass.  From  the 
upper  surface  of  these  cells,  on  a  level  with  the  apex  or  junction 
of  the  rods,  a  number  of  hair-like  processes  project.  A  delicate 
reticulated  membrane  lies  over  the  rods  and  the  cells,  and  seems 
to  be  lightly  attached  to  their  surface,  while  the  hairs  pass 
through  its  meshes. 

The  basilar  membrane  is  made  up  of  fibrous  bands  held  together 
by  a  delicate  membrane.  The  fibres  pass  transversely  across  the 
spiral  canal  of  the  cochlea,  so  as  to  subtend  the  bases  of  the  outer 
and  inner  rods.  The  basilar  membrane  gradually  becomes  wider 
as  it  passes  from  the  base  to  the  summit  of  the  cochlea.  The 
length  of  the  rods  also  increases  toward  the  summit  of  the  organ, 
their  bases  being  more  widely  separated  from  one  another  and 
their  point  of  junction  nearer  to  the  basilar  membrane,  this  form- 
ing a  lower  and  wider  tunnel.  The  number  of  rods  of  Corti  has 
been  estimated  at  6000  inner  and  4500  outer. 

STIMULATION  OF  THE  AUDITORY  NERVE. 

The  stimulation  of  the  nerve  of  hearing  by  sound  vibrations 
of  the  air  is  less  difficult  to  understand  than  the  excitation  of  the 
optic  nerve  by  light  waves  which  are  conveyed  by  an  imponder- 
able medium.  The  motions  of  the  membrane  of  the  drum,  being 
conveyed  in  the  manner  already  indicated  to  the  liquids  within 


STIMULATION    OF   THE   AUDITORY   NERVE.  611 

the  internal  ear,  pass  over  and  under  the  cells  connected  with  the 
nerve  terminals,  which  are  placed  on  the  elastic  basilar  mem- 
brane. The  transverse  fibres  are  set  in  motion  by  the  waves  in 
the  fluid,  and  as  they  vibrate  they  communicate  the  motion  to 
the  organ  of  Corti.  The  bases  of  the  inner  rods,  being  fixed  at 
the  inner  margin  of  the  basilar  membrane,  can  move  but  little, 
and  the  bases  of  the  outer  rods  being  placed  near  the  middle  of 
the  fibres  of  the  membrane,  where  the  motion  of  the  vibrations 
is  most  extensive,  a  slight  change  in  their  relative  positions,  and 
a  consequent  movement  of  the  apex  of  the  bow  takes  place.  This 
movement  at  the  apex  of  the  bow,  where  the  rods  join,  is  com- 
municated by  the  medium  of  the  reticular  membrane  to  the  hairs 
in  the  special  auditory  cells,  thence  to  the  nerves,  where  an 
excitation  is  produced  giving  rise  to  the  transmission  of  an 
impulse  to  the  brain. 

We  can. distinguish  differences  of  (1)  loudness,  (2)  pitch  and 
(3)  quality  in  sounds. 

Since  the  loudness  depends  simply  on  the  amplitude  of  the 
vibration,  we  have  no  difficulty  in  understanding  how  variations 
in  it  can  be  appreciated,  since  the  more  ample  the  vibration  the 
more  marked  the  motion,  and,  therefore,  the  more  intense  the 
stimulation  of  the  nerve  terminals.  What  we  call  the  loudness 
of  a  sound  simply  means  greater  or  less  intensity  of  stimulation 
of  the  nerve. 

The  perception  of  difference  of  pitch  presents  greater  difficulty. 
As  already  mentioned,  this  depends  on  the  rate  of  vibrations. 
We  know  that  most  bodies  capable  of  producing  sound  vibra- 
tions have  a  proper  tone,  i.  e.,  that  which  they  produce  when 
struck.  When  the  tone  proper  to  a  body  capable  of  vibrating, 
is  sounded  in  its  immediate  neighborhood,  it  also  is  set  vibrating 
through  the  medium  of  the  air.  If  a  clear  tone  be  sung  loudly 
over  the  strings  of  a  piano  a  kind  of  sympathetic  echo  will  be 
heard  to  come  from  the  strings  corresponding  to  the  notes  sounded. 
In  the  basilar  membrane  we  have  practically  a  series  of  strings 
of  different  length — since  the  membrane  gets  wider  as  it  passes 
from  below  upward  to  the  summit  of  the  cochlea — and  therefore 
a  great  variety  of  proper  tones.  With  a  high  note  a  fibre  of  one 


612  MANUAL   OF   PHYSIOLOGY. 

part  of  the  membrane  will  readily  fall  into  vibration,  and  with 
a  low  note  a  fibre  of  another  part.  Different  nerve  fibrils  are  in 
relation  to  these  different  parts,  and  we  may  conclude  that  tones 
of  different  pitch  stimulate  distinct  nerve  terminals,  and  are  con- 
veyed to  the  brain  by  separate  nerve  channels.  Impulses  arriv- 
ing at  certain  brain  cells  give  rise  to  the  idea  of  high  tones,  and 
impulses  coming  to  others  cause  the  impression  of  low  tones. 
There  are  about  a  sufficient  number  of  fibres  in  the  basilar  mem- 
brane for  all  the  notes  we  can  hear,  viz.,  from  about  33  to  38,000 
waves  in  the  second. 

The  rods  of  Corti  cannot  be  the  vibrating  agents,  because  they 
are  absent  in  birds  which  appreciate  and  reproduce  various  notes ; 
and  they  are  too  few  for  the  notes  we  hear.  Further,  the  rods 
are  not  elastic,  and  not  well  suited  for  vibration.  It  may,  there- 
fore, be  concluded  that  they  only  act  as  levers  which  convey  the 
vibrations  of  the  fibres  of  the  basilar  membrane  to  the  nerve 
endings  in  the  auditory  cells. 

The  explanation  of  our  wonderful  appreciation  of  the 'delicate 
shades  of  quality  of  tone  is  still  more  difficult.  Even  persons 
with  indifferently  good  ears,  as  musicians  say,  and  no  special 
musical  education,  can  at  once  distinguish  between  the  quality  of 
the  same  note  when  sounded  on  a  violin,  a  piano  and  a  flute. 
When  a  note  is  sung  against  the  strings  of  a  piano,  however  pure 
its  tone,  a  great  number  of  strings  are  set  vibrating.  Not  only 
does  the  string  of  that  note  vibrate,  but  also  all  those  that  have 
a  certain  simple  numerical  relation  to  its  vibrations.  In  fact,  all 
its  over-tones  resound.  In  the  cochlea  we  suppose  the  same  to 
take  place  with  the  fibres  of  the  basilar  membrane.  Not  only 
(}oes  the  one  fibre  whose  proper  tone  is  sounded  vibrate  in 
response,  but  also  all  those  which  represent  the  varied  over-tones 
or  harmonics.  It  has  already  been  pointed  out  that  the  quality 
of  a  tone  depends  on  the  relative  number,  force  and  arrangement 
of  harmonics,  which  invariably  accompany  any  musical  note  that 
possesses  a  definite  character. 

When  a  note  arrives  at  the  auditory  nerve  terminals,  one  of 
these  is  strongly  stimulated  by  the  wave  of  the  fundamental 
tone,  and  many  others  by  the  different  over-tones,  for  every 


STIMULATION    OF    THE    AUDITORY    NERVE.  613 

prominent  over-tone  stimulates  the  cochlea.  The  complexity  of 
the  impression  increases  with  the  impurity  of  the  tone,  and  so 
we  appreciate  the  quality  of  a  note.  Thus,  a  compound  of  im- 
pulses, corresponding  to  a  mixture  of  tones  of  varying  intricacy, 
is  transmitted  to  the  brain  cells,  where  it  gives  rise  to  the  impres- 
sion of  the  quality  which  we  by  experience  associate  with  that 
of  a  violin,  flute  or  piano,  as  the  case  may  be. 

With  regard  to  the  judgment  of  the  distance  of  sound,  it  need 
only  be  remarked  that  it  chiefly  depends  on  former  experience 
of  the  habitual  quality  and  intensity  of  sound.  A  faint  sound 
with  the  same  quality  that  we  familiarly  attribute  to  loud  sound 
seems  to  us  to  be  far  away.  Thus,  sounds  reaching  our  laby- 
rinths by  the  cranial  bones  appear  distant,  and  ventriloquists 
deceive  us  by  imitating  the  character  of  distant  sounds. 

The  direction  from  which  sound  comes  is  chiefly  judged  by  the 
difference  of  intensity  with  which  it  is  heard  by  one  or  other  ear. 
When  we  cannot  form  any  idea  of  whence  a  sound  comes  we 
turn  our  heads  one  way  or  the  other  in  order  to  present  one  ear 
more  directly  to  the  origin  of  the  sound.  When  a  sound  is 
either  directly  behind  or  before  us  we  cannot  judge  from  which 
position  it  really  comes,  unless  the  head  be  slightly  turned  to  one 
side  or  the  other  before  the  vibrations  have  ceased  to  be  audible. 


614  MANUAL   OF   PHYSIOLOGY. 


CHAPTEK  XXXIV. 

CENTRAL  NERVOUS  ORGANS. 

The  central  part  of  the  nervous  system,  or  cerebro- spinal  axis, 
consists  of  the  spinal  cord,  the  medulla  oblongata  and  the  brain. 

The  central  nervous  organs. are  composed  of  a  soft  texture, 
consisting  of  nerve  cells  and  nerve  fibres,  held  together  by  a 
peculiar  and  very  delicate  form  of  connective  tissue,  known  as 
Neuroglia*  With  the  naked  eye  the  central  nervous  organs  can 

FIG.  240. 


Transverse  section  of  nerve  fibres,  showing  the  axis  cylinders  cut  across,  and  looking 
like  dots  surrounded  by  a  clear  zone,  which  is  the  medullary  sheath.  Fine  connective 
tissue,  in  connection  with  neuroglia,  binds  the  fibres  into  bundles. 

be  seen  to  be  composed  of  two  distinct  kinds  of  substance  :  (1)  a 
white  substance,  found  by  the  microscope  to  be  composed  of  nerve 
fibres,  with  a  medullary  sheath,  and  (2)  a  gray  substance,  consist- 
ing of  a  dense  feltwork  of  naked  axis  cylinders,  with  numerous 
ganglion  cells  interspersed  between  them. 

In  the  brain  the  gray  substance  is  distributed  chiefly  on  the 
surface,  forming  a  kind  of  gray  cortex,  which  follows  all  the 
irregularities  of  the  convolutions. 

In  the  spinal  cord  the  gr^y  matter  is  situated  inside  and  the 
white  outside.  If  viewed  longitudinally  the  gray  substance  of 
the  cord  forms  separate  columns  on  either  side,  which  extend 


THE   SPINAL   CORD.  615 

throughout  its  entire  length  and  are  thicker  in  the  cervical  and 
lumbar  regions.  These  gray  columns,  together  with  their  con- 
nections with  the  roots  of  the  spinal  nerves,  divide  the  white 
substance  of  the  cord  into  more  or  less  distinct  regions  called  the 
posterior  and  antero-lateral  white  columns. 

The  general  properties  of  the  elements  of  nervous  tissue  have 
been  described  in  Chapter  xxvm.  The  functions  there  enumer- 
ated belong  also  to  the  fibres  and  cells  of  the  cerebro-spinal  axis, 
and  therefore  require  no  further  general  description  here. 

FIG.  241. 


Multipolar  cells  from  the  anterior  gray  column  of  the  spinal  cord  of  the  dog-fish  (a) 
lying  in  a  texture  of  fibrils;  (b)  prolongation  from  cells;  (c)  nerve  fibres  cut  across. 
(Cadiat.) 

Besides  having  the  power  of  conducting,  reflecting,  coordinating, 
inhibiting,  retaining  and  originating  impulses,  we  must  attribute 
to  the  activity  of  the  nerve  cells  of  the  brain  the  various  mental 
phenomena,  such  as  feeling,  thought,  volition,  memory,  etc., 
which  forms  of  activity  may  be  excited  either  by  impulses  arriv- 
ing from  without,  or  by  the  automatic  action  of  the  cells  of  the 
cerebral  cortex. 

THE  SPINAL  CORD. 

The  spinal  cord,  being  the  great  bond  of  connection  between 
the  brain  and  the  majority  of  the  peripheral  nerves,  is  neces- 


616 


MANUAL    OF    PHYSIOLOGY. 


sarily  a  conducting  apparatus  of  the  very  first  importance,  and 
from  the  quantity  of  nerve  cells  lying  in  its  gray  matter,  it  must 
also  exercise  the  function  of  a  governing  organ,  or  nerve  centre. 

SPINAL  CORD  AS  A  CONDUCTOR. 

Anatomical  Methods. — Anatomical  investigation  shows  that  the 
spinal  cord  is  not  merely  a  collection  or  aggregation  of  the  fibres 
that  pass  into  it.  In  the  first  place,  the  spinal  nerves,  if  bundled 


FIG.  242. 


p.m.f 


Diagram  illustrating  the  paths  probably  taken  by  the  fibres  of  the  nerve  roots  on  enter- 
ing the  spinal  cord.  (Scfid/er.) 

a.m.f.,  Anterior  median  fissure;  p.m.f.,  Posterior  median  fissure:  c.c.,  Central  canal; 
s.r.,  Substantia  gelatinosa  of  Rolando;  a.a.,  Funiculi  of  anterior  root  of  a  nerve;  p., 
Funiculusof  posterior  root  of  a  nerve.  By  following  the  fibres  1,2,  3,  etc.,  their  course 
through  the  gray  matter  of  the  spinal  cord  may  be  traced. 

together,  would  be  much  larger  than  the  cord,  even  at  its 
thickest  part ;  and  secondly,  it  does  not  taper  evenly  toward  its 
lower  extremity,  as  it  should  were  each  succeeding  pair  of  roots 
a  direct  loss  of  thickness.  The  question  then  arises,  How  are  the 
fibres  of  the  spinal  nerves  disposed  of  in  the  cord. 

The  posterior  roots  of  the   spinal   nerves   (Fig.    242,  jo)  pass 
through  the  white  substance  to  reach  the  posterior  gray  column 


SPINAL   CORD   AS   A    CONDUCTOR.  617 

(SK),  where  they  break  up  into  twigs,  some  of  which  are  distrib- 
uted to  neighboring  parts  of  the  gray  network  of  fibrils,  in  which 
they  are  lost  without  their  union  with  the  cells  being  obvious  or 
immediate,  while  others  pass  into  the  posterior  white  columns. 

The  fibres  of  the  anterior  roots,  in  irregular  bundles  (a,  a), 
traverse  the  superficial  white  part  of  the  cord  on  their  way  to 
reach  the  anterior  gray  columns,  into  the  cells  of  which  some  can 
be  directly  traced  ;  others  enter  the  gray  matter  without  joining 
the  cells.  The  other  processes  from  these  cells  pass  into  the 
fibrillar  network  which  makes  up  the  great  mass  of  the  gray 
substance,  and  are  in  communication  with  the  distant  parts  of 
the  cord  above. 

Anatomy  may  thus  be  said  to  leave  us  in  the  dark  in  regard 
to  the  paths  traversed  in  the  cord  by  the  impulses  on  their 
way  to  or  from  the  root  of  the  spinal  nerves. 

Histology  only  teaches  us  that  the  gray  and  white  matter  of 
the  cord  consist  of— (1)  innumerable  fibrils  and  cells,  and  (2) 
medullated  fibres  variously  connected  with  the  different  groups 
of  cells  and  the  roots  of  the  spinal  nerves. 

Since  ordinary  histological  research  fails  to  show  the  complex 
connections  of  the  fibres  of  the  spinal  cord,  other  methods  have 
been  resorted  to  in  attempting  to  discover  their  course.  Of  these 
the  two  following  have  given  good  results : — 

Developmental  Method. — It  was  found  that  the  medullary 
sheath  of  the  fibres  in  different  white  tracts  of  the  cord  was  per- 
fected at  different  ages  of  an  animal,  and  by  tracing  the  course 
of  the  imperfectly  developed  fibres  the  functional  relations  of 
certain  tracts  could  be  arrived  at. 

Degenerative  Method. — It  is  well  known  that  a  degenerative 
process  (sclerosis),  easily  recognized  histologically,  soon  follows 
the  section  of  nerve  fibres.  This  change  takes  place  first  in 
those  fibres  situated  at  the  side  of  the  section  toward  which  the 
impulses  travel ;  i.  e.,  at  the  peripheral  end  of  an  efferent  and 
central  end  of  an  afferent  fibre.  By  carefully  tracing  the  course 
of  the  degenerated  tracts  after  section  or  pathological  lesion  the 
function  of  the  fibres  and  their  connections  with  other  parts  of 
the  nervous  centres  can  be  determined. 
52  - 


FIG.  243. 


7)T 

\ 


>       Pf 


U,  s 

Transverse  section  of  the  lumbar  region  of  the  spinal  cord  (for  reference,  see  description 
under  t  ig.  245).     (Bevan  Lewis.) 

TTin     ->4/f 


Transverse  section  of  the  dorsal  region  of  the  spinal  cord.    (Sevan  Lewis.) 


SPINAL   CORD   AS   A   CONDUCTOR. 


619 


By  these  methods  the  following  tracts  of  white  fibres  have 
been  made  out  in  the  spinal  cord: — 

1.  The  direct  pyramidal  tracts  (Tiirck)  (Fig.  246,  T),  each  of 
which  is  continuous  with  the  pyramid  on  the  same  side  of  the 


FIG.  245. 


ai 


^Transverse  section  of  the  cervical  region  of  the  spinal  cord. 


A.  Anterior  gray  column. 

a.  Anterior  white  column. 

I.  Lateral  white  column. 

ac.  Anterior  commissure. 

ar.  Anterior  roots. 

of.  Anterior  median  fissure. 

il.  Intermedio-lateral  gray  column. 

vc.  Vesicular  column  of  Clarke.  -^ 

P.  Posterior  gray  column.     . 


p.  Posterior  white  column. 

pm.  Posterior  median  column. 

pc.  Posterior  commissure. 

cc.  Central  canal. 

pr.  Posterior  roots. 

pf.  Posterior  median  fissure. 

ae  and  ai.  External  and  internal  anterior 

vesicular  columns. 
sg.  Substantia  gelatinosa. 


medulla  oblongata.  They  lie  on  the  inner  aspect  of  the  anterior 
white  column,  and  form  the  immediate  boundaries  of  the  anterior 
median  fissure.  These  tracts  taper  from  above  downward  to 
nothing,  and  terminate  about  the  middle  of  the  dorsal  region. 


620  MANUAL   OF   PHYSIOLOGY. 

The  fibres  leaving  them  appear  to  cross  by  the  anterior  com- 
missure to  reach  the  anterior  gray  column  of  the  opposite  side. 

2.  The  crossed  or  lateral  pyramidal  tracts  (Fig.  246,  P)  are 
continuous  with  the  pyramids  of  the  opposite  side  of  the  medulla 
where  the  crossing  of  the  fibres  is  completed.  They  lie  in  the 
lateral  white  columns,  occupying  their  posterior  part  next  to  the 
posterior  gray  columns,  and  are  separated  from  the  surface  by 
the  direct  cerebellar  tract  (cfc).  The  crossed  pyramidal  tracts 

FIG.  246. 


Transverse  section  of  the  spinal  cord  of  embryo  at  five  months. 

G.  Columns  of  Goll.  T.  Direct  pyramidal  tracts.  P.  Crossed  pyramidal  tracts.  <ic. 
Direct  cerebellar  tract ;  ar.  Anterior  root  zones;  pr.  Posterior  root  zones;  ah.  Ante- 
rior gray  column ;  ph.  Posterior  gray  column. 

also  taper  toward  the  lower  part  of  the  cord,  but  can  be  traced 
to  the  lumbar  region.  The  fibres  are  connected  with  the  ante- 
rior gray  column  of  the  same  side. 

Section  of  these  tracts  in  the  cord  of  the  pyramids  in  the 
medulla,  or  the  channels  leading  from  the  motor  areas,  as  well  as 
destruction  of  the  motor  areas  in  the  cortex  of  the  brain,  is  fol- 
lowed by  descending  degeneration  of  both  these  pyramidal  white 
columns  along  their  entire  extent.  The  track  of  degeneration 


WHITE   TRACTS   IN   SPINAL   CORD.  621 

corresponds  with  the  limits  assigned  by  developmental  research 
to  these  pyramidal  tracts.  It  therefore  seems  clear  that  they 
are  the  channels  by  which  the  efferent  impulses  from  the  brain 
travel  to  the  cells  of  the  anterior  cellular  column  of  the  spinal 
cord.  There  seems  to  be  no  functional  difference  between  those 
fibres  that  cross  in  the  medulla  at  the  decussation  of  the  pyra- 
mids and  those  which  pass  directly  from  the  medulla  to  the  same 
side  of  the  cord,  and  then  gradually  cross  on  their  way  to  their 
destination. 

3.  The  direct  cerebellar  tracts  (c?c)  can  be  traced  from  the  infe- 
rior peduncle  of  the  cerebellum  to  the  superficial  part  of  the 
lateral  white  columns  of  the  cord,  where  they  form  a  flattened 
band  of  fibres  which  covers  in  the  crossed  pyramidal  tract.     The 
fibres  appear  to  be  connected  with  the  cells  of  Clarke's  column, 
to  be  described  below.      This  tract  tapers  toward  the  lumbar 
region,  about  the  upper  limit  of  which  it  disappears. 

After  section  of  the  cord  ascending  degeneration  can  be  traced 
along  these  tracts  through  the  restiform  bodies  of  the  medulla 
oblongata  to  the  vermiform  process  of  the  cerebellum.  Hence 
we  may  conclude  that  they  carry  centripetal  impulses. 

4.  The  posterior  median  columns  (Goll)  are  thick  strands  of  white 
fibres,  triangular  in  section  (Fig.  246,  G),  which  form  the  imme- 
diate boundary  of  the  posterior  median  fissure.     They  can  be 
traced  from  the  medulla  to  the  mid-dorsal   region,  where  they 
taper  to  a  point.     Some  of  the  fibres  of  the  posterior  roots  are 
connected  with  this  column. 

In  Goll's  posterior  median  column  ascending  degeneration  can 
be  traced,  after  section,  up  to  the  clavate  nucleus  of  the  medulla. 
This  tract  is  therefore  also  afferent. 

5.  Anterior  root  zone  (ar)  is  the  name  given  to  the  white  sub- 
stance next  to  the  anterior  gray  columns  not  included  in  the 
parts  just  described.     These  tracts  do  not  taper  toward  the  lower 
part  of  the  cord,  but  vary  in  thickness  with  that  of  the  roots  of 
the  spinal  nerves,  being  thickest  at  the  cervical  and  lumbar 
enlargements. 

After  transverse  section  of  the  cord  descending  sclerosis  can 
be  traced  a  short  distance  down  these  tracts.  The  direction  of 


622 


MANUAL   OF   PHYSIOLOGY. 


the  degeneration  shows  them  to  be  efferent,  but  from  the  limited 
extent  of  the  sclerosis  we  may  suppose  that  they  only  carry 
impulses  from  the  upper  to  the  lower  regions  of  the  spinal  cord. 
6.  Posterior  root  zones  (Burdach)  (pr)  surround  the  part  of 
the  gray  column  from  which  the  posterior  roots  spring,  and  vary 
in  proportion  to  -the  size  of  the  roots  of  the  spinal  nerves. 


Diagram  of  transverse  section  of  the  cervical  parts  of  the  spinal  cord,  showing  the  white 
tracts  supposed  to  be  functionally  distinct  by  differences  of  shading. 

A.  Anterior;  p.  Posterior  median  fissures,  dp.  Direct  pyramidal;  cp.  Crossed  pyra- 
midal tracts;^  dc.  Direct  cerebellar;  pm.  Posterior  median  column  (Goll);  az.  Ante- 
rior; pz.  Posterior  root  zones. 

The  mode  of  degeneration  (limited  ascending  sclerosis)  of  these 
fibres  teaches  us  that  they  are  afferent  channels  probably  carry- 
ing impulses  from  the  cells  of  the  lower  parts  of  the  posterior 
gray  columns  to  those  in  its  upper  segments. 

Experimental  Methods. — Besides  the  foregoing  anatomical  facts, 
we  learn  from  experimental  research  certain  facts  concerning  the 


WHITE   TRACTS   IN   SPINAL   CORD.  623 

loss  of  function  which  follows  transverse  sections  of  different 
extent,  of  the  spinal  cord. 

1.  Complete  section  of  the  cord  is  followed  by  loss  of  sensation 
and  the  power  of  voluntary  motion  in  the  parts  below  the  point 
of  section  (Galen). 

2.  Section  of  one  side  of  the  cord  is  followed  by  loss  of  sensa- 
tion on  the  side  of  the  body  opposite  to  and  below  the  section, 
and  loss  of  all  voluntary  motion  of  the  parts  below  and  on  the 
same  side  as  the  injury  of  the  cord.     Increased  sensitiveness  of 
the  parts  where  the  motor  paralysis  exists  is  also  said  to  be 
observed  in  some  of  the  lower  animals. 

3.  If  the  gray  matter  and  the  anterior  and  posterior  white 
columns  of  one-half  of  the  cord  be  cut  across,  i.  e.,  when  only 
the   lateral   white   columns   remain  intact   on   that   side,  both 
motor  and  sensory  impulses  (as  observed  in  the  rabbit)  seem  to 
be  transmitted  normally. 

4.  Longitudinal  section  of  the  commissures  which  unite  the 
two  sides  of  the  cord,  so  as  to  separate  the  lateral  halves,  is  said 
not  to  influence  voluntary  motion,  but  produce  an  ill-defined  loss 
of  sensation  below  the  lesion. 

5.  Experiments  consisting  of  partial  and  local  sections  were 
conducted  with  the  object  of  determining  the  exact  course  of  the 
impulses  giving  rise  to  different  kinds  of  sensation ;  and  it  was 
concluded  that  ordinary  sensory  impulses  (pain)  traveled  by  the 
gray  matter,  while  tactile  temperature  and  muscle  sense  traveled 
via  the  posterior  white  columns.     Though  pathological  observa- 
tion and  the  occurrence  of  "analgesia"  (unimpaired  tactile  sense 
with  local  loss  of  painful  impressions)  suggest  the  idea  of  such 
distinct  paths  for  different  kinds  of  sensation,  it  would  appear 
that  the  localization  of  pain  to  the  gray  matter  and  of  touch, 
etc.,  to  the  posterior  white  column  cannot  be  accepted  as  demon- 
strated experimentally. 

From  the  foregoing  we  may  draw  the  following  conclusions  : — 

1.  Voluntary   motor  impulses  from   the  cortex  of  the  brain 

travel  directly  to  the  pyramidal  tracts,  and  thence  to  the  cells  of 

the  anterior  gray  columns.      The  fibres  decussate  in  the  medulla 

or  in  the  upper  part  of  the  cord. 


624 


MANUAL   OF    PHYSIOLOGY. 


2.  Motor  impulses  travel  from  the  upper  to  the  lower  segments 
of  the  cord  in  the  white  fibres  around  the  anterior  gray  columns. 


FIG.  248. 


Diagram  illustrating  the  course  taken  by  the  fibres  in  the  spinal  cord.    (Ajter  Pick.) 
A,  B  and  c  represent  oblique  transverse  sections  of  the  cord,  the  1  issue  between  being 

supposed  to  be  transparent.    At  the  lowest  section  (c),  sensory  nerve  fibres  (a)  enter 

by  the  posterior  root,  and  are  connected  with  ganglion  cells  of  the  gray  matter,  and, 

through  the  posterior  white  column,  with  the  brain  (b). 
Impulses  arriving  by  the  same  posterior  root  may,  to  reach  other  parts,  traverse  the  finer 

fibrils  of  the  gray  matter— shown  by  the  fine  lines. 
When  an  impulse  comes  directly  from  the  brain  (voluntary  centres)  it  adopts  the  direct 

routes  (c  or  e),  passing  through  the  pyramidal  tracts,  to  excite  the  motor  ganglion  cells 

of  the  cord  to  coordinated  activity. 
From  many  parts  of  the  gray  matter  ganglion  cells  despatch  impulses  by  the  motor 

root  (d). 
Some  white  fibres  only  communicate  between  tha  cells  of  the  various  segments  of  the 

giay  matter  (/). 


3.  Various  afferent  impulses  cross  at  once  on  entering  the  cord 
to  the  posterior  gray  columns  of  the  other  side,  and  then  ascend 


SPINAL   CORD   AS   A   COLLECTION   OF   NERVE   CENTRES.    625 

by  the  neighboring  white  fibres  of  the  posterior  root  zones,  the 
direct  cerebellar  tract,  the  posterior  median  tract  of  Goll,  and 
probably  also  by  some  of  the  white  channels  of  the  lateral 
column. 

4.  Besides  their  numerous  thin  protoplasmic  connections  in 
the  various  segments  of  the  gray  matter,  all  the  cells  of  the  cord 
are  in  communication  with  their  more  distant  neighbors  by 
means  of  the  white  fibres  of  the  root  zones. 

SPINAL  CORD  AS  A  COLLECTION  OF  NERVE  CENTRES. 

In  the  gray  substance  there  is  still  greater  difficulty  in  tracing 
the  course  taken  by  the  various  kinds  of  impulses,  and  little  is 
known  on  the  subject  beyond  what  is  surmised  from  the  prox- 
imity of  the  different  parts  to  the  anterior  and  posterior  roots  and 
to  the  white  channels  the  function  of  which  is  known. 

Though  the  attempt  to  localize  the  different  functions  to  any 
anatomical  region  has  not  met  with  success,  histology  has  taught 
us  of  the  existence  of  certain  groups  of  cells  which,  when  viewed 
longitudinally,  may  be  called  vesicular  columns.  Of  these,  four 
may  be  named  as  distinctively  marked.  (Fig.  247.) 

1.  The    anterior   (motor)    cellular    columns   occupy   the  gray 
piatter  seen  in  sections  of  the  cord  as  the  anterior  cornua.    They 
extend  throughout  the  entire  length  of  the  cord,  the  cells  being 
specially  numerous  where  the  large  motor  roots  come  off.     The 
cells  are  characterized  by  their  great  size  and  the  number  of 
their  branches,  one  of  which  forms  the  axis  cylinder  of  one  of 
the  motor  fibres  passing  to  the  roots  of  the  spinal  nerves. 

2.  The  posterior  cellular  columns  situated  in  the  gray  matter  of 
the  posterior  cornua  are  much  less  obvious  than  the  anterior. 
The  cells  are  few,  small  and  mostly  spindle-shaped.     Their  pro- 
cesses are  not  readily  traced  to  the  roots  of  the  spinal  nerves. 

3.  The  poster o -median   cellular   column  (Clarke)   lies  on  the 
median  side  of  the  posterior  gray  column,  so  that  it  forms  the 
inner  part  of  the  posterior  coruua  near  its  base.     The  cells  are 
numerous,  but  much  smaller  than  those  of  the  anterior  vesicular 
column.    Clarke's  column  is  best  developed  at  the  junction  of  the 
lower  dorsal  and  upper  lumbar  nerves.     It  tapers  off"  above  and 

53 


626  MANUAL   OF    PHYSIOLOGY. 

below,  and  the  cells  cease  to  form  a  continuous  column  opposite 
the  seventh  cervical  nerve.  But  scattered  groups  of  cells  in  a 
corresponding  position  are  found  throughout  all  the  cervical 
cord,  and  seem  to  link  this  spinal  column  with  the  vagus  nucleus 
in  the  medulla. 

4.  The  intermedia-lateral  cellular  columns  lie  in  the  lateral  con- 
cavities seen  on  section  between  the  anterior  and  posterior  gray 
cornua.  They  thus  occupy  a  position  between  the  lateral  white 
column  and  the  central  part  of  the  gray  matter.  They  are  best 
marked  in  the  dorsal  region,  as  they  seem  fused  with  the  cells  of 
the  anterior  cornua  in  the  lumbar  and  cervical  enlargements. 

The  facts  that  cells  functionally  related  are  grouped  in  masses 
at  the  points  where  the  spinal  nerves  arise,  and  that  the  various 
regions  of  the  cord  can  respond  to  stimulus  when  severed  from 
the  rest,  seem  to  indicate  that  a  strict  homology  exists  between 
the  spinal  centres  of  vertebrate  and  the  central  nervous  system 
in  many  of  the  lower  animals,  which  consists  of  a  double  chain 
of  ganglia,  united  together  by  conducting  channels. 

We  may  then  suppose  the  gray  matter  of  the  spinal  cord  to 
be  made  up  of  a  series  of  segments,  corresponding  in  number  to 
the  vertebral  development,  fused  together  into  one  continuous 
organ.  These  segments  may  be  supposed  to  receive  the  afferent 
impulses  from  corresponding  parts  of  the  body,  and  send  efferent 
impulses  to  muscles  capable  of  moving  that  part,  just  as  the 
separate  ganglia  of  the  invertebrate  chain  preside  over  the  func- 
tions of  the  corresponding  somite  of  the  animal's  body. 

The  various  groups  of  cells  in  the  spinal  cord  are  in  more  or 
less  direct  union  with  the  roots  of  the  nerves  and  the  conducting 
fibrils  of  the  cord  itself,  so  that  they  participate  in  the  transmis- 
sion of  the  impulses  to  and  from  the  centres  situated  in  the  brain. 
In  the  transmission  of  these  impulses  the  cells  seem  to  have  a 
certain  directing  and  controlling  influence  which  deserves  special 
attention,  as  it  gives  us.  the  key  to  the  more  complex  mechanisms 
of  the  higher  centres.  Although  the  various  powers  exerted  by 
the  cells  of  the  spinal  cord  are  so  intimately  associated  together 
as  to  be  practically  inseparable,  it  is  found  convenient  to  consider 
their  functions  under  distinct  headings. 


REFLEX  ACTION.  627 

REFLEX  ACTION. 

When  an  afferent  impulse  arrives  at  the  cells  of  the  posterior 
column,  it  is  communicated  to  the  cells  in  the  same  segment,  and 
reaching  motor  cells  it  gives  rise  to  a  movement  of  the  muscles 
of  the  neighborhood  from  which  the  impulse  first  started.  At 
the  same  time  impulses  travel  to  the  brain,  and  there  give  rise  to 
a  consciousness  of  the  various  events  taking  place,  i.  e.,  a  local 
stimulation  and  a  local  movement.  The  action  of  the  cells  of 
the  cord  takes  place  without  the  aid  of  the  will,  and  occurs 
before  the  mind  is  conscious  of  it.  These  movements,  being  a 
turning  back  of  the  impulse,  are  called  reflex  acts. 

Reflex  action  forms  the  most  ordinary  function  of  the  cells  of 
the  spinal  cord.  Even  the  gentlest  stimulation  may  give  rise  to 
a  complex  movement,  the  execution  of  which  requires  many 
muscles  to  act  together,  as  it  were,  with  a  common  object.  An 
unexpected  touch  to  the  finger  causes  a  person  to  withdraw  the 
hand  quickly.  If  greater  or  more  prolonged  stimulus  be  applied, 
more  extensive  movements  occur ;  by  the  well-arranged  coopera- 
tion of  many  muscles,  a  forcible,  definite  and  familiar  action  is 
performed.  For  example,  if  the  burning  head  of  a  match  adhere 
under  the  thumb  nail,  more  than  a  mere  withdrawal  of  the  hand 
'takes  place.  The  entire  arm  is  violently  shaken,  before  the  will 
has  time  to  come  into  operation.  We  have  here  a  complex  form 
of  purposeful  muscular  movement,  the  immediate  result  of  an 
impulse  coming  from  a  single  point  of  the  skin,  owing  to  the 
spreading  of  the  impulse  to  the  cells  of  the  segments  in  the 
vicinity.  The  movement  is  regular,  performed  with  a  definite 
purpose,  as  if  it  were  the  result  of  thought,  but  since  there  is  no 
consciousness,  it  cannot  be  mental. 

If  the  degree  of  stimulation  be  carefully  regulated,  it  will  be 
found  that  the  results  obtained  by  peripheral  stimulation  depend 
on  (a)  the  strength  of  the  stimulus,  and  the  length  of  time  for 
which  it  is  applied ;  (6)  the  degree  of  excitability  of  the  cells, 
and  the  readiness  with  which  the  impulses  pass  along  the  thin, 
conducting  channels  to  the  gray  matter,  and  (c)  the  functional 
activity  of  the  muscles  which  act  as  indicators  of  the  reflex 
effects. 


628  MANUAL   OF    PHYSIOLOGY. 

All  these  points  may  be  easily  studied  on  a  frog  decapitated 
about  an  hour  beforehand.  If  the  animal  be  suspended  by  the 
lower  jaw  and  the  toe  touched,  the  foot  is  gently  withdrawn.  If 
the  toe  be  smartly  pinched,  the  entire  limb  is  forcibly  raised  ;  with 
intense  or  prolonged  stimulus  both  legs  are  violently  moved.  If 
a  fragment  of  blotting  paper,  moistened  in  weak  acid,  be  placed 
on  the  belly,  in  a  position  not  easily  reached  by  the  foot,  a  com- 
plex series  of  movements  follows.  The  muscular  action  is  both 
elaborate  and  purposeful,  and  the  movements  of  the  headless 
animal  might  almost  be  called  ingenious. 

Strength  and  Duration  of  Stimulus. — By  graduating  the  strength 
of  the  acid  used  to  moisten  a  square  millimetre  of  blottiW  paper, 
the  following  results  are  obtained :  When  very  weak\acjd  is 
employed  only  slight  local  and  unilateral  movement  is-caused. 
Stronger  acid  produces  a  series  of  reflex  movements,  spreading 
to  several  muscles  on  both  sides  of  the  body.  If  further  strength- 
ened, the  movements  become  violent  and  more  extended  until 
the  whole  body  is  thrown  into  convulsion.  The  movements 
spread  from  the  nerve  cells  to  their  neighbors,  and  then  to  those 
governing  the  corresponding  muscles  of  the  other  side,  in  which, 
however,  they  are  less  marked  than  in  tho^e  of  the  side 
stimulated. 

Slight  stimulation,  when  of  short  duration,  not  sufficient  to 
produce  immediate  response,  may,  after  a  time,  give  rise  to  defi- 
nite reflex  action,  as  if  the  weak  impulses  arriving  at  the  nerve 
cells  in  the  cord  were  stored  up  until  their  sum  sufficed  to  pro- 
duce a  definite  reflex  movement.  This  may  be  seen  in  animals 
whose  nerve  centres  are  intact,  for  the  cells  of  more  remote  parts 
exercise  a  kind  of  checking  influence  on  those  in  the  region 
receiving  the  stimulus,  and  thus  the  accumulative  action  (sum- 
mation) comes  more  effectively  into  play.  In  the  human  subject, 
where  slight  visceral  stimulations  ex-ist  for  a  long  time,  this 
summation  may  be  observed.  In  some  of  these  cases,  even  with- 
out sensory  appreciation  of  the  Ipcal  excitation,  an  amount  of 
energy  may  be  accumulated  in  fhe  gray^racts  of  the  cord,  that 
will  bring  on  the  most  extensive  forms  of  reflex  muscular  move- 
ment. These  movements  differ  often  from  the  regular  coordi- 


INHIBITION   OF   REFLEX   ACTION.  629 

.{(    ' 

nated  motion  resulting  at  once  from  skin  stimulation.  As  an 
example  of  this  may  be  named  the  convulsions  thaiKe^cur^j^ 
young  children  from  the  prolonged  irritation  of  intestinal 
worms,  or  during  the  painful  period  of  dentition. 

Exalted  Excitability  of  the  Cells. — In  certain  conditions  of  the 
nervous  system  convulsions  can  be  readily  excited.  As  most 
striking  among  these,  may  be  named  poisoning  with  the  alkaloid 
of  nux  vomica  (strychnia),  and  the  state  of  the  blood  which  is 
produced  by  cessation  of  the  respiratory  function  (asphyxia). 
These  toxic  conditions  bring  about  a  peculiar  excitable  state  of 
the  cells  or  conducting  fibres  of  the  spinal  cord,  in  which  im- 
pulses pass  with  unwonted  facility  from  one  part  to  another,  and 
give  rise  to  an  excessive  degree  of  action  even  in  response  to 
gentle  stimulation.  A  frog  poisoned  with  strychnia  is  thrown 
into  general  spasm  by  the  least  touch,  which  normally  would 
only  cause  it  to  withdraw  the  limb. 

On  the  other  hand,  there  are  many  poisons  which  deaden  the 
reflex  powers  of  the  cord  centres,  among  which  are  opium,  chloro- 
form, chloral,  digitalin,  etc.  The  condition  of  the  blood  (apncea) 
which  may  be  brought  about  by  very  rapid  movements  during 
artificial  respiration,  has  also  the  effect  of  lowering  the  excita- 
Jrility  of  the  spinal  nerve  cells,  and  slowing  respiration. 

INHIBITION  OF  REFLEX  ACTION. 

The  great  majority  of  reflex  actions  may  be  prevented  or  con- 
trolled by  the  will,  and  the  basal  ganglia  and  medulla  habitually 
exert  a  checking  or  inhibitory  influence  on  the  reflex  actions  of 
the  spinal  cord.  It  is  in  this  way  that  we  account  for  the  facts 
that  a  living  frog  when  stimulated  does  not  respond  with  the 
ordinary  reflex  movements,  and  that  a  human  being,  when 
asleep,  shows  reflex  action  in  response  to  a  slight  stimulus  that 
would  be  quite  ineffectual  were  he  awake.  For  some  little  time 
after  pithing  a  frog,  constant  or  regular  results  are  seldom  met 
with,  because  the  section  of  the  upper  part  of  the  spinal  cord 
acts  as  a  stimulus  to  those  channels  which  habitually  bear  im- 
pulses from  the  brain,  and,  by  exciting  them,  has  inhibitory 
effect.  Further,  artificial  stimulation  of  the  corpora  quadrigemina 


630  MANUAL   OF   PHYSIOLOGY. 

and  medulla  have  the  effect  of  checking  the  reflex  action  of  the 
cord. 

If,  while  the  cord  is  employed  in  reflex  action,  in  response  to 
gentle  cutaneous  stimulation,  the  central  end  of  a  large  sensory 
nerve  trunk  be  stimulated,  the  reflex  action  ceases.  In  short,  it 
may  be  accepted  that  strong  impulses  arriving  at  the  cord  from 
any  direction  have  the  effect  of  inhibiting  the  action  of  its 
reflecting  cells. 

The  theory  of  reflex  action  lies  at  the  bottom  of  all  nervous 

activities,  and  it  is  therefore  useful  to  attempt  to  work  out  the 

details  of  the  mechanisms  by  means  of  which  it  is  carried  on.     A 

simple  plan  of  the  channels  traversed  by 

FIG.  249.  .       r  .  J 

the  impulses  is  given  in  the  diagram  (I1  ig. 
249),  in  which  the  arrow  heads  show  the 
direction  of  the  afferent  impulse  passing 
along  the  posterior  root  to  reach  a  cell  in 
the  posterior  gray  column,  thence  it  passes 
to  a  cell  in  the  anterior  column,  to  reach 
the  efferent  fibre,  and  through  the  anterior 

S.  Sensory  receiving  organ  ° 

with    attached    afferent     motor  root  of  the  nerve  on  its  way  to  the 

nerve  fibre.  J 

G-C  Central  organs-ganglion  muscle.  It  has  been  suggested  that  the 
M.  Peripheral  organ  and  ef-  impulse  meets  with  considerable  resist- 

ferent  nerve.  .  . 

ance  in  passing  through  the  protoplasm 

of  the  cells,  and  that  owing  to  this  resistance,  the  effect  of  a  slight 
stimulus  remains  localized,  while  more  powerful  impulses  can 
overcome  the  resistance,  and  spread  to  a  greater  number  of  cells. 
Thus,  the  regular  radiation  in  the  cord  would  be  simply  depend- 
ent on  the  inability  of  the  impulses  to  affect  cells  other  than 
those  in  their  immediate  neighborhood.  Following  out  this  view, 
it  has  been  suggested  that  the  resistance  is  increased  by  impulses 
arriving  at  the  cells  from  a  different  direction,  and  the  inhibitory 
action  of  the  higher  centres,  or  peripheral  excitation  of  another 
part,  impedes  the  spreading  of  the  impulses. 

But  this  theory  of  resistance  to  and  interference  with  the  trans- 
mission of  impulses  in  the  nerve  cells  hardly  explains  all  the 
phenomena  observed  in  the  reflex  action  of  the  spinal  cord  and 
the  various  modifications  it  can  undergo. 


INHIBITION   OF   REFLEX   ACTION. 


631 


The  reflex  convulsions  that  occur  in  poisoning  with  strychnine, 
or  as  the  result  of  some  constant  but  slight  stimulation,  may  be 
explained  as  follows : — 


FIG.  250. 


Diagram  of  the  paths  taken  by  the  impulses  in  the  brain  and  cord.    MM.,  motor  chan- 
nels; ss.,  sensory  channels;  CR.,  cranial  nerves. 

Besides  the  resistant  protoplasmic  fibrils  in  the  gray  part  of 
the  cord,  there  exist  raedullated  fibres  in  the  root  zones — short 
cuts,  as  it  were — by  which  impulses  travel  from  one  part  of  the 
cord  to  another.  If  we  suppose  the  ordinary  reflex  traffic  of  the 


632  MANUAL   OF   PHYSIOLOGY. 

cord  cells  to  be  carried  on  without  the  assistance  of  these  direct 
lines  of  communication,  we  must  assume  that  there  is  some  special 
means  of  shutting  these  fibres  out  of  the  working  of  the  reflex 
machine.  Such  special  mechanisms  in  all  probability  exist,  and 
are  in  relationship  with  or  under  the  command  of  the  inhibitory 
cells  of  the  higher  centres.  We  may  then  suppose  that  strychnine 
removes  the  power  of  these  inhibitory  agents,  and  the  impulses 
finding  the  direct  ways  open,  take  these  routes,  and  are  simul- 
taneously and  irregularly  diffused  throughout  all  the  cell  terri- 
tories (independent  of  the  ordinary  paths  they  have  been  educated 
to  follow),  and  thus  convulsive  movements  are  excited  in  many 
parts  of  the  body. 

In  like  manner  the  unremitting  activity  necessary  to  keep  in 
check  the  impulses  arriving  from  a  constant  source  of  stimulation 
(such  as  intestinal  worms),  eventually  fatigues  the  active  ele- 
ments in  this  inhibitory  mechanism,  and  then — often  suddenly — 
the  force  of  the  accumulated  irritaticn  rushes  along  the  direct 
channels  to  all  parts  of  the  cord,  and  simultaneously  exciting 
them,  brings  many  discordant  muscles  into  spasmodic  action. 

The  reflexion  of  an  impulse  from  a  sensory  nerve,  through  the 
cells  of  the  spinal  cord  to  a  motor  nerve,  occupies  a  measurable 
length  of  time,  which  has  been  estimated  at  about  y1^  of  a  second. 
The  time  required  for  the  performance  of  a  reflex  act  varies  con- 
siderably in  the  same  individual  under  different  conditions;  of 
these,  high  temperature  and  intense  stimulation  shorten  the  time, 
and  fatigue  or  cold  lengthen  it. 

SPECIAL  REFLEX  CENTRES. 

Many  of  the  groups  of  nerve  cells  in  the  cord  are  employed  in 
executing  familiar  acts  essential  to  the  animal  economy  inde- 
pendent of  the  will.  Many  of  these  acts  are  very  complex,  and 
require  the  coordinated  action  of  certain  sets  of  muscles.  Such 
groups  of  nerve  cells  have  been  called  special  centres,  and  many 
of  them  have- already  been  described  in  the  preceding  chapters. 
The  more  important  are  : — 

1.  A  centre  for  securing  the  retention  of  the  urine  ty  the  tonic 
contraction  of  the  sphincter  muscle  of  the  bladder.  This  group 


COORDINATION.  633 

of  Derve  cells  is  probably  kept  in  action  by  impulses  arriving 
from  the  bladder  by  the  afferent  nerves  passing  from  its  walls  to 
the  spinal  cord.  The  more  distended  the  bladder  becomes,  the 
more  powerful  the  stimulus  sent  to  the  cord,  and  therefore  the 
more  firmly  the  sphincter  is  made  to  contract. 

2.  Nearly  related  to  the  former  is  the  centre  which  presides 
over  the  evacuation  of  the  bladder.     This  is  excited  by  impulses 
arriving  from  the  urethra,  near  the  neck  of  the  bladder.    It  then 
sets  the  detrusor  muscle  in  action,  while  the  sphincter  is  relaxed 
by  voluntary  inhibition. 

3.  The  ejaculation  of  the  semen  may  also  be  said  to  be  accom- , 
plished  by  a  special  spinal  centre,  capable  of  controlling  move- 
ments, in  which  involuntary  muscles  play  an  important  part. 

4.  In  parturition  a  number  of  motions  are  called  into  play  (as 
well  as  the  uterine  contraction)  which  are  so  regularly  coordi- 
nated, though  involuntary,  as  to  entitle  us  to  suppose  that  they 
are  arranged  by  a  special  centre  in  the  spinal  cord. 

5.  The  act  of  defecation  is  accomplished  by  means  of  a  spinal 
centre  also.     The  action  of  this  centre  might  (like  that  presiding 
over  the  urinary  bladder)  be  divided  into  two  parts — retention 
and  evacuation — in  which  volition  and  intestinal  peristalsis  play 
a  very  important  part. 

COORDINATION. 

From  what  has  been  said  concerning  the  more  complex  reflex 
actions,  it  is  clear  that  the  cells  of  the  spinal  cord  are  capable  of 
arranging  the  discharge  of  nerve  impulses,  so  as  to  bring  about 
definite  purposeful  movements.  This  power  of  coordinating  im- 
pulses, which  is  so  striking  in  some  reflex  actions  after  the  brain 
has  been  destroyed,  is  equally  important  in  arranging  efferent 
impulses  and  accomplishing  ordinary  voluntary  movements.  In 
fact,  most  of  the  details  of  the  mode  of  working  of  the  muscles  are 
under  the  control  of  the  cells  of  the  spinal  cord. 

It  will  help  us  in  formulating  the  mechanism  if  we  suppose  the 
resistance  in  the  gray  part  of  the  cord  to  be  much  greater  than 
that  in  the  medullated  nerve  channels,  and  that  throughout  it 
the  paths  are  so  numerous  that  each  individual  nerve  cell  might 
be  in  communication  with  every  other  nerve  cell.  These  paths 


634  MANUAL   OF   PHYSIOLOGY. 

are  made  passable  by  use ;  the  oftener  an  impulse  traverses  a 
given  route  the  more  adapted  such  a  route  becomes  for  future 
traffic.  Thus,  by  practice,  we  constantly  freshen  certain  chan- 
nels of  intercommunication  between  the  various  cells  of  the  cord 
and  thus  make  beaten  tracks,  along  which  impulses  can  pass 
without  hindrance.  In  a  similar  way  certain  groups  of  nerve 
cells  acquire  the  habit  of  working  together  and  exciting  complex 
movements  which  at  first  were  impossible.  The  nerve  paths, 
along  which  the  impulses,  producing  common  movements,  have 
to  pass,  are  no  doubt  prepared  by  the  long  practice  of  our  ances- 
tors, and  the  power  of  performing  these  actions  is  transmitted  to 
us  ready  for  immediate  application.  Other  paths  connecting 
groups  of -cells  required  for  the  production  of  unusual  combina- 
tions of  movements  have  to  be  practiced  by  the  individual,  and 
much  of  the  difficulty  of  learning  any  trade  of  special  manual 
dexterity  depends  on  the  necessity  of  making  impulses  readily 
traverse  definite  directions,  so  as  to  excite  certain  groups  of  cells 
to  act  synchronously  and  set  the  required  combination  of  mus- 
cles in  accurately  coordinated  motion.  Indeed,  the  delicacy  of 
manipulation  required  by  some  trades  cannot  be  attained  in  the 
lifetime  of  one  individual ;  thus,  it  is  said  to  require  three  gene- 
rations to  make  a  perfect  glassblower ;  the  grandson  having  the 
benefit  of  the  hereditary  tendency  to  accomplish  certain  coordi- 
nations acquired  by  the  lifelong  habit  of  the  parents. 

The  importance  of  this  technical  education  of  the  cells  of  the 
spinal  cord  in  the  execution  of  delicate  manipulations  will  be 
felt  if  one  attempt  to  imitate  the  movements  of  precision  which 
a  skilled  craftsman  executes  without  attention  or  voluntary  effort 
even  in  the  most  careless  exercise  of  his  craft.  The  practice 
required  for  such  education  is  experienced  by  any  one  who  attains 
skill  in  the  simplest  special  manipulation,  from  writing  to  play- 
ing the  violin. 

AUTOMATISM. 

Besides  being  excited  to  action  by  impulses  coming  from  the 
brain — volition — and  from  the  surface — reflexion — the  groups  of 
cells  in  the  spinal  cord  may  act  without  any  obvious  incoming 
impulse ;  that  is  to  say,  some  of  the  cells  appear  to  be  capable  of 


AUTOMATISM.  635 

independent  activity.  Such  groups  of  nerve  cells  are  commonly 
called  automatic  centres;  the  more  important  of  those  found  in 
mammalia  may  be  classified  as  follows : — 

1.  Vasomotor  centres:    Though  the  central  point  controlling 
the  contraction  of  the  blood  vessels  is  situated  in  the  medulla, 
there   is   no   doubt  that  even  in   man,  centres  are  distributed 
throughout  the  gray  matter  of  the  spinal  marrow,  which  are 
capable  of  keeping  up  the  arterial  tone  in  the  regions  over  which 
they  preside.     As  evidence  of  this  may  be  mentioned  the  fact 
that  the  dilatation  of  the  arteries,  which  follows  the  severance  of 
the  lumbar  part  of  the  cord  from  the  medulla,  only  lasts  a  few 
days,  after  which  the  vessels  again  contract  in  a  distinctly  tonic 
manner.    The  arterial  tonus  only  disappears  completely  and  per- 
manently when  the  spinal  cord  is  destroyed.     Thus,  it  would 
appear — although  habitually  all  the  vessels  of  the  body  are  regu- 
lated by  a  centre  in  the  medulla,  nearly  related  to  the  cardiac 
centre — that  every  vascular  region  has  a  nervous  mechanism  of 
its^pwn  in  the  cord,  which  suffices  to  keep  up  the  tonic  contrac- 
tion of  the  muscular  coat  of  its  vessels. 

2.  Sweating  centres :  Though  closely  related  to  the  preceding, 
the  centres  which  preside  over  the  secretion  of  sweat  in  the  lower 
part  of  the  body  and  hinder  extremities  must,  for  many  reasons 
which  cannot  now  be  mentioned,  be  regarded  as  separate  centres. 

3.  Some  smooth   muscle  fibres  appear   to   be   influenced   by 
centres  in  the  cord.     In  the  lower' part  of  the  cervical  cord  is  a 
group  of  nerve  cells  which  keep  the  sphincter  muscle  of  the  iris 
in  check;  narrowing  of  the  pupil  has  been  described  as  follow- 
ing injury  of  this  region. 

4.  The  gray  matter  of  the  cord  is  also  said  to  keep  the  skeletal 
muscles  in  a  state  of  slight  tonic  contraction ;  elongation  of  the 
muscles  is  said  to  follow  section  of  the  anterior  roots.     When 
this  muscular  tone  is  absent  the  phenomenon  known  as  "  tendon 
reflex  "  is  wanting,  as  the  tap  on  the  tendon  ceases  to  excite  the 
toneless  muscle. 

5.  So-called  trophic  centres  are  also  said  to  exist  in  the  spinal 
cord.     The  best  evidence  in  this  matter  is  derived  from  the 
skeletal  muscles.     If  the  motor  nerves  or  roots  be  cut,  or  the 


636  MANUAL   OF   PHYSIOLOGY. 

anterior  gray  motor  columns  injured,  the  paralyzed  muscles  soon 
undergo  fatty  degeneration,  which  does  not  depend  on  mere  inac- 
tivity, for  it  does  not  follow  cerebral  paralysis,  in  which  the 
integrity  of  the  muscle  can  be  preserved  by  suitable  electric 
stimulation.  Similar  trophic  agencies  probably  influence  the 
other  tissues.  Thus,  many  affections  of  the  skin,  herpes,  etc.,  are 
attributed  to  nervous  lesions. 

On  account  of  the  elaborate  and  purposeful  reflex  movements 
performed  by  decapitated  frogs  or  eels,  it  has  been  suggested  that 
in  the  lower  vertebrates  the  spinal  cord  is  capable  of  sensation 
and  volition — mental  activity — but  to  follow  this  assumption  we 
should  have  to  modify  our  ideas  of  volition  and  sensation,  for 
which  consciousness  is  commonly  taken  to  be  a  necessary  factor. 
It  is,  however,  important  to  note  that  the  lower  we  go  in  the 
scale  of  vertebrate  animals  the  less  powerful  are  the  mental 
faculties,  and  the  more  important  are  the  functions  presided  over 
by  the  spinal  marrow. 


MEDULLA   OBLONGATA.  637 


CHAPTER  XXXV. 
THE  MEDULLA  OBLONGATA. 

The  direct  continuation  of  the  spinal  cord  is  called  the  medulla 
oblongata.  It  consists  of  representatives  of  the  various  parts  of 
the  cord,  with  some  additional  gray  matter.  The  relationship  of 
the  different  parts  of  the  medulla  to  those  of  the  spinal  cord 
may  be  best  understood  by  supposing  the  posterior  median  fissure 
and  underlying  nerve  substance  at  its  upper  limit  to  be  split 
vertically  down  to  the  central  canal,  and  the  lateral  masses  sepa- 
rated, so  that  the  gray  part  becomes  spread  out  on  the  posterior 
surface,  and  there  forms  the  floor  of  the  fourth  ventricle.  The 
gray  matter  of  the  medulla  oblongata  consists  of  two  sets  of  nuclei  ; 
one  being  the  continuation  of  the  gray  columns  of  the  spinal 
marrow,  and  the  other  made  up  of  certain  additional  gray 
nodules'embedded  here  and  there  among  the  white  strands. 

The  anterior  motor  gray  columns,  which  are  cut  off  from  the 
central  gray  substance  by  the  passage  of  the  pyramidal  tract  to  the 
opposite  side,  are  continued  along  the  floor  of  the  fourth  ventricle 
near  the  median  line.  The  posterior  gray  columns  are  continued 
upward  to  form  the  nucleus  of  Rolando,  and  are  spread  out  on  the 
lateral  part  of  the  floor  of  the  ventricle.  Important  nuclei  of 
gray  matter  lie  in  the  olivary  bodies,  and  numerous  collections 
of  cells  forming  the  nuclei  from  which  arise  the  chief  cranial 
nerves.  For  an  adequate  description  of  these  groups  of  nerve 
cells  and  their  connections,  works  on  anatomy  must  be  consulted. 

The  various  white  columns  of  the  spinal  cord  are  so  distributed 
in  the  medulla  that  their  course  gives  some  indication  of  the 
channels  by  which  impulses  are  carried  through  it. 

In  ascending  to  the  medulla  the  posterior  white  columns  become 
differentiated  into  three.  (1)  Goll's  column  is  more  distinctively 
marked  off,  and  enlarges  to  form  the  funiculus  gracilis,  containing 
the  clavate  nucleus;  the  funiculus  gracilis  tapers  away  to  nothing 
above.  (2)  Burdach's  column  widens  in  a  wedge-like  fashion 


638 


MANUAL   OF   PHYSIOLOGY. 


FIG.  251. 


and  is  called  funiculus  cuneatus,  which  contains  the  cuneate  nu- 
cleus. It  passes  on  to  help 
to  form  the  inferior  pe- 
duncle of  the  cerebellum. 
(3)  By  the  projection  of 
the  enlarged  posterior 
gray  column,  Tubercle  of 
Rolando,  a  prominence  is 
produced  called  thefunic- 
ulus  of  Rolando.  This  also 
helps  to  form  the  inferior 
peduncle  of  the  cerebel- 
lum. 

The  greater  part  of  the 
lateral  white  columns  of  the 
spinal  cord  pass,  at  the  de- 
cussation  of  the  pyramids, 
to  the  opposite  side  to  form 
the  pyramidal  prominence 
on  the  front  of  the  medul- 
la, and  are  thence  con- 
tinued upward  directly  to 
the  motor  areas  of  the  cor- 
tex. The  direct  cerebellar 

Diagram  of  Brain  and  Medulla  Oblongata.    (Cleland.)   ,        ,         i«   r       f  ,1 

a,  Spinal  cord  ;  b,  b,  Cerebellum  divided,  and  above  tr€LCt      whlch       foi>mS  the 

it  the  valve  of  Vieussens  partially  divided;   c,  ennprfioifll      narf     of  thp 

Corpora  quadrigemina;    *,£,  Optic  thalami:    e',  SUP61 

pineal  body;/,/,  Corpora  striata;  gr,. 9    Cerebral  lateral     Column     joins   the 
hemispheres  in  section;   h,  Corpus  callosum;  i,  J 

Fornix ;  /,  /,  Lateral  ventricles ;  3,  Third  ventricle ;  cuneate      and       Kolando's 
4,  Fourth  ventricle ;  5,  Fifth  ventricle,  bounded  on 

each  side  by  septum  lucidum.  bands  to  form  the  inferior 

cerebellar  peduncle. 

The  majority  of  the  fibres  of  the  anterior  gray  columns  pass  into 
the  medulla  beneath  the  pyramids  by  which  they  are  quite  con- 
cealed. They  can  be  traced  some  distance  through  the  pons 
Varolii.  The  fibres  of  the  direct  pyramidal  tracts  join  the  pyra- 
mid of  their  own  side. 

It  must  be  remembered  that  the  medulla  is  the  only  route 
between  the  spinal  cord  and  the  upper  nerve  centres. 


MEDULLA   OBLONGATA. 


639 


Through  it  all  the  afferent  and  efferent  channels  must  pass,  as 
they  do  through  the  spinal  cord.  From  it,  and  the  prolongation 
of  its  gray  nuclei  in  the  pons  Varolii,  several  cranial  nerves  take 
origin.  Thus,  the  medulla  is  to  the  cranial  nerves  (from  the  fifth 
to  the  ninth)  as  the  spinal  cord  is  to  the  spinal  nerves,  but  their 
mode  of  distribution  is  different. 

THE   MEDULLA  OBLONGATA  AS  A  CENTRAL  ORGAN. 
A  number  of  groups  of  ganglion  cells  with  special  duties  are 
located  in  the  medulla.     Those  acts  which  are  most  important 

FIG.  252. 


0 


c 


• 


Diagram  showing  the  position  of  the  nuclei  of  the  cranial  nerves  in  the  medulla  ob- 
longata,  etc.,  as  if  seen  in  antero-posterior  .section,  looking  from  the  median  line 
toward  the  right  side.  The  nuclei  near  the  median  line  are  more  darkly  shaded. 

Py,  Pyramidal  tracts;  PyKr,  Pyramidal  decu'ssation ;  0,  Olivary  body;  Os,  Superior 
olivary  body;  V,  motor;  V,  Middle  sensory  ;  and  F",  Lower  sensory  nuclei  of  fifth 
nerve  ;  R  v.  Roots  of  fifth  nerve  j-vi,  Nucleus  of  sixth  nerve  \  R  VI,  Root  of  sixth 
nerve;  vii,  Nucleus  ;  Gf,  Knee;  and  R  vn,  Root  of  |ortio  dura  of  seventh  nerve; 
Vin,  Auditory  ;  ix,  Glosso-pharyngeal ;  x,  Vagus;  xi,  Accessorius  ;  and  xn,  Hypo- 
glossal  nuclei ;  Kz,  Clavate  nuclei. 

for  the  execution  of  the  vegetative  functions,  are  arranged  and 
governed  by  the  nerve  cells  of  the  medulla.  Some  of  these 
centres  are  called  automatic,  though  they  are  variously  affected 


640  MANUAL   OF   PHYSIOLOGY. 

by  many  impulses  arriving  from  distant  points,  while  others  are 
purely  reflex  in  their  action. 

The  former  are  the  more  essential,  and  will  therefore  be  first 

considered. 

RESPIRATORY  CENTRE. 

The  centre  which  regulates  the  respiratory  movements  is  situ- 
ated in  the  floor  of  the  fourth  ventricle,  at  the  upper  and  back 
part  of  the  medulla.  Flourens  long  since  showed  that  injury  of 
this  spot — the  vital  point — was  followed  by  almost  instant  cessa- 
tion of  respiration. 

This  is  a  good  example  of  a  so-called  automatic  centre;  that 
is  to  say,  the  blood  flowing  through  the  medulla  and  nourishing 
the  cells  suffices  to  supply  them  with  the  energy  necessary  for 
their  activity.  Even  slight  variations  in  the  quality  or  tempera- 
ture of  the  blood  reaching  this  part  modifies  the  activity  of  the 
cells.  The  less  oxygen  and  waste  products  contained  in  the 
blood,  the  more  powerfully  does  it  act  as  a  stimulant  on  the 
centre. 

Although  we  take  the  respiratory  centre  as  an  example  of  an 
automatic  centre,  its  working  is  arranged  by  afferent  impulses, 
so  that  the  normal  rhythm  of  breathing  is  regulated  by  reflex 
action.  The  mechanical  state  of  the  lungs — whether  distended 
as  in  inspiration  or  contracted  as  in  expiration — seems  to  excite 
the  terminals  of  certain  fibres  of  the  vagus,  which  carry  impulses 
to  the  centre,  and  thus  excite  or  restrain  movements. 

This  automatic  centre  can  also  be  influenced  by  the  higher 
centres  of  the  brain,  for  by  our  will  we  can  regulate  our  breath- 
ing movements  or  stop  breathing  altogether  for  a  time.  Inde- 
pendent of  volition  the  higher  centres  control  the  respiratory 
rhythm,  as  seen  in  sleep,  when  their  action  is  partially  in  abey- 
ance while  the  vagi  are  active,  and  respiration  becomes  periodic, 
or  when  the  brain  functions  are  impaired  and  respiration  becomes 
intermittent  (Cheyne-Stokes  respiration).  Further,  the  action  of 
the  respiratory  centre  can  be  altered  by  impulses  arriving  from 
the  surface,  as  may  be  seen  by  the  gasping  inspirations  which 
involuntarily  follow  the  sudden  application  of  cold. 

Again,  the  activity  of  the  centre  may  be  altered  by  stimula- 


VASOMOTOR   CENTRE.  641 

tions  of  certain  parts  of  the  air  passages ;  so  much  so,  that  con- 
vulsive actions  of  the  respiratory  muscles  are  brought  about, 
which  induced  some  to  speak  of  a  sneezing  centre  and  a  coughing 
centre  in  the  medulla.  Bat  sneezing  and  coughing  may  be 
equally  well  explained  as  a  peculiar  form  of  activity  of  the 
respiratory  centre,  or  a  reflex  alteration  in  the  respiratory  rhythm, 
caused  by  irritation  of  the  nasal  or  laryngeal  mucous  membranes. 

Though  the  action  of  the  respiratory  centre  can  be  modified 
by  (1)  the  will  and  (2)  various  peripheral  stimulations,  and  is 
habitually  regulated  from  the  periphery  through  (3)  the  vagi  by 
the  state  of  the  lungs,  the  condition  of  the  blood  supplied  to  the 
centre  may  be  such  that  these  remoter  influences  are  quite  power- 
less. This  uncontrollable  condition  of  the  centre  is  established 
when  the  blood  flowing  through  it  is  abnormally  venous  and  the 
cells  become  over-stimulated.  We  know  how  short  a  time  we 
can  hold  our  breath  by  voluntary  checking  of  the  centre,  and 
most  people  have  had  occasion  to  observe  the  inordinate  and 
painful  efforts  of  a  person  whose  respiration  is  interfered  with  by 
disease,  ^yhen  the  dyspnoea  becomes  intense,  nearly  all  the 
muscles  in  the  body  are  called  into  action.  Thus,  in  quiet 
breathing  comparatively  few  nerve  cells  in  the  medulla  carry  on 
the  work  of  respiration,  but  under  certain  emergencies  they  can 
call  to  their  aid  the  entire  motor  areas  of  the  gray  substance  of 
the  spinal  cord,  and  thus  give  rise  to  a  general  effort.  Hence, 
we  often  hear  of  a  convulsive  centre  in  the  medulla  being  placed 
in  close  relation  to  the  respiratory  centre.  In  some  cases,  irrita- 
tion of  the  air  passages  or  imperfect  oxidation  of  the  blood,  the 
convulsive  centre  comes  under  the  command  of  the  cells  of  the 
respiratory  centre,  which  can  then  excite  coughing,  sneezing  or 
convulsive  inspiratory  effort. 

As  already  mentioned,  the  convulsions  of  asphyxia  may  be 
explained  by  the  impure  blood  acting  as  a  stimulus  on  the  cells 
of  the  cord  itself. 

THE  VASOMOTOR  CENTRE. 

It  has  already  been  stated  that  groups  of  cells  exist  in  the 
gray  part  of  the  spinal  cord,  which,  according  to  the  class  of 
animal,  have  more  or  less  direct  influence  upon  the  muscles  in 
54 


642  MANUAL   OF    PHYSIOLOGY. 

the  coats  of  the  vessels.  Thus,  in  a  frog  whose  brain  and 
medulla  have  been  destroyed,  in  some  hours  the  vessels  of  the 
web  regain  a  considerable  degree  of  constriction,  which  is  again 
lost  if  the  cord  be  destroyed.  In  the  dog  the  vessels  of  the 
hinder  limb  recover  their  tone  more  or  less  perfectly  in  a  few 
days  after  the  spinal  cord  has  been  cut  in  the  dorsal  region, 
although  just  after  the  section  they  are  widely  dilated  from  the 
paralysis  of  their  muscular  coats.  In  a  few  days  the  cells  of  the 
cord  can  learn  to  accomplish,  of  their  own  accord,  work  which 
they  have  been  in  the  habit  of  doing,  only  under  the  direction  of 
the  higher  centre.  From  this  we  conclude  that  though  the  cord 
contains  local  vasomotor  centres  distributed  throughout  its  gray 
matter,  these  are  normally  under  the  control  of  the  vasomotor 
centre  in  the  medulla,  and  this  centre  is  really  the  chief  station 
from  which  impulses  destined  to  affect  all  the  blood  vessels  must 
emanate. 

This  arrangement  is  quite  comparable  with  that  by  which  the 
ordinary  muscles  are  made  to  contract.  When  the  will  causes  a 
muscular  contraction,  the  impulse  starting  from  the  cerebral  cor- 
tex does  not  travel  directly  to  the  muscle,  but  it  passes  from  the 
brain  to  certain  cells  in  the  cord  and  thence  to  the  muscles.  In 
fact,  to  these  spinal  agents  the  ultimate  arrangement  and  coor- 
dination of  the  act  is  confided.  So,  also,  the  chief  vasomotor 
centre  in  the  medulla  executes  its  function  through  the  medium 
of  numerous  under  centres  placed  at  various  stations  along  the 
cord. 

The  vasomotor  centres— like  nearly  all  other  controlling 
groups  of  ganglion  cells — may  be  considered  as  composed  of  two 
parts  antagonistic  one  to  the  other,  viz.,  a  constricting  and  dilat- 
ing centre,  the  impulses  from  which  travel  by  separate  nerve 
channels.  The  constricting  impulses  are  mainly  distributed  by 
the  sympathetic  nerve,  while  the  dilating  impulses  accompany 
those  which  are  employed  in  calling  forth  the  ordinary  function 
of  the  part  in  question. 

From  what  has  been  said  as  to  the  wide  distribution  of  centres 
influencing  the  blood  vessels,  an  attempt  to  localize  exactly  the 
position  of  the  medullary  vasomotor  cells  is  not  satisfactory.  In 


CARDIAC   CENTRE.  643 

lower  animals — frogs— the  cells  are  evenly  diffused  throughout 
the  medulla  and  cord.  In  man  the  localization  is  difficult  to 
demonstrate,  though  we  have  reasons  for  thinking  it  much  more 
definitely  circumscribed.  In  the  rabbit  it  has  been  localized  to 
the  floor  of  the  fourth  ventricle  in  the  immediate  neighborhood 
of  the  respiratory  and  cardiac  centres.  From  this  the  nerves 
pass  by  the  cord  to  the  spinal  roots,  by  which  they  reach  the 
sympathetic. 

The  vasomotor  centre  exerts  a  tonic  or  continuing  action  on 
the  vessels,  holding  them  in  a  state  of  partial  constriction  or  lone. 
In  this  it  may  be  said  to  have  an  automatic  action.  Although 
this  tonic  state  of  activity  of  the  centre  may  be  called  automatic, 
it  is  really  under  the  control  of  many  reflex  influences,  which 
constantly  vary  the  general  tone,  or  effect  local  changes  in  the 
degree  of  constriction  of  this  or  that  vascular  area.  Among  the 
most  striking  afferent  regulating  impulses  are  those  arriving  from 
the  heart,  the  digestive  organs  and  the  skin.  In  some  animals, 
a  special  nerve — the  depressor — has  been  discovered,  which, 
passing  from  the  heart  to  the  medulla,  keeps  the  vasomotor 
centre  informed  as  to  the  degree  of  tension,  etc.,  of  the  heart 
cavities.  When  the  heart  becomes  over-full,  impulses  pass  from 
it,  and  check  the  tonic  power  of  the  centre,  so  as  to  reduce  the 
arterial  pressure  against  which  the  ventricle  has  to  act.  Elec- 
tric stimulation  of  this  nerve  causes  a  remarkable  fall  in  the 
general  blood  pressure.  The  vasomotor  centres  regulate  the 
distribution  of  blood  to  the  viscera  and  skin,  according  to  the 
condition  of  activity  of  these  parts  as  described  in  another 
chapter  (xxxr). 

THE  CARDIAC  CENTRE. 

Although  the  heart  beats  periodically  when  cut  off  from  the 
nervous  centres,  its  normal  rhythm  is  under  the  control  of  a 
group  of  nerve  cells  in  the  medulla,  from  which  some  fibres  of 
the  vagus  carry  special  regulating  impulses  to  the  heart.  The 
action  of  this  centre  is  habitually  that  of  a  restraining  agent 
lessening  the  rate  of  the  heart's  contractions,  and  is,  hence,  called 
a  tonic  inhibitory  centre.  The  activity  of  the  centre  is  in- 
fluenced by  the  condition  of  many  distant  parts,  such  as  the 


644  MANUAL   OF   PHYSIOLOGY. 

cortex  of  the  brain,  the  abdominal  viscera,  etc.,  which  exert  a 
kind  of  reflex  action  on  the  heart  through  the  centre.  The 
degree  of  inhibitory  power,  as  well  as  the  share  taken  in  the 
action  of  the  centre  by  automatism  and  reflexion,  differs  in  dif- 
ferent animals.  A  centre  (accelerator)  antagonistic  to  the  latter 
also  exists  in  the  medulla.  It  is  weaker  in  action  than  the 
inhibitory  centre,  and  is  not  tonic. 

In  the  medulla  there  also  exist  many  other  centres  connected 
with  the  organic  functions.  Among  these,  the  centres  for  swal- 
lowing and  vomiting  may  be  mentioned.  For  further  details  on 
this  subject,  the  reader  may  consult  the  chapter  on  Digestion. 


THE   BRAIN.  645 


CHAPTER  XXXVI. 

THE  BEAIN. 

As  we  pass  upward  in  attempting  to  trace  the  conducting 
channels  of  the  medulla,  we  come  to  the  more  elaborate  system 
of  nervous  textures  which,  together,  are  called  the  brain.  This 
is  anatomically  the  most  highly  developed,  and  physiologically 
the  most  intricate,  part  of  the  central  nervous  organs.  Besides 
the  nerve  cells  and  various  kinds  of  conducting  channels  with 
which  we  have  already  become  familiar  in  the  cord,  etc.,  there 
are  in  the  brain  a  vast  number  of  smaller  elements  which  do  not 
possess  the  distinctive  characteristics  of  nerve  cells.  These 
granular  bodies  are  tightly  packed  together  in  many  parts  of  the 
centres,  and  must  have  some  important  function. 

To  form  a  general  idea  of  the  plan  of  construction  of  the 
brain,  it  is  well  to  follow  its  development  in  the  earlier  stages  of  the 
embryo,  from  the  time  when  it  forms  an  irregular  and  thickened 
part  of  the  tube  of  tissue,  from  which  is  developed  the  cerebro- 
spinal  axis.  From  this  it  will  be  seen  that  the  brain  is  but  a 
modified  part  of  the  primitive  nervous  tube,  in  which  swellings 
may  be  observed  at  an  early  period  of  embryonic  life.  These 
swellings  are  called  the  fore-brain,  mid-brain  and  hind-brain, 
and  in  the  future  development  of  the  parts  give  rise  to  (1)  the 
hemispheres  and  basal  ganglia ;  (2)  the  corpora  quadrigemina, 
pons  and  cerebellum  ;  and  (3)  the  medulla  oblongata.  The  great 
mass  of  the  brain — the  hemispheres — is  formed  by  an  excessive 
development  of  bud-like  processes  which  grow  out  from  the  sides 
of  the  fore-brain  at  an  early  period,  and  become  elaborately 
folded,  so  that  in  the  adult  it  is  difficult  to  trace  the  relationship 
to  the  original  form.  For  further  details  of  the  development  of 
the  brain,  vide  chapter  on  that  subject. 

The  cells  of  the  brain  are,  like  those  of  the  cord,  grouped 
together  in  the  complex  gray  substance,  while  the  white  part  is 
made  up  of  conducting  fibres.  The  gray  substance  is  distributed 


FIG.  253. 


Diagram  illustrating  the  progressive  changes  in  the  development  of  the  brain. 

1.  Shows  the  first  step;  the  formation  of  three  vesicles. 

2.  Shows  the  budding  forward  of  the  hemispheres  (cr),  upward  of  the  pineal  gland  («/), 
and  downward  of  the  pituitary  body  (pt)  from  the  fore-brain  (a),  and  the  thickening 
of  the  mid  (ft)  and  hind-brain  (c). 

3.  Shows  the  backward  turn  of  the  hemispheres  and  their  cavity  (lateral  ventricle)  (/). 
The  development  of  the  corpora  quadrigemina  (q)  and  crura(?)  from  the  mid-brain  (ft) 
and  the  cerebellum  (erf)  and  pons(p)  from  the  hind-brain;  their  cavities  being  nar- 
rowed into  the  tier  a  tertio  ad  quartum  venti-lculum. 

4.  The  hemispheres  now  extend  backward  and  form  the  temporal  lobe  (#),  which  after- 
ward grows  downward  and  forward.     The  tbrnix    (/)  approaches  its  final  position. 
The  space  (x)  between  the  fornix  and  velum  is  closing,  so  that  the  outside  of  the  brain 
(morphologically)  becomes  practically  its  very  centre. 

Explanation  of  tetters.— a,  Fore-brain ;  b,  Mid-brain  ;  o.  Hind-brain  ;  cb,  Cerebellum;  cr. 
Cerebrum;  d,  Cavity  of  medulla;  /,  Fornix;  #,  Lateral  vent  rifle;  m,  Medulla  oblongata; 
ma,  Corpora  mammillaria;  o,  Olfactory  lobe;  />,  P.ms  Varolii  ;pf,  Pineal  gland;  pt,  Pitui- 
tary body;  q,  Corpora  quadrigemina  :  r,  Crura  cerebri ;  t,  Lamina  terminalis;  tl,  Tem- 
poral lobe  of  cerebrum;  x,  Space  enclosed  by  the  extension  backward  of  the  cerebrum. 


THE   BRAIN. 


647 


in  four  distinct  regions.      (1)  Of  these  one. can  be  traced  along 
the  floor  of  the  fourth  ventricle,  from  the  gray  matter  of  the 


FIG.  254. 


Diagram  of  some  of  the  paths  taken  by  nerve  impulses  in  the  brain  and  spinal  cord. 
c.  Gray  substance  of  cerebral  cortex,    c'.  Gray  substance  of  cerebellum. 
cr.  Cranial  nerves,  some  afferent  and  some  efferent. 
M.  Motor  (efferent)  spinal  nerves,    s.  Sensory  (afferent)  spinal  nerves. 

cord  to  the  base  of  the  brain,  a*s  far  forward  as  the  tuber  cine- 
reum,  so  that  it  may  be  considered  representative  of  the  gray 


648  MANUAL   OF   PHYSIOLOGY. 

matter  forming  the  inner  lining  of  the  primitive  nervous  tube. 
(2)  The  ganglia  of  the  brain  are  isolated  masses  of  gray  sub- 
stance within  the  brain,  known  as  the  corpora  quadrigemina,  optic 
thalami,  corpora  striata,  etc.  (3)  The  gray  substance  of  the 
cerebellum  and  of  the  corpora  quadrigemina  is  derived  from  the 
upper  part  of  the  mid-brain.  (4)  The  cortex  of  the  hemi- 
spheres of  the  brain  is  the  most  extensive  gray  district,  and  must 
be  regarded  as  distinct  from  the  preceding. 

Connecting  the  various  parts  of  these  gray  regions  are  sets  of 
fibres,  which  may  be  classified  as  follows : — 

1.  Those  which  act  as  channels  of  intercommunication  for  the 
different  parts  of  the  same  region.     These  may  be  divided  into 
unilateral,  which  connect  together  the  cells  of  a  single  hemi- 
sphere, and  bilateral,  or  commissural  fibres,    which    unite   the 
corresponding  masses  of  gray  matter  on  both  sides  of  the  brain. 

2.  Those   which  connect   the  different  regions  one  with  an- 
other.    Under  this  head  fall  (1)  those  fibres  which  pass  between 
the  cortex  and  the  basal  ganglia,  or  anterior  gray  column  of  the 
spinal  cord  ;  (2)  those  running  from  the  cortex  or  the  spinal 
cord  to  the  cerebellum  ;    and   (3)  those  connecting  the  above 
with  the  spinal  gray  matter. 

THE  MESENCEPHALON  AND  CEREBELLUM. 

In  examining  the  functions  of -the  brain,  we  may  consider  the 
various  parts  in  the  order  they  are  found  in  proceeding  from 
the  medulla  toward  the  cerebral  hemispheres.  Between  the 
medulla  oblongata  and  the  hemispheres,  we  come  to  the  pons 
Varolii  and  cerebellum,  the  crura  cerebri,  and  corpora  quad- 
rigemina, which,  being  developed  from  the  mid-brain,  m.ay  be 
called  the  mesencephalon.  The.  duties  of  these  parts  of  these 
nervous  centres  can  be  investigated  by  observing  the  actions  of 
lower  animals  in  which  the  hemispheres  have  bqen  removed  or 
the  parts  directly  stimulated,  and  by  noting  the  symptoms  pro- 
duced in  man  by  lesions  of  this  part  of  the  brain.  The  former 
method  gives  the  most  definite  results,  and  therefore  deserves 
most  attention. 

In  all  these  parts  there  are  innumerable  fibres  capable  of  con- 


THE   MESENCEPHALON    AND   CEREBELLUM. 


649 


ducting  impulses  in  many  directions,  and  numerous  masses  of 
ganglionic  cells  distributed  throughout  the  white  substance.     In 
the  cerebellum    a   remarkable  layer  of  large    branching  cells 
divides  the  central  from  the  cortical  tissue. 


FIG.  255: 


Section  through  a  part  of  the  cerebellum. 

a,  Molecular  layer  into  which  pass  the  branches  of  Purkinje's  cells,  6;  d,  Medullary 
centre  from  which  medullated  fibres  pass  through  the  granular  layer  of  nervous  and 
neuroglia  cells  to  reach  the  cells  of  Purkinje. 

When  the  cerebral  hemispheres  have  been  removed  from  a 
frog,  the  animal  retains  the  power  of  carrying  out  coordinated 
55 


650  MANUAL   OF   PHYSIOLOGY. 

motions  of  much  greater  complexity  than  those  performed  by 
the  spinal  cord  alone.  But  this  power  is  not  exercised  spon- 
taneously. That  is  to  say,  the  animal  can  balance  itself  accu- 
rately, jump,  swim,  swallow,  etc.,  but  it  only  attempts  these  acts 
when  excited  to  do  so  by  stimulations  from  its  outer  surround- 
ings. Thus,  on  a  flat  surface  it  sits  upright,  but  does  not  stir 
from  the  spot  where  it  has  been  placed  ;  if  the  surface  upon 
which  it  sits  be  inclined,  so  that  its  head  is  too  low,  it  turns 
round  to  regain  its  ordinary  position.  If  the  surface  be  further 
inclined,  it  at  first  crouches  so  as  not  to  slip  off,  and  then  crawls 
upward  to  find  an  even  resting  place.  Plunged  into  water,  it 
swims  perfectly,  but  on  arriving  in  a  shallow  part  it  either  rests 
quietly  with  its  nose  out  of  the  water,  and  its  toes  touching  the 
ground,  or  crawls  out  to  sit  on  the  water's  edge,  where  it  can  find 
its  balance.  When  touched  on  the  leg,  it  jumps  away  from  the 
stimulus,  and  in  so  doing  avoids  any  obvious  dark  obstacle.  It 
swallows  if  a  substance  be  put  in  its  mouth,  but  does  not  attempt 
to  eat  even  if  surrounded  with  food.  In  short,  all  movements, 
even  the  most  complex,  may  be  brought  about  by  adequate 
stimulation — spontaneity  only  is  wanting.  The  pupil  responds  by 
reflex  contraction,  when  the  retina  is  exposed  to  light ;  the  eyes 
are  closed  if  the  light  be  intense  ;  and  the  head  may  follow  the 
motions  of  a  flame  moved  from  side  to  side.  A  sudden  or  loud 
noise  causes  it  to  move.  From  the  foregoing  facts,  and  the 
power  such  a  frog  has  of  avoiding  a  dark  object,  we  may  conclude 
that  the  impulses  arriving  from  the  special  sense  organs  are  all 
duly  received,  and  excite  more  or  less  elaborate  response,  but 
that  the  consciousness  of  the  arrival  of  these  impulses  no  longer 
exists. 

The  removal  of  the  hemispheres  of  birds  and  rabbits  leaves 
the  animal  in  a  somewhat  similar  condition ;  but  the  response 
to  the  special  sense  impulses  is  not  so  definite  or  well  marked, 
since  the  animal  flies  or  runs  against  even  the  most  obvious 
obstacles. 

We  may  conclude,  then,  that  the  medulla  controls  the  coordi- 
nated movements  absolutely  necessary  for  the  vegetative  func- 
tions, and  that  the  mid-brain  (including  the  cerebellum  of  birds 


THE    MESENCEPHALON    AND    CEREBELLUM.  651 

and  mammals)  controls  the  complex  associations  of  coordinated 
movements  necessary  for  the  perfect  performance  of  such  acts  as 
standing  and  walking. 

The  enormous  number  of  muscles  simultaneously  used  in  some 
of  our  commonest  daily  actions,  concerning  which  we  have  but 
little  thought,  and  take  no  voluntary  trouble,  shows  the  great 
importance  of  this  part  of  the  brain.  If  we  take  a  simple  ex- 
ample, that  of  standing  in  the  upright  position  (equilibration) 
(see  page  481),  we  find  that  a  great  number  of  muscles  have  to 
act  together  with  the  most  exact  nicety  to  accomplish  what,  even 
in  man,  is  an  unconscious,  if  not  quite  involuntary,  action.  In 
the  frog,  as  has  been  seen,  equilibration  is  performed  by  reflex 
action  alone.  In  man  the  nervous  mechanisms  are  probably 
more  complicated  by  his  erect  attitude  and  the  addition  of  the 
cerebellum,  etc.,  but  they  are  nevertheless  comparable  with  those 
of  the  frog.  It  may,  therefore,  be  instructive  to  examine  the 
details  of  the  mechanisms  in  a  frog  deprived  of  its  cerebral 
hemispheres. 

The  optic  lobes  of  the  frog's  brain  (which  correspond  to  the 
corpora  quadrigemina,  and  also  take  the  place  of  the  cerebellum 
of  the  higher  animals)  form  the  great  centres  of  equilibration, 
locomotion,  etc.  If  these  lobes  be  destroyed,  the  animal  can  no 
longer  sit  upright,  jump  or  swim.  The  first  point  to  determine 
is,  whence  do  the  impulses  arrive  which  bring  about  these  com- 
plex coordinations.  The  first  set  is  that  coming  from  the  tactile 
sense  of  the  skin  of  those  parts  touching  the  ground.  A  second 
set  arrives  from  the  acting  muscles  indicating  to  the  centres  the 
amount  of  work  done  (muscular  sense).  A  third  set  comes  from 
the  eyes,  by  which  the  position  of  the  surrounding  objects  is 
gauged.  Finally  comes  the  fourth  set  from  the  semicircular  canals 
of  the  internal  ear,  which  communicate  to  the  equilibrating 
centres  the  position  of  the  head. 

By  depriving  a  frog  of  these  several  portals  by  which  incom- 
ing stimuli  direct  the  balancing  centres,  it  can  be  rendered  inca- 
pable of  any  of  the  acts  requiring  equilibration,  even  when  the 
regulating  centres  are  intact.  In  our  own  bodies  we  can  con- 
vince ourselves  of  the  importance  of  these  afferent  regulating 


652  MANUAL   OF   PHYSIOLOGY. 

impulses  arriving  from  the  eye,  ear,  skin  and  muscles.  When 
the  eyes  are  shut  and  heels  together  we  cannot  stand  as  steadily 
as  when  we  keep  our  eyes  fixed  on  something ;  even  with  care 
not  to  move,  we  sway  slowly  to  and  fro.  If,  having  bent  our 
head  to  the  handle  of  a  walking  stick,  the  end  of  which  is  fixed 
on  the  ground,  we  run  three  or  four  times  around  this  axis  so  as 
to  disturb  the  fluid  in  the  semicircular  canals,  and  then  attempt 
to  walk  straight,  we  find  how  helpless  our  volition  becomes  when 
deprived  of  the  aid  naturally  coming  from  special  mechanisms  in 
the  internal  ear. 

When  the  feet  are  "  asleep  "  or  benumbed,  standing  or  walking 
can  only  be  performed  in  a  most  awkward  manner,  showing  the 
necessity  of  tactile  sensation  for  perfect  equilibration.  In  a 
disease  known  as  locomotor  ataxy  the  muscular  sense  is  lost,  and 
the  power  of  standing  or  walking  correspondingly  impaired. 

CRURA  CEREBRI. 

Passing  above  the  Pons  Varolii,  we  come  to  an  isthmus,  com- 
posed of  two  thick  strands  of  nerve  substance  connecting  the  pons 
Varolii  with  the  cerebral  hemispheres.  These  are  called  the 
crura  cerebri.  They  diverge  slightly  in  their  upward  course 
toward  the  hemispheres,  and  lie  just  below  the  corpora  quadri- 
gemina,  already  referred  to.  Minute  examination  of  these  crura 
brings  to  light  an  anatomical  difference  which  corresponds  with 
a  physiological  separation  between  the  paths  taken  by  the  sen- 
sory and  motor  impulses  in  each  crus.  The  lower  and  more 
anterior  part,  which  can  be  seen  on  the  base  of  the  brain,  is 
called  the  base  or  crusta.  This  is  made  up  of  efferent  nerve 
channels  only.  The  posterior  or  upper  part,  which  lies  next  to 
and  is  connected  with  the  corpora  quadrigemina,  is  called  the 
tegmentum,  and  is  composed  of  afferent  fibres.  Anatomically, 
the  separation  between  the  two  is  indicated  by  some  scattered 
nerve  cells  (locus  niger}.  The  base,  or  crusta,  which  is  the  great 
bond  of  union  between  the  spinal  cord  and  the  cerebral  motor 
centres,  passes  into  the  corpus  striatum  ;  and  the  tegmentum,  or 
great  sensory  tract,  is  directly  connected  with  the  optic  thalamus. 


BASAL    GANGLIA. 


653 


FIG.  256. 


BASAL  GANGLIA. 

On  the  floor  of  the  lateral  ventricles  are  the  corpora  striata 
and  optic  ihalami,  which  together  are  spoken  of  as  the  basal 
ganglia.  The  exact  rela- 
tionship borne  by  their 
functions  to  those  of  the 
mesencephalon  and  cere- 
bral cortex  is  not  perfectly 
understood.  The  follow- 
ing are  some  of  the  more 
important  points  in  the 
evidence  on  the  subject : — 

CORPORA  STRIATA. — 
The  motor  tracts,  coming 
from  below,  lie  in  the  lower 
part  of  the  crus  cerebri, 
and  thence  one  on  each 
side  passes  into  the  corres- 
ponding corpus  striatuin. 
Anatomically,  this  part 
may  be  regarded  as  the 
ganglion  of  the  motor 
tract. 

Destructive  lesion  of 
one  corpus  striatum  is 
followed  by  loss  of  motion 
of  the  other  side.  This 

„  1  Diagram  of  Brain  and  Medulla  Oblongata.    (Cleland.) 

IS    equally   true    Ol   lesions  a,Spinal  cord;  b,b,  Cerebellum  divided,  and  above 

it  the  valve  of  Vieussens  partially  divided;  c, 
Corpora  quadrigemina;  rf,  rf,  Optic  thalami:  e, 
pineal  body;/,/,  Corpora  striata;  g,  g,  Cerebral 
hemispheres  in  section;  A,  Corpus  callosum;  i, 
Fornix;  1,1,  Lateral  ventricles;  3,  Third  ventricle; 
4,  Fourth  ventricle;  5, Fifth  ventricle, bounded  on 
each  side  by  septum  lucidum. 


artificially  produced  in 
animals,  and  those  result- 
ing from  disease  in  man. 
When  the  crura  on  both 


sides    are    destroyed,   the 

animal  remains  motionless  and  prostrate. 

Electrical  stimulation  of  one  corpus  striatum  causes  movement 
of  the  other  side.  This  fact,  however,  does  not  teach  us  much 
concerning  the  functions  of  the  particular  cells  of  its  gray 


654  MANUAL   OF   PHYSIOLOGY. 

matter,  since  the  stimulus  cannot  be  kept  from  affecting  the 
fibres  passing  through  the  corpus  striatum  and  forming  the 
direct  motor  tract. 

In  dogs,  and  still  more  in  rabbits,  the  corpora  striata  seem  to 
be  able  to  carry  out  some  complex  motions  which  in  man  are 
believed  to  require  the  cooperation  of  the  higher  cerebral  cen- 
tres. It  has  been  stated  that  a  dog  whose  cerebral  cortex  is 
completely  destroyed  can  perform  movements  that  in  man  can 
only  be  evoked  by  the  cortex  of  the  hemispheres. 

It  would  appear  that  the  gray  matter  of  the  corpus  striatum 
is  motor,  being  nearly  related  in  function  to  the  cerebral  cortex. 
The  cells  of  this  ganglion  are  probably  agents  working  under 
the  direction  of  the  cortical  centres,  organizing  and  distributing 
certain  motor  impulses.  In  animals  whose  hemispheres  are  less 
complexly  developed,  such  as  the  dog  or  rabbit,  the  "  basal 
agent "  seems  capable  of  carrying  on  more  elaborate  work,  inde- 
pendent of  the  guidance  of  the  higher  motor  centres  in  the  gray 
matter  of  the  brain. 

OPTIC  THALAME. — The  evidence  concerning  the  function  of 
these  ganglionic  masses  is  far  from  being  even  as  satisfactory  or 
conclusive  as  that  relating  to  the  corpora  striata.  - 

Anatomically,  their  relationship  is  equally  clear;  they  are  the 
ganglia  of  the  sensory  tracts,  since  the  tegmentum  or  sensory 
parts  of  the  crura  pass  directly  into  them.  They  form  the  chief 
routes  by  which  impulses,  giving  rise  to  different  sensory  im- 
pressions, arrive  at  the. cerebral  cortex.  But  the  evidence  we 
obtain  by  the  physiological  examination  of  sensory  impressions 
is  indistinct  compared  with  the  results  when  motor  tracts  are 
excited.  In  the  complete  absence  of  all  motion,  it  is  impossible 
to  know  whether  an  animal  feels  or  not,  as  we  have  no  other 
signs  of  the  stimulus  taking  effect.  It  is  difficult,  as  has  been 
already  seen,  to  stimulate  any  sensory  tract  without  the  impulse 
being  reflected  to  its  motor  neighbors,  so  a  muscular  movement 
often  results  from  stimulation  of  a  group  of  cells  purely  sensory 
in  function,  and  yet  is  not  decisive  evidence  of  conscious  sensation. 

When  we  take  into  consideration  the  foregoing  points,  and  the 
fact  that  it  is  difficult,  if  not  impossible,  to  destroy  a  portion  of 


BASAL   GANGLIA. 


655 


brain  substance  with- 
out irritating  it  and 
the  neighboring 
structures,  we  can- 
not be  surprised  that 
experimenters  have 
arrived  at  very  con- 
tradictory results, 
both  by  stimulating 
and  destroying  the 
optic  thalami.  Some 
find  that  electric 
stimulation  causes 
muscular  move- 
ments ;  others  find 
that  it  does  not. 

Some  authorities 
state  that  destruction 
of  the  optic  thalami 
interrupts  only  the 
incoming  sensory  im- 
pressions ;  others  say 
it  gives  rise  to  motor 
paralysis. 

Human  pathology 
helps  us  but  little, 
for  it  is  impossible 
to  say  whether  a 
lesion  simply  abol- 
ishes the  function  or 
acts  as  an  irritant,  or 
produces  both  these 
effects.  Local  lesions 
of  the  optic  thalami 
have  been  met  with, 
in  some  of  which 
sensory,  in  others 


FIG.  257. 

^fP^T"'^ 


&$&$3$$ft. 

••  .".jJU  '*•>  «f,  .•"»•!";•  C 

ISIK 

»»§ 

ImfSf- 

A!»i*C*N*i  *j  n  * 

'HSfflS 

5t|/-.»^iKV4*'«   r'i 


.*,•'  -..i      ..J  s'*^1  A*  . 

w^'ffoittssifc^y 


IhWffiW 

•;»]>  wa^i*.1 


Peripheral. 


Small  angular  cells. 


Pyramidal  cells. 


•t  Granular  stratum. 


Ganglionic  cells. 


G  Spindle  cells. 


Section  through  the  cor- 
tex of  temporal  lobe 
of  monkey,  showing 
the  series  of  layers  of 
nervous  cells  with  dif- 
ferent characters. 


656  MANUAL   OF   PHYSIOLOGY. 

both   sensory  and    motor,  defects   have   been   observed   in  the 
patients. 

We  must  remember  that  the  occurrence  of  motion  as  the 
result  of  stimulation,  or  the  absence  of  muscular  power  as  the 
result  of  destruction  of  the  optic  thalami,  must  not  be  accepted 
as  good  evidence  of  the  motor  function  of  the  nerve  cells  of  this 
part,  because  these  results  may  depend  on  the  indirect  influence 
of  the  sensory  impulses  coming  from  these  cells. 

CEREBRAL  HEMISPHERES. 

It  is  now  universally  regarded  as  a  recognized  fact  that  the 
hemispheres  of  the  brain  are  the  seat  of  the  mental  faculties— 

FIG.  258. 


Upper  surface  of  the  hemispheres  of  monkey,  showing  details  of  motor  areas.    Kefer- 
ences  as  in  next  figure.    (Ferrier.) 

perception,  memory,  thought  and  volition.  The  cerebral  cortex 
is  the  part  of  the  nervous  system  in  which  the  subjective  percep- 
tion of  the  various  sensory  impulses  takes  place,  and  in  which 


CEREBRAL   HEMISPHERES.  657 

impulses  are  converted  into  impressions  or  mental  operations.  It 
is  in  the  cortical  nerve  cells  that  the  so-called  voluntary  impulses, 
causing  movement  of  the  skeletal  muscles,  have  their  origin.  It 
is  thus  a  sensory  and  a  motor  organ.  But  it  has  a  far  wider 
range  of  function  than  is  expressed  by  saying  it  is  both  sensory 
and  motor.  Thus  restricted,  its  function  would  be  no  higher  than 

FIG.  259. 


Left  hemisphere  of  monkey,  showing  details  of  motor  areas  indicated  by  the  movements 
following  stimulation  of, — 

1.  Superior  parietal  lobule  ;  exciting  advance  of  the  hind  limb. 

2.  Top  of  ascending  frontal  and  parietal  convolutions;   flexion  and  outward  rotation  of 
thigh  ;  flexion  of  toes. 

3.  On  ascending  frontal  convolution  near  semi-lunar  sulcus;   movements  of  hind  limb, 
tail  and  extremity  of  trunk. 

4.  On  adjacent  margins  of  ascending  frontal  and  parietal  convolution ;  adduction  and 
extension  of  arm,  pronation  of  hand. 

5.  Top  of  ascending  frontal  near  superior  frontal  convolution ;  forward  extension  of 
arm. 

a,  b,  c,  rf,  On  ascending  parietal  ;  movements  of  various  muscles  of  the  fore-arm. 

6.  Ascending  frontal  convolution;  flexion  of  fore-arm  and  supination  of  hand  which  is 
brought  toward  mouth. 

7.  Retraction  and  elevation  of  corner  of  mouth. 

8.  Elevation  of  nose  and  lip. 

9  and  10.  Opening  mouth  and  motions  of  tongue. 

11.  Retraction  of  angle  of  mouth, 

12.  Middle  and  superior  frontal  convolutions;  movements  of  head  and  eyelids. 

13  and  13'.  Anterior  and  posterior  limbs  of  angular  gyrus;  movements  of  eyeballs. 

14.  Superior  tempero-sphenoidal  convolution,  ear  pricked  and  head  moved. 

15.  Movement  of  lip  and  nostril.    (Ferrier.) 

that  of  the  nerve  centres  in  the  spinal  cord,  etc.  The  cells  of  the 
cortex  of  the  brain  seem  to  differ  from  those  of  the  lower  nerve 
centres,  which  can  only  receive,  and  at  once  send  out  corres- 
ponding impulses,  in  this :  when  an  impulse  arrives  at  certain 
cerebral  cells,  it  there  excites  a  change,  which,  besides  producing 


658  MANUAL   OF   PHYSIOLOGY. 

an  immediate  effect,  leaves  a  more  or  less  permanent  impression. 
The  impression  persisting,  if  the  cell  be  well  supplied  with 
chemical  energy  in  the  shape  of  nutriment,  it  may  be  reproduced 
at  a  subsequent  period.  This  revival  of  impressions,  the  effects 
of  past  stimulations,  or  "recollection"  is  exclusively  the  prop- 
erty of  the  cerebral  cortex,  and  to  it  the  hemispheres  owe  their 
mental  faculties.  During  our  lifetime  sensory  impulses  are  con- 
tinually streaming  into  the  cells  of  the  cortex  of  the  brain  from 
the  peripheral  sensory  organs.  Thus  innumerable  impressions 
are  stored  up  in  the  nerve  cells.  The  effect  of  the  continuing 

FIG.  260. 


Dark  shading  indicates  the  extent  of  a  lesion  of  the  gray  matter  of  the  right  hemisphere 
of  a  monkey  followed  by  complete  motor  paralysis  of  the  limbs  of  the  opposite  side 
without  impairment  of  sensation.  (Ferrier.) 

c,  Fissure  of  Rolando;  d,  Postero-parietal  lobule  ;  e,  Ascending  frontal  convolution. 

presence  of  these  impressions  in  the  active  cells  is  memory,  and  by 
association,  arrangement  or  analysis  of  these  persisting  impres- 
sions, the  activity  of  the  cells  gives  rise  to  thought  or  ideation. 

In  close  relation  and  connection  with  these  cells  of  the  cortex, 
in  which  permanent  impressions  are  stored  and  ideation  is  accom- 
plished, are  those  other  groups  of  cells  which  have  been  mentioned 
as  being  in  direct  communication  with  the  spinal  motor  centres, 
and  can  by  the  medium  of  the  latter  execute  voluntary  move- 
ment. 


CEREBRAL   HEMISPHERES.  659 

It  is  a  very  remarkable  fact,  that  one  side  of  the  brain  is  suf- 
ficient for  the  perfect  performance  of  the  mental  faculties. 
Memory,  consciousness  and  thought  can  all  be  operative  in  a 
perfectly  normal  way,  when  one  side  of  the  brain  is  rendered 
incapable  of  performing  its  functions  by  disease  or  injury. 

This  is  not  the  case  as  regards  sensory  impressions,  or  volun- 
tary movement,  both  of  which  are  destroyed  on  the  side  of  the 
body  opposite  to  that  of  the  injured  hemisphere.  The  difference 
between  the  mental  powers  and  mere  motor  and  sensory  functions 
of  the  brain  can  be  seen  in  those  cases  of  paralysis  known  as 

FIG.  261. 


The  dark  shading  shows  the  region  of  the  angular  gyrus  of  a  monkey,  injury  of  which 
is  followed  by  transient  blindness.    (Ferrier.) 


hemiplegia.  The  patient  is  frequently  fully  conscious,  and  may 
possess  unimpaired  powers  of  thought  and  memory,  yet  he  is 
unable  to  perceive  the  sensory  impulses  coming  from  one  side  of 
his  body,  or  to  send  voluntary  impulses  to  the  muscles  of  the 
paralyzed  side. 

The  cells  which  act  as  the  immediate  receivers  of  afferent,  and 
dispensers  of  efferent  impulses,  to  one  or  other  side  of  the  body, 
are  then  localized  to  one  hemisphere,  viz.,  that  of  the  opposite 
side,  while  mental  operations  are  diffused  over  the  cortex  of  both 
hemispheres. 


660  MANUAL   OF   PHYSIOLOGY. 

LOCALIZATION  OP  THE  CEREBRAL  FUNCTIONS. 

Whether  the  entire  surface  of  the  hemispheres  can  be  mapped 
out  into  areas,  each  of  which  is  set  apart  for  a  definite  immutable 
function,  is  a  question  surrounded  with  difficulty,  and  which,  up 
to  the  present,  cannot  be  answered  with  certainty.  The  experi- 
mental evidence  hitherto  brought  forward  on  the  subject  seems, 
in  some  points,  to  be  contradictory,  a  fact  which  may  be  explained 
partly  by  the  difficulties  with  which  such  experiments  are  beset, 
and  partly  by  different  observers  being  anxious  to  uphold  with 
too  great  fervor  either  the  localization  or  non-localization  theory. 

The  evidence  on  this  subject  may  be  briefly  summarized  as 
follows.  In  favor  of  localization  are  the  facts  that — 

1.  Lesion  of  a  certain  part  of  the  cortex  of  the  frontal  lobe 
of  the  left  hemisphere  of  man  (posterior  part  of  the  third  frontal 
convolution)  has  been  so  frequently  followed  by  the  loss  of  mem- 
ory of  words  necessary  for  the  faculty  of  speech — aphasia — that 
pathologists  now  call  that  spot  the  speech  centre. 

2.  Destruction  of  the  cortical  surface  of  the  angular  gyri,  or 
of  the  posterior  lobes,  causes  transitory  loss  of  the  perception  of 
visual  impressions,  and  if  an  area  including  both  angular  gyri 
and  posterior  lobes  be  destroyed,  the  result  is  permanent  blind- 
ness. 

3.  Destruction  of  the  convolutions  around  and  in  the  neigh- 
borhood of  the  fissure  of  Rolando  gives  rise  to  loss  of  power  in 
the  limbs  of  the  other  side,  voluntary  motion  being  abolished 
when  an  extensive  area  is  destroyed.     This  loss  of  power  is  more 
obvious  in  animals  with  complex  brains  (man  and  monkey)  than 
in  those  less  highly  organized  (dog,  cat,  rabbit),  which  gradually 
recover. 

4.  The  function  of  the  partr  destroyed  may  be  lost  forever,  and 
the  nerve  channels,  which  formerly  carried  the  impulses  to  or 
from  the  injured  centre,  become  degenerated. 

5.  Stimulation    of    the    convolutions   around    the    fissure   of 
Rolando  gives  rise  to  definite  coordinated  movements  of  muscles 
of  the  other  side.     Local  groups  of  muscle  respond  with  striking 
constancy  to   the   electric  stimulation   of  certain  definite  and 
limited   areas   of  the   cortex.     These   convolutions   have   been 


LOCALIZATION    OF    THE    CEREBRAL    FUNCTIONS.  661 

mapped  out  into  motor  centres  for  hind  limb,  fore  limb,  face, 
etc.     Compare  Figs.  258  and  259. 

From  the  foregoing  we  may  safely  conclude  (1)  that  certain 
parts  of  the  cortex  of  the  hemispheres  are  the  agents  for  the 
reception  of  definite  sensory  impressions ;  (2)  that  others  (the 
neighborhood  of  the  fissure  of  Rolando)  are  related  to  the  dis- 
charge of  voluntary  motor  impulses ;  and  (3)  though  we  cannot 
say  that  the  anterior  lobes  are  immediately  subservient  to  either 

FIG.  262. 


The  dark  shading  shows  areas  which  were  destroyed  in  a  monkey  without  giving  rise  to 
any  functional  defect  that  could  be  detected.     (Ferrier.) 

the  sensory  or  motor  functions,  a  portion  of  one  of  them  seems 
devoted  to  the  memory  of  words. 

As  objections  to  the  soundness  of  these  conclusions,  it  has  been 
urged  : — 

1.  Considerable  discordance  still  exists  in  the  results  arrived 
at  by  different  experimenters. 

2.  The  function  returns  after  the  lapse  of  a  variable  interval, 


662 


MANUAL   OF   PHYSIOLOGY. 


particularly  in  unilateral  destruction  of  the  cortical  centres.  In 
some  instances  the  loss  of  function  only  continues  for  a  few  hours 
after  the  operation;  in  others  (those  in  which  the  injury  is  exten- 
sive and  deep,  or  the  animal  belongs  to  a  class  with  high  mental 
organization)  the  recovery  is  slow  and  may  extend  over  several 
weeks  or  months. 

3.  Certain  tracts  of  the  cortex  of  the  hemispheres,  notably  the 
anterior  and  posterior  lobes,  may  be  extensively  injured,  by  acci- 

FlG.  263. 


Extirpation  of  the  posterior  lobes.  Lesion  followed  by  only  transient  disturbance  of 
vision.  In  a  few  hours  no  defect  could  be  detected  in  any  of  the  animal's  functions. 

•  Some  observers  have  found  total  blindness  after  a  lesion  but  slightly  greater  than  that 
here  shown.  (Ferrier.) 

dent  or  experiment,  without  interfering  with  the  cerebral  func- 
tions in  any  marked  or  tangible  way.  Both  men  and  animals 
have  lived  for  years  after  the  loss  of  a  considerable  quantity  of 
brain  substance,  without  showing  impairment  of  either  mental  or 
bodily  faculties. 

4.  Certain  areas  of  the  brain  surface  may  be  stimulated  me- 
chanically, chemically,  or  electrically,  without  the  least  response 


LOCALIZATION   OF   THE   CEREBRAL   FUNCTIONS.1 


being  shown  by  the  animal,  to  indicate  either  sensory  or 
excitations. 

In  spite  of  this  negative  evidence  from  the  facts  just  adduce 
— viz.,  that  certain  groups  of  muscles  respond  regularly  to  the 
stimulation  of  local  areas  of  the  brain  surface,  and  that  loss  of 
function  of  some  organ  occurs  when  a  given  point  is  injured — it 
seems  definitely  fixed  that  certain  local  parts  of  the  brain  surface 
are  in  more  immediate  connection  with  definite  peripheral  organs 
than  with  others,  and  that  these  local  areas  have  been  in  the 
habit  of  receiving  or  sending  out  special  impulses. 

The  evidence  is  strongest  in  support  of  the  motor  areas  situated 
around  the  fissure  of  Rolando  in  the  central  region  of  the  hemi- 
spheres. Here  (1)  limited  stimulus  excites  definite  action,  (2) 
circumscribed  lesion  is  followed  by  local  paralysis,  and  (3)  after 
lesion  a  tract  of  degeneration  unites  the  cortical  and  spinal  cen- 
tres engaged  in  the  production  of  voluntary  movement. 

With  regard  to  the  sensory  centres  the  areas  are  not  so  per- 
fectly localized.  We  can  hardly  suppose  that  all  the  gray  matter 
of  the  angular  gyri  and  occipital  lobes  are  devoted  exclusively 
to  the  reception  of  visual  impressions.  Yet  it  has  been  stated  all 
this  region  must  be  destroyed  to  annihilate  the  visual  function. 

From  the  fact  that  its  stimulation  causes  movements  of  the 
pinna  and  its  destruction  is  said  to  abolish  hearing,  the  superior 
temper o-sphenoidal  convolution  has  been  allotted  the  function  of 
an  auditory  centre.  From  somewhat  similar  evidence,  and  that 
gained  from  anatomy,  the  hippoeampal  lobule  is  said  to  be  the 
seat  of  smell. 

Ordinary  sensation  has  been  localized  to  the  inferior  tempero- 
sphenoidal  convolution  and  the  hippoeampal  region,  owing  to  the 
anaesthesia  found  after  destruction  of  these  parts. 

From  the  other  facts  mentioned — viz.,  the  absence  of  func- 
tional disturbance  after  cortical  lesion  and  the  recovery  of 
function  after  injury,  we  must  conclude  that  there  are  extensive 
tracts  containing  cells  to  which  we  can  assign  no  localized  func- 
tion, and  that  the  local  areas  to  which  a  function  can  be  assigned 
are  not  the  only  agents  which  can  carry  on  the  business  of 
receiving  impulses  from  the  periphery,  and  sending  voluntary 


664  MANUAL   OF   PHYSIOLOGY. 

impulses  to  the  muscles ;  but  that  there  are  many  groups  of  nerve 
cells  which  can  take  on  the  duty  of  the  injured  cells  and  act  for 
them  in  receiving  sensory  and  discharging  motor  impulses. 

It  has  already  been  pointed  out  that  the  function  of  any  given 
nerve  fibre  depends  on  the  relationship  of  its  terminals.  The 
fibre  itself  is  merely  a  conducting  agent.  In  somewhat  the  same 
way  the  functions  of  any  given  nerve  cell  must  depend  on  the 

FIG.  264. 


The  shading  shows  the  great  extent  of  surface  destroyed  before  permanent  cortical 
blindness  followed  by  retinal  atrophy  was  produced ;  i.  e.,  all  posterior  lobes  and  angu- 
lar gyri. 

number  and  character  of  its  connections.  If  it  be  connected 
with  a  motorial  end  plate  in  a  muscle,  it  can  only  excite  impulses 
that  give  rise  to  motion  :  if  it  be  connected  with  a  sensory  ter- 
minal, it  can  only  be  a  receiver  of  sensory  impulses.  But,  in  the 
gray  matter  of  the  spinal  cord,  and  still  more  sq  in  that  of  the 
cerebral  cortex,  we  may  assume  that  all  the  cells  are  in  more  or 
less  intimate  connection  with  innumerable  other  cells.  In  fact, 


LOCALIZATION  OF  THE  CEREBRAL  FUNCTIONS.     665 

we  must  imagine  that  the  gray  matter  of  both  cord  and  brain  is 
interwoven  into  a  complex  texture  of  fibrils  and  cells,  no  part  of 
which  is  isolated  from  the  rest,  but  all  the  elements  form  part  of 
a  continuous  system,  and  within  certain  limits  can  subsidize  each 
other's  functions. 

When  we  excise,  cauterize  or  stimulate  a  given  point  of  the 
complex  cortex,  we  do  not  know  in  what  way  we  interfere  with 
the  perfect  action  of  that  wonderful  nervous  nexus  which  controls 
the  organism,  for  we  can  only  judge  of  the  effects  we  produce  by 
results  limited  to  those  few  functions  the  activity  of  which  is 
obvious. 
56 


666  MANUAL   OF   PHYSIOLOGY. 


CHAPTER   XXXVII. 

KEPKODUCTION. 
MALE  AND  FEMALE  GENERATIVE  ELEMENTS. 

One  of  the  chief  characteristics  of  living  beings  is  their  power 
of  reproduction ;  that  is  to  say,  organisms  can,  under  favorable 
conditions,  form  other  individuals  with  lives  and  habits  similar 
to  their  own. 

In  the  lowest  forms  of  animal  life  this  propagation  of  species 
may  take  place  by  the  division  of  a  single  cell :  thus  an  amoeba 
reproduces  by  the  cleavage  of  its  mass  of  protoplasm,  separating 
the  main  body  into  two  amoebae.  Such  a  method  of  reproduction 
is  purely  asexual,  each  individual  having  the  intrinsic  power  of 
reproduction. 

As  we  ascend  the  animal  scale,  we  find  that,  just  as  other  func- 
tions are  executed  by  certain  specially  differentiated  groups  of 
cells,  so  reproduction  is  performed  by  certain  collections  of  cells 
endowed  with  specific  powers.  Further,  we  find  that  the  produc- 
tion of  a  new  being  requires  the  cooperation  of  two  kinds  of 
generative  elements,  each  of  which  is  produced  by  a  distinct 
organ.  In  the  higher  organisms  these  reproductive  elements  are 
produced  by  different  individuals  of  the  same  species,  thereby 
dividing  them  into  two  sexes.  This  is  termed  sexual  reproduction. 

The  sexual  method  of  reproduction  is  met  with  in  all  the  more 
highly  developed  forms  of  animal  and  vegetable  life.  The  male 
organ  produces  active  elements — the  spermatozoa ;  the  female 
organ  produces  the  ovum,  which,  when  fertilized  by  the  sperma- 
tozoa, develops  embryo. 

In  mammalia  the  uterus  is  a  most  important  subsidiary  organ, 
as  it  becomes  modified  to  allow  of  the  development  and  growth 
of  the  embryo;  its  earlier  functions,  however,  can  be  performed 
by  other  organs,  as  seen  in  cases  of  extra-uterine  fcetation,  when 
the  ovum  develops  in  some  unusual  situation,  such  as  the  Fallo- 
pian tube  or  the  abdominal  cavity. 


REPRODUCTION. 


667 


The  spermatozoa  are  formed  indirectly  from  the  cells  lining  the 
tubuli  seminiferi  of  the  testicle.  These  cells,  cubical  masses  of 
protoplasm,  give  rise  to  others  (spermatoblasts\  which  form 
another  layer  and  undergo  rapid  proliferation.  The  nuclei 
divide,  and  from  each  part  arises  the  "head  of  a  spermatozoon, 
the  body  being  developed  from  the  protoplasm  of  the  cell.  The 
spermatic  elements  escape  into  the  tubes,  and  pass  down  the  vasa^ 
deferentia  into  the  vesiculce  seminales,  where  they  either  undergo 
retrograde  change  or  are  cast  out  of  the  body. 


FIG.  2G5. 


Section  of  the  tubuli  seminiferi  of  a  rat.    (Scfittfer.) 

a.  Tubuli  in  wbich  the    spermatozoa  are  not  fully  developed,    b.  Spermatozoa  more 
developed,    c.  Spermatozoa  fully  developed. 

The  ovum  arises  from  the  differentiation  of  a  cell  from  the  gertn 
epithelium  covering  the  surface  of  the  ovary.  A  group  of  these 
cells  entering  the  periphery  of  the  ovary,  becomes  there  embedded 
in  a  kind  of  capsule  derived  from  the  surrounding  areolar  tissue 
of  the  strorna,  and  forms  an  immature  Graafian  follicle.  A  cen- 
tral cell  grows  rapidly  to  form  the  ovum,  the  rest  increase  in 
number  to  form  the  small  cells  of  the  granular  tunic.  As  the 
follicle  develops,  it  works  its  way  toward  the  centre  of  the  ovary, 


668 


MANUAL   OF   PHYSIOLOGY. 


and  subsequently  approaches  the  periphery  of  the  organ  as  a 
fully-developed  Graafian  follicle. 

Microscopically, it  is  seen  to  be  surrounded  by  a  capsule,  tunica 
fibrosa,  which  is  ill-defined   from    the  stroma   of   the  ovary  in 


FIG.  266. 


Section  of  the  ovary  of  a  cat,  showing  the  origin  and  the  development  of  Graafian 

follicles.     (Cadiat.) 

a.  Germ  epithelium.  e.  Ovum. 

&.  Graafian  follicle  partly  developed.  /.  Vittlline  membrane. 

c.  Earliest  form  of  Graafian  follicle.  g.  Veins. 

d.  Well-developed  Graafian  follicle.  h,  i.  Small  vessels  cut  across. 

which  it  lies.  Outside  this  is  a  layer  of  capillary  blood  vessels, 
tunica  vasculosa,  and  to  these  two  coats  collectively  the  term 
tunica  propria  is  applied. 


MENSTRUATION   AND    OVULATION. 

Inside  the  tunica  propria  are  granular  cells  of  small  size, 
which  occupy  a  considerable  space  in  the  follicle ;  they  are 
heaped  up  at  one  spot  round  the  ovum,  which  lies  embedded  in 
their  midst.  These  cells  receive  the  name  of  the  tunica  granulosa, 
and  their  projecting  portion,  which  encircles  the  ovum,  is  called 
the  discus  proligerus.  The  remainder  of  the  follicle  is  filled  with 
a  fluid,  liquor  folliculi.  The  surface  of  the  ovary  is  covered  by 
columnar  cells,  germ  epithelium,  continuous  with  the  endothelial 
cells  of  the  peritoneum.  When  the  follicle  is  fully  matured,  it 
lies  at  the  periphery  of  the  ovary  beneath  this  layer  of  cells, 
which  separates  it  from  the  abdominal  cavity. 

MENSTRUATION  AND  OVULATION. 

After  puberty,  at  intervals  averaging  about  four  weeks,  the 
genital  organs  of  the  female  become  congested,  and  at  the  same 
time  a  Graafian  follicle  is  ruptured  and  its  contained  ovum  set 
free.  Coincideutly  with  the  rupture  of  the  follicle,  the  fim- 
briated  extremity  of  the  Fallopian  tube  becomes  closely  approxi- 
mated to  the  spot  where  the  follicle  lies,  so  that  the  ovum,  instead 
of  falling  into  the  abdominal  cavity,  passes  into  the  canal  of  the 
Fallopian  tube,  down  which  it  is  conveyed  to  the  uterus. 

The  usual  place  for  the  ovum  to  meet  the  spermatozoa,  and  to 
be  impregnated,  is  the  Fallopian  tube. 

When  the  ovum  reaches  the  uterus,  if  it  be  unimpregnated,  it 
is  cast  out  with  the  surface  cells  of  the  mucous  membrane  of  the 
uterus,  which  are  destroyed,  and  escape  along  with  a  sanious 
fluid.  The  whole  of  these  phenomena  constitute  a  menstrual 
act. 

If,  however,  the  ovum  become  impregnated,  it  remains  in  the 
Fallopian  tube  some  days,  during  which  time  the  mucous  mem- 
brane of  the  uterus  becomes  so  hypertrophied  as  to  retain  the 
ovum  when  it  reaches  that  organ. 

The  human  ovum  is  a  cell  consisting  of  a  mass  of  protoplasm 
enclosing  a  nucleus  and  nucleolus,  and  surrounded  by  a  cell  wall. 
On  its  outer  surface  is  an  irregular  layer  of  cells,  the  remains  of 
that  part  of  the  tunica  granulosa  which  encircled  the  ovum  in 
the  Graafian  follicle.  The  cell  wall  of  the  ovum  is  called  the 


670 


MANUAL   OP   PHYSIOLOGY. 


vitelline  membrane  or  zona  pellucida,  and  the  mass  of  granular 
protoplasm  ft  encircles,  the  vitellus  or  yolk,  and  in  this  is  a 
nucleus — the  germinal  vesicle,  which  contains  a  nucleolus — the 
germinal  spot. 

Beneath  the  outer  covering  of  calcareous  material  of  the  hen's 
egg  there  is  a  white  membrane,  which  encloses  a  transparent 
albuminous  substance  known  as  the  white  of  egg.  Inside  this 
is  a  yellow  fluid  mass,  the  yolk,  surrounded  by  a  delicate  mem- 
brane, vitelline  membrane.  The  yolk  is  made  up  of  two  varieties 
of  material  of  different  shades  of  color,  the  white  and  the  yellow 
yolk.  Of  these  the  yellow  forms  the  greater  part,  the  white 
being  arranged  in  thin  layers,  which  separate  the  yellow  yolk 
into  strata.  In  the  centre  of  the  yolk  it  forms  a  flask-shaped 
mass,  with  its  neck  turned  to  the  upper  surface,  upon  which  a 
portion  of  the  yolk  rests  called  the  cicatrieula.  This  cicatricula, 
which  lies  between  the  vitelline  membrane  und  the  white  yolk,  is 
the  active  growing  part  of  the  egg,  and  out  of  it  is  developed  the 
chick  and  the  embryonic  membranes. 

Extending  through  the  albumin  from  the  vitelline  membrane 

to  the  ends  of  the  egg  are  two 
twisted  membranous  cords — the 
chalazce,  which  fix  and  protect 
the  delicate  yolk  from  shocks, 
but  allow  it  to  rotate,  so  that  the 
cicatricula  is  always  the  upper- 
most part  of  the  yolk  when  the 
egg  is  on  its  side. 

The  main  structural  differences 
between  the  human  ovum  and 
that  of  a  fowl  are  apparent  from 
the  above  description :  the  essen- 
tial peculiarity  of  the  develop- 
ment of  the  hen's  egg  is  that 
only  a  portion  of  the  yolk  is 
engaged  in  the  formation  of  the 
first  signs  of  the  chick  and  its  membranes,  by  far  the  greater  part 
of  the  egg,  both  yolk  and  albumin,  being  utilized  as  nourish- 
ment during  the  subsequent  stages  of  development. 


FIG.  267. 


Ovum.    (Robin.) 

a.  Zona  pellucida  or  vitelline  membrane. 

b.  Yolk. 

c.  Germinal  vesicle  or  nucleus. 

d.  Germinal  spot  or  nucleolus. 

e.  Interval  lelt  by  the  retraction  of  the 

vitellus  from  the  zona  pellucida. 


CHANGES   IN   THE   IMPREGNATED   OVUM. 


671 


After  the  egg  has  been  laid,  it  obtains  no  help  from  the  outside 
world,  except  the  oxygen  of  the  air  and  the  heat  of  the  mother's 
body ;  it  is,  as  it  were,  fenced  in  with  a  protecting  membrane, 
garrisoned  with  the  quantity  of  provisions  required,  and  by  the 
warmth  of  the  hen's  body  stimulated  to  growth  and  activity. 

The  whole  of  the  human  ovum,  on  the  other  hand,  undergoes 
segmentation  and  differentiation  in  the  primary  formation  of  the 
embryo,  which  subsequently  is  supplied  with  the  necessary  nour- 


FIG.  268. 


y-y- 


ch.l. 


C/l.1. 


Diagram  of  a  section  of  an  unimpregnatei  fowl's  egg.    (From  Foster  and  Balfour,  after 

Allen  Thomson.) 
bl.  Blastoderm  or  cicatricula. 
w.y.  White  yolk. 
y.y.  Yellow  yolk. 
ch.l.  Chalaza. 
i.s.m.  Inner  layer  of  shell  membrane. 


*.  Shell. 

a.ch.  Airspace. 

w.  The  white  of  the  egg. 

vl.  Vitelline  membrane. 

x.  The  denser  albuminous  layer  which  lies 


s  m.  Outer  layer  of  shell  membrane. 


next  to  the  vitelline  membrane. 


ishment  from  the  maternal  circulation.  The  life  and  growth  of 
the  human  embryo  depend  upon  supplies  from  the  mother,  the 
ovum  not  having  within  itself  any  store  of  nutrient  material. 

CHANGES  IN  THE  OVUM  SUBSEQUENT  TO  IMPREGNATION. 
The  first  changes  in  the  ovum   independent  of  impregnation 
consist  in  the  shrinking  of  the  yolk  from  the  vitelline  membrane, 


672 


MANUAL   OF   PHYSIOLOGY. 


and  the  extrusion  from  it  of  certain  granular  bodies  which  lie 
between  it  and  the  vitelline  membrane,  and  are  called  the  polar 
globules.  The  germinal  spot  and  germinal  vesicle  also  disappear, 
and  are  said,  by  some  observers,  to  form  these  polar  globules. 


FIG.  269. 


ent.  ^~~~^ 


Xf. 


Sections  of  the  ovum  of  a  rabbit,  showing  the  formation  of  the  blastodermic  vesicle. 

(E.  Van  Beneden.) 

a,  ft,  c,  d,  are  ova  in  successive  stages  of  development.  zp.  Zona  pelhicida. 

ect.  Ectomeres,  or  outer  cells.  ent.  Entomeres,  or  inner  cells. 

After  the  union  of  the  male  and  female  elements,  a  new  nucleus 
appears  in  the  vitellus  which  forms  what  is  called  the  segmentation 
sphere.  This  divides  at  first  into  two  segments,  then  into  four, 
eight,  sixteen,  and  so  on,  until  a  large  mass  of  cells  occupies  the 


CHANGES  IN  THE  IMPREGNATED  OVUM. 


673 


yolk.  To  this  condition  the  name  of  morula  is  given,  from  its 
supposed  likeness  to  a  mulberry.  Fluid  now  collects  among  the 
cells,  and  separates  some  of  them  from  the  others,  and  they 
arrange  themselves  into  an  outer  and  inner  layer,  consisting  of 
different  kinds  of  cells.  The  inner  cells  finally  become  aggre- 
gated at  one  part  of  the  ovum  in  contact  with  the  outer  cells. 
The  ovum  now  receives  the  name  of  the  blastodermic  vesicle. 

In  the  hen's  egg  the  cleavage  is  confined  to  the  cicatricula  or 
blastoderm,  and  does  not  include  the  rest  of  the  yolk.  From  the 
fact  that  the  cleavage  of  the  yolk  is  only  partial,  such  an  ovum 
receives  the  name  of  meroblastie.  The  human  ovum,  which  un- 
dergoes complete  segmentation,  is  called  holoblastic. 

The  cells  in  the  blastodermic  vesicle  become  arranged  into 

FIG. 270. 


Transverse  section  of  the  medullary  groove,  and  half  the  blastoderm  of  a  chick  of 
teen  hoard.    (Fo-tter  and  Balfour.) 

A.  Epiblast.  mf.  Medullary  fold. 

B.  MesoMast.  me.  Medullary  groove. 
c.  Hypoblast.  c/i.  Notochord. 

three  definite  layers,  which  are  called  respectively,  from  their 
position  in  the  blastoderm,  the  epiblast,  the  mesoblast,  and  the 
hypoblast. 

From  these  layers  are  developed  the  embryo  and  the  mem- 
branes surrounding  it,  each  layer  being  developed  exclusively 
into  certain  tissues. 

Thus  from  the  epiblast,  or  outer  layer,  arise  the  epidermis  of  the 
skin,  the  brain  and  spinal  cord,  and  certain  parts  of  the  organs 
of  special  sense  ;  while  it  also  aids  in  the  formation  of  the  choriou 
and  the  amnion.  From  the  mesoblast  are  developed  the  skeleton, 
connective  tissues,  muscles,  and  nerves,  in  addition  to  the  vascu- 
lar system  and  the  supporting  tissue  of  the  glands  ;  one  kind  of 
57 


674 


MANUAL   OF   PHYSIOLOGY. 


tessellated  cells  arises  from  this  layer,  viz.,  the  endothelium, 
forming  the  surface  of  all  serous  membranes.  From  the  hypo- 
blast  spring  the  epithelial  lining  of  the  alimentary  canal,  that 
of  the  glands  which  are  diverticula  from  it,  and  of  the  lungs ;  it 
also  forms  the  lining  membrane  of  the  allantois  and  yolk  sac. 

The  blastoderm  of  the  hen's  ovum,  which  is  comparatively 
easily  studied,  consists  of  a  small,  clear,  central  portion,  called  the 
area  pellucida,  from  which  the  body  of  the  chick  arises.  Sur- 
rounding the  area  pellucida  is  a  much  larger  zone,  which  appears 
less  transparent ;  this,  the  area  opaca,  is  devoted  to  the  forma- 
tion of  the  membranes. 


FIG.  271. 


N.C. 


F.So. 


Diagrammatic  longitudinal  section  through  the  axis  of  an  emhryo  chick.  (Foster  and 

Bilfnur.) 

N.  C.  Neural  canal.  Ch.  Notochord.  7>.  Foregut.  F.So.  Somatopleure.  F.  Sp.  Splanch- 
noi-leure.  ftp.  Splanchnopleuie  forming  the  lower  wall  of  the  foregut.  Hi.  Heart. 
pp.  Pleuroperitoneal  cavity.  Am.  Amniotic  fold.  ^l.Epiblast.  B.  Mesoblast.  C.  Hypo- 
blast. 


The  embryo  is  developed  from  the  rest  of  the  blastoderm  in 
the  following  manner.  At  the  front  of  the  area  pellucida  a  fold, 
or  dipping  in  of  the  blastoderm  takes  place;  this  consists  of  a 
projecting  part  above  and  a  groove  below,  and  constitutes  the 
cephalic,  or  head  fold.  The  upper  projecting  portion  of  the  fold 
tends  to  grow  forward,  while  the  groove  grows  gradually  back- 
ward. Later  on,  another  fold  appears  at  the  posterior  part  of 
the  area  pellucida;  this  is  the  tail  fold.  At  the  sides  of  the  area 
pellucida  folds  appear,  which  tend  to  grow  downward  and  in- 
ward so  as  to  reach  the  under  surface  of  the  blastoderm  and 
unite  with  the  head  and  tail  folds. 


FORMATION    OF   THE   MEMBRANES.  675 

By  the  approximation  of  all  these  folds  a  canal  is  formed — 
the  embryonal  sac — which  is  closed  above  by  the  main  portion  of 
the  area  pellucida,  in  front  by  the  head  fold,  behind  by  the  tail 
fold,  at  the  sides  by  the  lateral  folds,  while  below  it  is  open  to  the 
vitellus.  This  canal  ultimately  becomes  subdivided  into  an  inner 
tube,  the  alimentary  tract,  and  an  outer  one,  which  forms  the 
body  walls,  the  final  place  of  union  of  the  folds  being  marked 
by  the  umbilicus.  It  must  be  clearly  understood  that  these 
primary  folds  which  form  the  embryo  include  in  their  layers  the 
epiblast,  the  whole  thickness  of  the  mesoblast,  and  the  hypoblast, 
whereas  the  folds  giving  rise  to  the  membranes  do  not  compre- 
hend all  these  layers: 

FORMATION  OF  THE  MEMBRANES. 

(1)  The  Amnion. — The  mesoblast  around  the  embryo  becomes 
thickened,  and  is  split  into  two  distinct  layers  ;  this  cleavage  is 
at  first  confined  to  the  neighborhood  of  the  embryo,  but  gradu- 
ally spreads  over  the  whole  blastoderm. 

The  upper  of  these  two  layers  of  the  blastoderm  receives  the 
name  of  the  somatopleure,  and  is  engaged  in  the  formation  of  the 
body  walls  of  the  embryo  and  the  amnion.  The  lower  one  is 
called  the  splanchnopleure,  and  forms  the  walls  of  the  alimentary 
canal,  the  allantois,  and  the  yolk  sac.  The  space  intervening 
between  these  layers  is  called  the  pleuroperitoneal  cavity.  At  a 
point  in  front  of  the  cephalic  fold,  an  upward  projection  of  soma- 
topleure takes  place,  conveying  with  it  the  overlying  epiblast. 
Along  the  sides  of  the  embryo  and  behind  the  caudal  fold,  pro- 
jections of  the  somatopleural  mesoblast  and  epiblast  also  occur. 
Thus  folds  are  developed  consisting  of  somatopleural  mesoblast  and 
of  epiblast,  which  tend  to  grow  upward  and  meet  over  the  back 
of  the  embryo.  These  are  the  amniotic  folds,  and  each  presents 
two  surfaces,  one  looking  toward  the  embryo  and  the  other 
toward  the  vitelline  membrane.  As  they  meet  over  the  back 
of  the  embryo  the  folds,  become  fused,  the  membranes  looking 
toward  the  embryo  joining  to  form  the  amnion  proper,  while 
those  next  the  vitelline  membrane  unite  to  form  the  false  amnion, 
which,  separating  from  the  amnion  proper,  retires  toward  the 


676 


MANUAL   OF   PHYSIOLOGY. 


vitelline  membrane,  with  which  it  unites  to  form  the  primitive 
chorion. 

The  proper  amnion  then  is  a  sac  formed  of  an  outer  layer 
derived  from  the  mesoblast  and  an  inner  layer  derived  from  the 
epiblast.  The  false  amnion  likewise  consists  of  mesoblast  and 
epiblast,  but  here  the  epiblast  is  external.  The  amnion  proper 


FIG.  272. 


FIG.  272  and  the  following  two  woodcuts  are  diagrammatic  views  of  sections  through  the 

developing  ovutn,  showing  the  formation  of  the  membranes  of  the  chick.    (Foster  and 

Balfour.) 
A,  B,  C,  D,  E  and  F  are  vertical   sections  in  the  long  axis  of  the  embryo  at  different 

periods,  showing  the  stages  of  development  of  the  amnion  and  of  the  yolk  sac. 
I,  II,  III  and  IV  are  transverse  sections  at  about  the  same  stages  of  development. 
i,ii  and  iii  give  only  the  posterior  part  of  the  longitudinal  section,  to  show  three  stages 

in  the  formation  of  the  allantois. 

e.  Embryo,    y.  Yolk.    pp.  Pleuroperitoneal  fissure,    vt.  Vitelline  membrane, 
a/.  Amniotic  fold.     al.  Allantois. 

is  continuous  with  the  skin  of  the  embryo,  and  when  the  foetus 
is  mature,  the  connection  may  be  traced  by  the  umbilical  cord, 
round  which  it  forms  a  sheath  continuous  with  the  skin  at  the 
umbilicus.  This  membranous  sac  enlarges,  and  in  mammalia 
eventually  becomes  the  large  bag  of  liquid  which  contains  the 


FORMATION   OF   THE   MEMBRANES. 


677 


foetus.  The  amniotic  liquid  is  of  low  specific  gravity,  consisting 
mainly  of  water  containing  traces  of  nitrogenous  matter,  phos- 
phates and  chlorides. 

It  contains  albumin  and  some  other  nitrogenous  constituents, 


iii. 


e.  Embryo,    a.  Aranion.    a'.  Alimentary  canal,    vt.  Vitelline  membrane,    af.  Amniotic 
fold.     ac.  Amniotic  cavity,    y.  Yolk.    al.  Allantois. 

and  a  minute  quantity  of  urea,  which  is  thought  to  be  derived 
from  the  foetal  kidneys. 

This  fluid  preserves  the  child  from  the  effects  of  any  jolts  or 
jars  caused  by  the  movements  of  the  mother,  and  similarly  pro- 


678 


MANUAL   OF   PHYSIOLOGY. 


tects  the  uterus  of  the  mother  by  acting  as  a  buffer  between  the 
foetus  and  the  uterine  wall.  Before  delivery  it  helps  to  dilate 
the  os  uteri,  so  that  when  the  amnion  is  ruptured  the  head  of  the 
foetus  occupies  the  opening  which  has  been  gradually  made  by 
the  fluid  wedge.  The  outer  part  of  the  amniotic  membrane, 
derived  from  the  mesoblast,  is  of  a  tougher  character  than  the 
inner  epithelial  layer,  possesses  muscular  fibre  and  is  capable  of 
rhythmical  contractions. 


Diagrammatic  sections  of  an  embryo,  showing  the  destiny  of  the  yolk  sac,  ys.  vt.  Vitel- 
line  membrane,  pp.  Pleuroperitoneal  cavity,  ac.  Cavity  of  the  amnion.  a.  Amnion. 
a'.  Alimentary  canal,  ys.  Yolk  sac. 

(2)  The  Yolk  sac  is  that  part  of  the  blastoderm  which  grows 
and  envelops  the  yolk,  which  previously  was  only  surrounded  by 
the  vitelline  membrane.  After  the  mesoblast  has  split  into  two 
layers,  the  splanchnopleure  becomes  bent  inward  at  a  point  some 
distance  from  its  origin,  carrying  with  it  the  hypoblast.  By  this 
curve  an  upper  constricted  canal  is  differentiated  from  the  large 
lower  cavity.  This  upper  canal  becomes  eventually  the  aliment- 


FORMATION   OF   THE    MEMBRANES. 


679 


ary  tract,  the  lower  cavity  the  yolk  sac,  while  the  constricted 
portion  leading  from  one  to  the  other  is  the  canal  leading  from 
the  intestine  to  the  yolk,  called  the  ductus  vitello-intestinalis. 

At  first  the  splanchnopleure  encloses  only  the  upper  part  of 
the  yolk,  but  as  development  proceeds  it  grows  around,  and  at 
last  completely  encircles  it.  The  yolk  sac  is  thus  derived  from 
the  splanchnopleural  layer  of  the  mesoblast,  and  its  lining  hypo- 
blast. 

The  yolk  is  continually  used  up  for  the  nutrition  of  the 
embryo,  and  its  covering  shrinks  in  size,  becoming  smaller  with 
the  growth  of  the  foetus,  until  eventually  it  forms  but  a  shriveled 
protrusion  from  the  intestine,  lying  in  the  umbilical  cord. 

FIG.  273. 


Diagrammatic  longitudinal  section  of  a  chick  on  the  fourth  day.     (Allen  Thomson ) 
ep.  Epiblast.     hy.  Hypoblast.    sm.  Somatopleure.     vtn   Splanchnopleure      af,  pf.  Folds 
of  the  aiunion.     pp.  Pieuroperitoneal  cavity,    am.  Cavity  of  arnnion.    al.  AHantois. 
a.  Position  of  the  future  anus.    A.  Heart,    i.  Intestine,    vi.  Vitelline  duct.    ys.  Yolk. 
s.  Foregut.    m.  Position  of  the  mouth,    me.  Ttie  mesentery. 

The  importance  of  the  yolk  sac  differs  largely  in  mammalia 
and  birds.  In  man  it  is  not  highly  developed,  as  its  place  is 
early  supplied  by  the  placenta.  In  birds  it  develops  to  a  much 
higher  degree,  being  the  seat  of  a  special  circulation,  which 
carries  nourishment  from  the  yolk  to  the  chick.  The  vessels  are 
developed  in  the  mesoblastic  portion  of  the  membrane,  and  are 
called  the  omphalo-mesenteric  vessels,  which  convey  blood  to  and 
from  the  primitive  heart. 

(3)  The  AHantois,  or  urinary  vesicle,  in  the  chick  is  of  import- 
ance, as  the  vessels  developed  in  it  are  used  for  respiratory  pur- 


680  MANUAL   OF   PHYSIOLOGY. 

poses,  being  spread  out  beneath  the  porous  shell.  In  the 
mammalian  embryo  it  is  still  more  important,  as  it  is  the  seat  of 
the  circulation,  which  performs  the  chief  function  of  the  foetal 
placenta.  The  allantois  arises  at  the  tail  of  the  embryo,  as  a 
budding  outward  of  a  portion  of  the  splanchnopleure  forming 
the  wall  of  the  primitive  intestine.  It  is  lined  by  hypoblast,  and 
projects  into  the  pleuro-peritoneal  cavity.  As  it  grows  away 
from  the  embryo  it  extends  between  the  layers  of  the  true  and 

FIG.  274. 


Diagram  of  an  embryo,  showing  the  relationship  of  the  vascular  allantois  to  the  Tilli  of 

thechorion.     (Cadial.) 

a.  Lies  in  the  cavity  of  the  amnion  under  the  embryo,    b.  Yolk  sac.    c.  Allantois. 
d.  Vessels  of  the  allantois  dipping  into  the  villi  of  the  choriou.    e.  Chorion. 

false  amnion  and  approaches  toward  the  vitelline.  membrane, 
but  remains  connected  to  the  intestine  by  a  narrow  tube.  When 
it  reaches  the  periphery  of  the  ovum,  it  spreads  over  the  chorion 
as  a  complete  lining,  and  sends  processes  into  the  villi  of  that 
organ.  It  becomes  chiefly  developed  at  that  part  of  the  chorion 
which  is  opposite  the  decidua  serotina  of  the  mother.  In  the 
mesoblastic  layer  of  the  allantois  blood  vessels  arise  which  are 
connected  with  large  trunks,  proceeding  from  the  primitive  aortse, 


THE    PLACENTA.  681 

called  the  umbilical  arteries ;  these  will,  however,  be  further 
described  when  treating  of  the  fetal  placenta. 

As  the  foetus  becomes  developed,  the  part  of  the  allantois  in 
connection  with  the  body  becomes  gradually  obliterated.  A  part 
of  it  remains  as  the  urinary  bladder,  and  the  rest  forms  a  fibrous 
cord,  which  runs  from  the  apex  of  the  bladder  to  the  umbilicus, 
and  is  known  as  the  urachus. 

(4)  The  Chorion  is  the  external  covering  of  the  ovum.  At 
first  it  consists  simply  of  the  zona  pellucida  or  vitelline  mem- 
brane, and  then  it  is  called  the  primitive  chorion.  Later  it  is 
supplemented  by  that  part  of  the  somatopleure  removed  from 
the  embryo  in  the  process  of  forming  the  amnion.  This  blends 
with  the  primitive  chorion  and  strengthens  it,  and  while  lying 
beneath  the  zona  pellucida,  receives  the  name  of  the  subzonal 
membrane.  The  chorion  at  first  is  a  smooth  membrane,  but 
villous  processes  early  grow  out  from  it.  These  villi  are  chiefly 
developed  at  its  upper  part,  where  they  aid  in  the  formation  of 
the  foetal  placenta. 

The  allantois,  when  it  has  spread  over  the  chorion,  becomes 
blended  with  this  membrane,  and  fills  the  villous  processes  with 
the  blood  vessels  it  contains. 

THE  PLACENTA. 

The  Placenta  is  an  organ  most  important  to  the  mammalian 
embryo.  It  conveys  not  only  nourishment  but  also  oxygen  from 
the  maternal  blood  to  that  of  the  foetus.  It  is,  of  course,  neces- 
sary that  the  animals  whose  ova  do  not  contain  large  stores  of 
food  should  in  some  way  provide  the  substances  necessary  for 
the  life  of  their  embryo,  and  it  is  by  means  of  the  placenta  that 
this  is  brought  about.  The  embryo  of  oviparous  animals  does 
not  require  a  placenta  for  its  nutrition,  since  there  is  inside  the 
egg  a  large  store  of  highly  nutritious  albuminous  and  fatty  ma- 
terials ;  the  shell  is  pervious  to  air,  and  the  chick's  blood  can  in 
the  allantois  be  oxidized  by  the  air  directly.  A  bird's  egg  con- 
tains in  itself  all  the  necessaries  which  the  placenta  supplies,  and 
when  impregnated  only  requires  the  heat  of  the  mother's  body  to 
develop  a  chick. 


682  MANUAL    OF    PHYSIOLOGY. 

While  an  ovum  is  descending  the  Fallopian  tube,  the  mucous 
membrane  of  the  uterus  becomes  turgid,  and,  as  before  mentioned, 
if  the  ovum  be  unirapregnated  it  is  cast  out  of  the  body,  part 
of  the  substance  of  the  lining  membrane  of  the  uterus  is  des- 
quamated and  discharged  with  a  fluid  largely  composed  of 
blood.  This  takes  place  approximately  every  four  weeks,  and 
hence  is  called  menstruation.  If  the  ovum  be  impregnated, 
however,  the  mucous  membrane  of  the  uterus  not  only  becomes 
turgid,  but  its  cells  proliferate,  and  considerable  thickening  of 
the  tissue  takes  place.  The  mucous  membrane  is  then  called  the 
decidua.  When  the  ovum  reaches  the  uterus  it  ordinarily  be- 
comes embedded  in  that  part  of  the  decidua  which  occupies  the 
fundus  of  the  uterus.  The  decidua  here  grows  excessively,  and 
becomes  much  thickened,  and  on  either  side  of  the  ovum  a 
projection  is  sent  from  the  decidua  which  meets  below  the  ovum, 
and  completely  encircles  it. 

The  name  decidua  vera  is  given  to  the  membrane  lining  the 
general  cavity  of  the  uterus,  while  that  part  lining  the  fundus, 
to  which  the  ovum  is  attached,  is  called  the  decidua  serotina,  and 
its  processes  surrounding  the  ovum  receive  the  name  of  the 
decidua  reflexa. 

The  placenta  is  developed  from  two  sources,  one  arising  from 
the  membranes  of  the  foetus,  and  the  other  belonging  to  the 
mother. 

Relation  of  the  Foetal  to  Maternal  Placenta. — The  maternal  part 
is  formed  from  the  decidua  serotina,  which  becomes  much  thick- 
ened and  very  vascular  where  the  placenta  is  attached.  The 
foetal  placenta  is  derived  from  the  chorion,  which  sends  out  a 
number  of  finger-like  processes,  which  subdivide  and  into  which 
the  allantois,  as  it  spreads  over  the  chorion,  sends  prolongations. 
The  mesoblastic  layer  of  the  allantois  gives  rise  to  the  capillaries 
which  are  in  these  processes.  The  capillaries  spring  from  the 
branches  of  the  umbilical  arteries  which  pass  along  the  umbilical 
cord  to  reach  the  chorion.  The  vessels  of  the  decidua  serotina 
or  maternal  placenta  end  in  large  sinuses,  lined  by  endothelial 
cells.  The  blood  is  carried  to  these  sinuses  by  the  uterine 
arteries,  and  from  them  by  the  uterine  veins.  The  walls  of  the 


THE   PLACENTA. 

FlG.  275. 


683 


Series  of  diagrams  representing  the  relationship  of  the  decidua  to  the  ovum  at  different 
periods.  The  decidua  are  colored  black,  and  the  ovum  is  shaded  transversely.  la  4 
and  5  the  vascular  processes  of  the  chorion  are  figured  (copied  from  Daltori). 

1.  Ovum  entering  the  congested  mucous  membrane  of  the  fundus — decidua  serotina. 
2.  Decidua  reflexa  growing  round  the  ovum.  3.  Completion  of  the  denidua  around 
the  ovum.  4.  General  growth  of  villi  of  the  chorion.  5.  Special  growth  of  villi  at 
placental  attachment,  and  atrophy  of  the  rest. 


684 


MANUAL   OF   PHYSIOLOGY. 


sinuses  are  provided  with  unstriped  muscular  tissue,  which  can 
close  the  inlets  from  the  arteries,  and  thus  shut  out  the  blood. 
The  villi  of  the  foetal  placenta,  dipping  into  these  uterine  sinuses, 
are  covered  with  a  single  layer  of  thin,  scaly  cells,  so  that  the 
foetal  blood  is  only  separated  from  the  maternal  by  the  walls  of 

FIG.  276. 


Antero-posterior  section  through  a  gravid  uterus  and  ovum  of  five  weeks  (serai-diagram- 
matic). (Allen  Thomson.) 

a.  Anterior  wall  of  uterus,  p.  Posterior  wall  of  uterus,  m.  Muscle  substance,  g.  Glandu- 
lar layer,  us.  Decidua  serotina.  r.  Decidua  reflexa.  v.  Decidua  vera.  ch.  Chorion. 
u.u.  Uterine  cavity,  c.  Cavity  of  the  cervix. 

the  capillaries  and  these  thin  cells,  and  thus  the  interchange  of 
nutrient  materials  and  gas  readily  go  on  between  them  ;  it  is 
very  similar  to  the  conditions  of  the  lung  alveoli,  where  the 
blood  is  separated  from  the  air  with  which  it  interchanges  gases 
by  the  cells  of  the  capillary  wall  and  of  the  lung  alveolus. 


THE   PLACENTA.  685 

Though  the  capillaries  of  the  foetus  are  in  such  close  relation 
to  the  blood  of  the  mother,  it  must  be  distinctly  understood  that 
there  is  no  direct  communication  between  the  vessels  of  the 
foetus  and  those  of  the  mother,  and  therefore  it  is  not  possible  to 
inject  the  vessels  of  the  mother  through  those  of  the  foetus,  or 
vice  versa. 

The  nutrient  materials  from  the  maternal  blood  together  with 
oxygen  diffuse  through  the  walls  of  the  foetal  capillaries,  the 
effete  matter,  on  the  other  hand,  passing  from  the  capillaries  to 
the  blood  in  the  veins  which  surrounds  and  bathes  these  vessels. 
The  placenta  increases  with  the  growth  of  the  foetus  till  shortly 
before  birth,  when  it  is  said  to  undergo  a  certain  amount  of 
degeneration.  It  is  cast  out  of  the  uterus  after  the  expulsion  of 
the  foetus  with  the  membranes  attached  to  it.  It  is,  however, 
only  the  superficial  layer  of  the  maternal  placenta  (which  is 
intimately  connected  with  the  foetal  placenta)  that  is  cast  off,  the 
deeper  layer  remaining  in  the  uterus,  and  undergoing  various 
changes  during  the  reduction  of  this  organ  to  its  normal  size. 

After  ligature  of  the  umbilical  cord,  the  intimate  relation- 
ships of  the  maternal  and  foetal  circulation  cease,  and  it  is 
thought  that  this  causes  the  inlets  of  the  uterine  sinuses  to  con- 
tract, so  that  when  the  placenta  separates  from  the  uterine  walls, 
the  arterioles  leading  to  the  sinuses  are  contracted  and  possibly 
occluded  with  clots.  The  uterine  blood  current  is  thus  pre- 
vented from  escaping  into  the  uterine  cavity  after  parturition, 
and  causing  profuse  hemorrhage. 

The  uses  of  the  placenta  may  be  briefly  summed  up  as— 

(1)  Alimentary,  as  it  supplies  the  place  of  the  organs  of  diges- 
tion by  supplying  the  foetal  blood  with  nutritive  material. 

(2)  Respiratory,  as  it  performs  the  function  of  the  lungs,  the 
foetal  blood  receiving  oxygen  from  the  oxyhsemoglobin  of  the 
mother,  to  which  it  gives  up  its  CO2. 

(3)  Excretory,  as  it  does  duty  for  the  kidneys,  removing  the 
urea,  etc.,  from  the  foetal  blood. 


686 


MANUAL   OF    PHYSIOLOGY. 


FIG.  277. 


CHAPTER  XXXVIII. 

DEVELOPMENT  OF  THE  SPECIAL  SYSTEMS. 
DEVELOPMENT  OF  THE  VERTEBRAL  AXIS. 

The  earliest  evidence  of  the  differentiation  of  the  blastoderm 
consists  in  the  appearance  of  the  primitive  streak  which  forms  the 

first  sign  of  the  embryo.  This 
is  a  line  which  appears  near 
what  is  to  be  the  tail  end  of 
the  embryo,  and  runs  for- 
ward. This  primitive  line  or 
streak  is  due  to  the  thicken- 
ing of  the  mesoblast,  and  it 
becomes  converted  into  a 
groove  by  a  depression  ap- 
pearing in  its  centre,  forming 
the  primitive  groove.  This 
extends  in  a  forward  direc- 
tion, but  never  reaches  the 
head  fold  of  the  embryo, 
which,  in  the  chick,  appears 
a  few  hours  after  the  forma- 
tion of  the  primitive  groove. 
In  front  of  the  primitive 
groove,  and  stretching  back- 
ward to  overlap  it  at  the 

Viewoftheareapellucidaofachickofeigh-         .,  .  „  ,  ,          „     , 

teen  hours,  seen  from  above.    (Foster  and      SIQCS,    arise    tWO    folds    OI   the 

.4.  Medullary  folds.  epiblast,   called   the    laminse 

me.  Medullary  groove. 


pr.  Primitive  streak  and  groove. 


dorsales    or     the     medullary 

folds. 

These  are  elevations  of  the  epiblast,  beneath  which  the  meso- 
blast is  thickened.  They  arise  in  front,  where  they  are  joined 
immediately  behind  the  head  fold,  while  posteriorly  they  diverge, 


DEVELOPMENT  OF  THE  VERTEBEAL  AXIS. 


687 


and  passing  on  either  side  of  the  primitive  groove,  gradually 
become  lost.  Between  the  two  folds  is  a  furrow  lined  by  epi- 
blast,  which  is  called  the  medullary  groove. 

The  medullary  folds  growing  upward,  turn  in  toward  one  an- 
other, and  eventually  coalesce  at  their  line  of  meeting,  convert- 

FlG.278. 


Transverse  section  of  the  embryo  of  a  chick  at  the  end  of  the  first  day.    (K&Uiker.) 
sp.  Mesoblast.    dd.  Hypoblast.    TO.  Medullary  plate,    h.  Epiblast.    Pv.  Medullary  groove. 
Rf.  Medullary  fold.    ch.  Chorda  dorsalis.    uwp.  Proto-vertebral  plate,    uwh.  Division 
of  niesoblast. 

ing  the  medullary  groove  into  a  channel — the  medullary  canal; 
this  union  of  the  folds  takes  place  from  before  backward. 

The  medullary  canal  thus  formed  lies  in  the  axis  of  the  em- 
bryo on  the  uncleft  mesoblast ;  it  is  covered  in  superficially  by 

FIG.  279. 


Transverse  section  of  an  embryo  of  a  chick  at  the  latter  end  of  the  second  day. 

(KOUiker.) 

rw.  Medullary  fold.  rf.  Medullary  groove,  h.  Epiblast.  ao.  Aorta,  dd.  Hypoblast. 
p,  Pleuroperitoneal  cavity,  sp.  External  plate  of  niesoblast  dividing,  uwp.  Proto- 
vertebral  plate. 

several  layers  of  epiblastic  cells,  which  also  line  its  walls.  The 
canal  is  the  earliest  representative  of  the  nervous  centres,  and 
eventually  becomes  the  brain  and  spinal  cord.  The  front  part 
of  the  canal,  when  completely  closed  in,  becomes  dilated  into  a 


688  MANUAL   OF   PHYSIOLOGY. 

bulb,  thus  forming  the  earliest  indication  of  the  brain.  The 
hind  part  of  the  medullary  groove  remains  unclosed  considerably 
later  than  the  fore  part.  It  gradually  becomes  converted  into  a 
canal  at  the  tail  end,  and  as  it  extends  backward  obliterates  the 
primitive  streak  and  groove,  which  are  lost,  and  take  no  perma- 
nent part  in  the  formation  of  the  embryo. 

Beneath  the  medullary  canal  the  cells  of  the  mesoblast  are 
altered  to  form  a  rod-shaped  cellular  body,  which  following  the 
line  of  the  canal  lies  in  the  axis  of  the  embryo;  this  is  the  chorda 
dorsalis  or  notochord. 

Supporting  the  medullary  canal  on  either  side  of  the  chorda 
dorsalis  are  masses  of  mesoblast,  somewhat  quadrangular  in 
section,  which  are  termed  the  vertebral  plates;  continuous  with 


Transverse  section  through  the  embryo  of  a  chick  on  the  second  day,  where  the  medul- 
lary canal  is  closed.    (KO  liker.) 
mr.  Medullary  canal    h.  Epiblast.    uwh.  Cavity  of  protovertebra  uw.    ung.  Wolffian  duct. 

mp.  Meaoblast  dividing,   hpl.  Somatopleure.  df.  S^lanchuopleure.  sp.  Pieuroperitoneal 

cavity,    dd.  Hypoblast.    ch.  Notochord. 

these  externally  are  other  thinner  masses  of  mesoblast  called  the 
lateral  plates. 

The  lateral  plates  become  divided  into  an  upper  part  or 
somatopleure,  which  is  in  close  relationship  to  the  epiblast,  and  a 
lower  part,  the  splanchnopleure,  which  is  next  to  the  hypoblast ; 
the  space  between  these  being  the  pleuroperitoneal  cavity.  The 
vertebral  plates  become  separated  from  the  lateral  plates  by  a 
longitudinal  partition,  so  that  on  either  side  of  the  neural  canal 
is  a  mass  of  undivided  mesoblast  extending  laterally  toward  the 
divided  mesoblast. 

In  each  vertebral  plate  there  appear  transverse  vertical  inter- 
ruptions at'  definite  intervals,  which  split  the  plate  up  into  a 


DEVELOPMENT  OF  THE  VERTEBRAL  AXIS. 


689 


number  of  quadrangular  blocks  of  mesoblast,  known  as  the 
protovertebrce ;  the  number  of  these  corresponds  to  the.  number 
of  vertebrae  of  the  animal. 

These  protovertebrse  become  subdivided  by  transverse  fissures 
into  external  parts,  the  muscle  plates,  which  form  eventually  the 

FIG. 282. 


Embryo  chick  at  the  end  of  the 
second  day,  seen  from  below. 
(KQdiker.) 

Vh.  Forebrain. 

Ab.  Optic  vesicles. 

Ch.  Notochord. 

H.  Heart. 

om.  Omphalo-mesenteric  veins. 

Vd,  Lower  opening  of  foregut. 


Division  of  the  vertebral  column 
of  a  chick.  (EOlliker  after 
Kemak.) 

1.  Notochord. 

2.  Points  of  separation  of  the  ori- 
ginal protovertebrse. 

3.  Points  of  division  of  the  per- 
manent vertebrae. 

4.  Arches  of  the  vertebrae. 

5.  Spinal  ganglia. 

c.  Body  of  first  cervical  vertebra. 

d.  One  of  the  lower  vertebrae. 


dorsal  and  other  muscles,  and  internal  parts  which  become  the 
permanent  vertebrae. 

From  these  inner  portions  processes  of  mesoblast  grow  upward 
over  the  medullary  canal  to  meet  with  processes  from  the  pro- 
tovertebrse of  the  opposite  side.     Mesoblastic  tissue  also  grows 
58 


690 


MANUAL   OF   PHYSIOLOGY. 

FIG.  283. 


Transverse  section  through  the  dorsal  region  of  an  embryo  chick  of  forty-five  hours. 

(Foster  and  Balfour.) 
A.  Epiblast.     M.c.  Medullary  canal.     P.v.  Proto-vertebra?.     W.d.  Wolffian  duct.     p.p. 

Pleuroperitoneal  cavity.      S.o.  Somatopleure.      S.p.  Splanchnopleure.      c.v  Vessels. 

a.o.  Aorta.    B.  Mesoblast.     C.  Hypoblast.    o.p.  Line  of  union  of  opaque  and  pellucid 


areas,    w.  Spheres  of  the  white  yolk 


DEVELOPMENT   OF   THE   CENTRAL   NERVOUS   SYSTEM.      691 

inward  between  the  medullary  canal  and  the  notochord,   and 
between  the  notochord  and  subjacent  hypoblast. 

These  projections  beneath  the  notochord  meet  with  others  from 
a  mass  of  the  mesoblast,  lying  between  the  proto vertebrae  and  the 
cleft  mesoblast,  and  known  as  the  intermediate  cell  mass. 

The  portions  of  the  protovertebrse  above  the  medullary  canal 
form  the  arches  of  the  vertebrae ;  from  those  surrounding  the 
notochord  the  bodies  of  the  vertebrae  are  developed. 

The  outer  part  of  each  protovertebra  divides  into  an  anterior 
or  pre-axial  part,  from  which  arises  the  ganglion  of  a  spinal 
nerve,  and  into  a  posterior  or  post-axial  part. 

After  this  the  original  lines  of  separation  between  the  proto- 
vertebrse disappear,  and  the  spinal  column  is  fused  into  a  cartila- 
ginous mass.  New  segmentation  now  appears  in  the  centre  of 
each  original  protovertebra,  midway  between  the  primary  divi- 
sions. Thus  the  vertebral  column  is  divided  into  a  number  of 
component  parts,  each  of  which  is  destined  to  become  a  perma- 
nent vertebra. 

The  vertebrae  do  not  then  correspond  to  the  original  protover- 
tebra, but  rather  to  the  posterior  half  of  that  which  lay  in  front 
of  the  primary  division  joined  to  the  anterior  half  of  the  one 
behind.  The  ganglia  of  the  spinal  nerves,  by  this  arrangement, 
instead  of  belonging  to  the  front,  become  joined  to  the  posterior 
part  of  the  vertebra  to  which  they  belong. 

The  notochord  atrophies  with  ossification  of  the  vertebrae,  and 
finally  is  represented  only  by  a  mass  of  soft  cells  in  the  centre  of 
an  intervertebral  disc. 

In  connection  with  the  vertebras  in  the  dorsal  region,  processes 
grow  horizontally,  these  are  the  rudiments  of  the  ribs. 

DEVELOPMENT  OF  THE  CENTRAL  NERVOUS  SYSTEM. 
SPINAL  CORD. 

Soon  after  the  closure  of  the  medullary  or  neural  canal  at  its 
anterior  or  cranial  end,  it  is  dilated  in  this  region  into  three 
vesicles,  known  as  the  first,  second,  and  third  cerebral  vesicles, 
from  which  the  brain  is  developed.  The  spinal  cord  is  formed 
from  that  part  of  the  medullary  canal  which  lies  over  the  chorda 


692 


MANUAL   OF   PHYSIOLOGY. 


FIG.  284. 


dorsalis.  The  medullary  canal  is  lined  by  columnar  cells  derived 
from  the  epiblast,  which,  shortly  after  they  are  shut  off  from  the 
general  epiblast,  develop  at  the  sides  of  the  canal,  so  as  to  narrow 
the  lumen  of  the  tube  by  the  increase  in  thickness  of  its  sides. 
The  upper  and  lower  parts  of  the  canal  do  not  become  thickened. 

The  side  walls  approximate 
to  the  centre,  decreasing 
laterally  the  lumen  of  the 
canal,  which  becomes  narrow 

^ r-rt — -£^,  N  in  the   middle  with  a  dilata- 

,,\  f*w*nH?^v          \  tion  above  and  below.     The 

lateral  walls  of  the  canal,  thus 
approximated,  unite  in  the 
centre,  and  convert  the  medul- 
lary canal  into  two  separate 
tubes,  a  dorsal  and  a  ventral. 
The  lower  or  ventral  tube  of 
the  divided  canal  becomes  the 
central  canal  of  the  spinal 
cord,  and  the  columnar  cells 
of  the  epiblast  form  a  lining  of 
ciliated  columnar  epithelium. 
The  epiblast  at  the  lower 

Transverse  section  of  the  spinal  column  of  the  part    of     the     Canal     becomes 
human  embryo  of  from  nine  to  ten  weeks.  .     i     •     .        .1 

(KGiHker.)  converted   into   the   anterior 

gray  columns,  in  connection 
with  which  arise  the  anterior 
roots  of  the  spinal  nerves ; 
while  at  the  upper  part  the 
posterior  gray  columns  are 
formed  in  connection  with  the 
posterior  roots  of  the  spinal 
nerves  and  their  ganglia. 

The  white  columns  are  thought  by  some  authors  to  be  derived 
from  the  mesoblast  surrounding  the  canal,  but  by  others  they  are 
assigned  to  the  epiblast. 

The  upper  or  dorsal  canal  becomes  converted  into  a  fissure  by 


dm.  Dura  mater. 

p'.  Column s  of  Goll. 

p.  Posterior  column. 

pr.  Posterior  root. 

na.  Arch  of  vetrebra. 

g.  Ganglion  of  a  ppinal  nerve. 

a.  Anterior  column. 
cir.  Anterior  root. 
ch.  Notochord. 

b.  Body  of  the  vertebra. 
n.  Spinal  nerve. 

c.  Central  canal. 

e.  Epithelium  of  canal. 


DEVELOPMENT    OF    THE    BRAIN. 


693 


the  absorption  of  its  roof,  and  is  thus  changed  into  the  posterior 
fissure  of  the  spinal  cord. 

The  anterior  fissure  is  formed  by  the  down-growth  of  the  anterior 
columns,  which  diverge,  leaving  between  them  an  interval  which 
becomes  occupied  by  the  pia  inater. 

The  commissures  are  not  formed  between  the  lateral  halves  of 
the  cord  until  later.  The  gray  commissure  appears  first. 


lew 


etc 


Transverse  section  of  the  spinal  cord  of  a  chick  of  seven  days.  (Foster  and  Balfour.) 
ep.  Epithelium  lining  the  medullary  canal,  pf.  Part  of  the  cavity  of  the  medullary 
canal  which  becomes  the  posterior  fissure,  spc.  Permanent  medullary  tube  or 
central  canal  of  the  spinal  cord.  age.  Anterior  gray  commissure,  af.  Anterior  fissure, 
not  yet  well  formed,  c.  Tissue  filling  in  the  upper  part  of  the  posterior  fissure,  pc. 
Cells  forming  the  posterior  gray  matter,  pew.  Posterior  white  column,  ct.  Mesoblast 
surrounding  thespinal  cord,  lew.  Lateral  white  column,  acw.  Anterior  white  column. 
«c.  Cells  forming  the  anterior  gray  matter. 

THE  BRAIN. 

Anterior  Cerebral  Vesicle. — As  already  mentioned,  the  brain  is 
formed  from  the  primitive  neural  canal,  the  anterior  part  of  which 
dilates  into  three  little  swellings  called  the  anterior,  middle  and 
posterior  cerebral  vesicles.  From  the  anterior,  or  first  cerebral 


694 


MANUAL   OF   PHYSIOLOGY. 


FIG.  286. 


vesicle,  at  an  early  period  spring  two  processes,  which  become  the 
optic  vesicles.  These  ultimately  develop  into  the  retina,  and  other 
nervous  parts  of  the  eye,  with  the  history  of  which  the  changes 
occurring  in  them  will  be  described. 

The  optic  vesicles  are  pushed  downward  by  two  large  processes 
growing  forward  from  the  anterior  vesicle  (the  primitive  cerebral 
hemispheres).  The  anterior  part  of  the  brain  then  appears  to  be 
composed  of  two  divisions,  the  anterior  of  which  is  subsequently 
developed  into  the  cerebral  hemispheres,  corpora  striata,  and  the 
olfactory  lobes,  as  a  whole  called  prosencephalon,  while  the  hinder 
part,  representing  the  anterior  vesicle,  receives  the  name  of 
thalamencephalon. 

The  cavity  of  the  thalamencephalon  opens  behind  into  the 
cavity  of  the  middle  cerebral  vesicle,  and  in 
front  communicates  with  the  hollow  rudiments 
of  the  cerebral  hemispheres,  and  eventually  it 
becomes  the  cavity  of  the  third  ventricle.  The 
floor  of  the  thalamencephalon  is  ultimately 
developed  into  the  optic  chiasma,  part  of  the 
optic  nerves,  and  the  infundibulum.  The  lat- 
ter comes  in  contact  with  a  process  from  the 
mouth,  uniting  with  which  it  ultimately  forms 
the  pituitary  body.  From  the  posterior  part 
of  the  roof  of  the  thalamencephalon  is  devel- 
oped the  pineal  gland — a  peculiar  outgrowth, 
of  unknown  function,  more  elaborately  devel- 
oped in  some  of  the  lower  vertebrates.  The 
anterior  part  of  the  roof  of  the  thalamen- 
cephalon becomes  very  thin,  and  its  place  is 
finally  occupied  by  a  thin  membrane  contain- 
ing a  vascular  plexus,  which  persists  in  the  roof 
of  the  third  ventricle  (choroid  plexus).  From  the  sides  of  the 
thalamencephalon,  which  become  extremely  thickened,  are  devel- 
oped the  optic  thalami. 

The  primitive  cerebral  hemispheres  first  appear  as  two  lobes 
growing  from  the  anterior  part  of  the  first  cerebral  vesicle.  The 
floor  of  these  lobes  thickens,  and  forms  the  corpora  striata,  while 


Diagram  of  the  cere- 
bral vesicles  of  the 
brain  of  a  chick  at 
the  second  day.  (Ca- 
diat.)  1,  2,  3.  Cere- 
bral vesicles.  0.  Op- 
tic vesicles. 


DEVELOPMENT   OF   THE   BRAIN. 


695 


v-zzr 


Diagram  of  a  vertical  longitudinal  section  of  the  developing  brain  of  a  vertebrate  ani- 
mal, showing  the  relation  of  the  three  cerebral  vesicles  to  the  different  parts  of  the 
adult  brain.  (Huxley.) 

Olf.  Olfactory  lobes.  Fm.  Foramen  of  Monro.  Os.  Corpus  striatum.  Th.  Optic  thala- 
nms.  Pn.  Pineal  gland.  M.b.  Mid  brain.  Cb.  Cerebellum.  Mo.  Medulla  oblongata. 
Hmp.  Cerebral  hemispheres.  Th.E.  Thalamencephalon.  Py,  Pituitary  body.  CQ. 
Corpora  Quadrigemina.  C.C.  Crura  cerebri.  PV.  Pons  Varolii.  I.— XII.  Regions 
from  which  spring  the  cranial  nerves.  1.  Olfactory  ventricle.  2.  Lateral  ventricle. 
3.  Third  ventricle.  4.  Fourth  ventricle. 


FIG. 


M.O. 


Diagram  of  a  horizontal  section  of  a  vertebrate  brain.     (Huxley.') 
Olf.  Olfactory  lobes.    Lt.  Lamina  terminal!*.     Cs.  Corpus  striatum.     Th.  Optic  thalamus. 
Pa.   Pineal  gland.    Mb.    Mid  brain.      Cb.   Cerebellum.      Mo.  Medulla  oblongata.    1. 
Olfactory  ventricle.     2.   Lateral  ventricle.    3.  Third  ventricle.     4.   Fourth  ventricle. 
-+•  Iter  a  tertio  ad  quartum  ventriculum.     FM.  Foramen  of  Munro.    //.  Optic  nerves. 


696 


MANUAL    OF   PHYSIOLOGY. 


the  roof  develops  into  the  hemispheres  proper.     The  cavities  of 
these  lobes  become  the  lateral  ventricles,  and  are  connected  by 

the  foramen   of  Munro, 
which     at 
periods 


FIG.  289. 


L'JS 


IS 


the    earlier 
very   wide, 

but  subsequently  nar- 
rows to  a  mere  slit. 
The  cerebral  hemi- 
spheres are  separated 
at  an  early  stage  by  a 
fold  of  connective  tis- 
sue, which  ultimately 
forms  into  the  falx 
cerebri.  The  hemi- 
spheres are  greatly  en- 
larged in  the  backward 
direction,  so  that  they 
quite  overlap  the  thala- 
mencephalon  and  the 
parts  developed  from 
the  middle  cerebral 
vesicle.  The  corpus  cal- 
losum  is  subsequently 
formed  by  the  fusion  of 
the  juxtaposed  parts  of 
the  hemispheres. 

From  the  anterior 
part  of  the  cerebral 
hemispheres  arise  two 
prolongations,  which  de- 
velop into  the  olfactory 
bulbs  ;  these  grow  for- 
ward, and  soon  lose  their 
cavities,  which  at  first 

communicated  with  those  of  the  ventricles. 

Middle  Cerebral  Vesicle. — By  the  cranial  flexure  the  brain  is 

bent  at  the  junction  of  the  first  and  second  cerebral  vesicles; 


Chick  on  the  third  day,  seen  from  beneath  as  a  trans- 
parent object,  the  head  being  turned  to  one  side. 
(Foster  and  Bui  Jour.) 

a'.  False  amnion.  a.  Amnion.  CU.  Cerebral  hemis- 
phere. FB.,MB.,  HB.,  Anterior  Middle,  and  Pos- 
terior cerebral  vesicles,  op.  Optic  vesicle,  ot. 
Auditory  vesicle,  ofv.  Omphalo-mesenteric  veins. 
Hi.  Heart.  Ao.  Bulbus  arteriosus.  Ch.  Notocbord. 
Of.a.  Omphalo-mesenteric  arteries.  Pv.  Proto- 
vertebrse.  x.  Point  of  divergence  of  the  splanchno- 
pleural  folds,  y.  Termination  of  the  foregut,  V. 


THE   ALIMENTARY   CANAL.  697 

the  first  is  thus  turned  downward,  leaving  the  second  as  the  most 
anterior  part  of  the  brain. 

The  upper  walls  of  the  middle  cerebral  vesicle  are  developed 
into  the  corpora  quadrigemina. 

The  cavity  of  this  vesicle  persists  as  a  narrow  channel,  forming 
a  communication  between  the  third  ventricle  in  front  and  the 
fourth  ventricle  behind,  and  receives  the  name  in  the  adult  brain 
of  the  Her  a  tertio  ad  quartum  ventriculum.  The  crura  cerebri 
arise  from  the  lower  wall  of  this  middle  vesicle. 

Posterior  Cerebral  Vesicle. — This  is  divided  into  an  anterior  and 
a  posterior  part.  From  the  roof  of  the  anterior  division  arises 
the  cerebellum,  and  from  its  floor  the  pons  Varolii. 

The  posterior  division  gives  rise  to  the  medulla  oblongata. 

The  cavity  of  this  vesicle  is  called  the  fourth  ventricle.  It  is 
continuous  with  the  central  canal  of  the  spinal  cord.  Its  upper 
wall  is  thinned  and  forms  the  valve  of  Vieussens.  It  communi- 
cates with  the  subarachnoid  space  through  the  foramen  of 
Majendie. 

THE  ALIMENTARY  CANAL  AND  ITS  APPENDAGES. 

When  the  blastoderm  is  bent  at  its  anterior  extremity  to  form 
the  cephalic  fold,  it  closes  and  forms  the  anterior  boundary  of  a 
short  canal,  the  upper  wall  of  which  is  formed  by  the  general 
blastoderm,  and  the  lower  by  that  part  of  the  splanchnopleure 
which  runs  backward,  leaving  the  somatopleure  to  form  the 
pleuroperitoneal  space.  It  then  turns  forward  to  meet  the 
uncleft  mesoblast,  forming  the  wall  of  the  yolk  sac,  which  com- 
municates freely  with  this  rudimentary  part  of  the  alimentary 
tract. 

This  canal  becomes  closed  in  for  a  considerable  extent,  and  is 
called  the  fore  gut.  It  is  the  precursor  of  the  pharynx,  the  lungs, 
the  oesophagus,  the  stomach,  and  the  duodenum.  The  mouth, 
which  at  this  period  is  unformed,  is  developed  later  by  an  invo- 
lution of  the  epiblast  and  the  removal  of  the  tissue  between  the 
fore  gut  and  the  buccal  cavity. 

The  tail  fold,  in  a  somewhat  similar  manner,  shuts  off  a  canal 
called  the  hind  gut,  which  becomes  developed  into  the  posterior 
59 


698 


MANUAL   OF   PHYSIOLOGY. 


part  of  the  alimentary  canal.  This  hind  gut,  until  the  further 
development  of  the  bladder,  etc.,  is  in  connection  with  the  allan- 
tois,  which  arises  as  a  bud  from  the  lower  part  of  the  rudimentary 
hind  gut. 

Between  these  two  canals  an  intermediate  one  is  formed  by  the 
splanchnopleure,  which,  at  a  distance  from  its  origin,  becomes 
constricted,  and  shuts  off  an  upper  canal,  the  mid-gut,  from  the 


FiG. 290. 


Alimectary  canal  of  an  embryo  while  the  rudimentary  mid-gut  is  still  in  continuity  with 
the  yolk  sac.    (KOlliktr  ajter  Bischoff.) 


A.  View  from  below . 

a.  Pharyngeal  plates. 

b.  The  pharynx. 

c.c.  Diverticula  forming  the  lungs. 

d.  The  stomach. 

/.  Diverticula  of  the  liver. 

g.  Membrane  torn  from  the  yolk  sac. 

h.  Hind-gut. 


B.  Longitudinal  Section. 
a.  Diverticulum  of  a  lung. 
6.  Stomach, 
c.  Liver, 
rf.  Yolk  sac. 


lower  larger  yolk  sac,  the  connection  between  the  two  forming 
the  ductus  vitello-intestinalis. 

Thus  the  primitive  alimentary  canal  consists  of  an  anterior 
and  a  posterior  blind  canal,  which  are  closed  below,  and  a  third 
intermediate  between  these,  which  opens  at  its  lower  surface  into 
the  yolk  sac. 

As  the  placental  circulation  becomes  more  developed,  the 
yolk  sac  shrinks  and  atrophies,  until  it  is  represented  by  a  fold 


THE   ALIMENTARY    CANAL. 

FIG.  291. 


699 


Position  of  the  various  parts  of  the  alimentary  canal  at  different  stages.    A.  Embryo  of 

five  weeks;  B.  Of  eight  weeks;  C.  Of  ten  weeks.    (Allen  Thomson.) 
/.Pharynx  with  the  lungs;    s.  Stomach;    i.  Small  intestine;   i.   Large  intestine;    g. 
Genital  duct ;  «.  Bladder ;  cl.  Cloaca ;    c.  Caecum ;  vi.  Ductus  vitello-intestinalis ;  si. 
Urogeuital  sinus  ;  v.  Yolk  sac. 

FIG.  292. 


Longitudinal  section  of  a  foetal  sheep.    (Cadiat.) 

a.  Pericardium;    b.  Commencement  of  diaphragm ;   c.    Heart;   d.  Branchial  arches 
e.  Pharynx;  /.  Origin  of  lung;  g.  Liver. 


700 


MANUAL   OF    PHYSIOLOGY. 


FIG.  293. 


of  tissue  connected  with  the  primitive  intestine.  The  ductus 
vitello-intestinalis  accordingly  becomes  obliterated,  and  thus  the 
mid-gut  is  closed  at  its  lower  aspect. 

The  primitive  intestine  placed  at  the  inferior  aspect  of  the 
embryo,  just  below  the  protovertebne,  is  lined  by  hypoblast  and 
covered  by  mesoblast.  The  cephalic  or  anterior  extremity  of 
the  canal  is  formed  by  uncleft  mesoblast ;  the  rest  of  the  canal 
is  formed  by  the  splanchnopleural  layer. 

A  dilatation  of  a  part  of  the  fore  gut  gives  origin  to  the  primi- 
tive stomach  ;  this  is  quite  straight  at  first,  lying  below  the 
vertebral  column,  with  which  it  is  connected  by  mesoblast.  After 
a  time  the  stomach  becomes  turned  to  the  right  side,  so  that  the 
left  surface  of  the  organ  lies  anteriorly  and  the  right  posteriorly, 
the  mesoblast  connecting  it  with  the  vertebral  column,  being  de- 
veloped into  the  peritoneal  processes  of  the  organ. 

The  lower  part  of  the  fore  gut  is  of  much  smaller  calibre  than 
the  dilated  portion  forming  the 
stomach  ;  it  becomes  the  duodenum, 
in  connection  with  which  arise  two 
important  viscera,  the  liver  and  the 
pancreas.  The  mid-gut  and  hind  gut 
form  the  small  and  large  intestines, 
these  being  at  first  one  straight  tube, 
of  which  the  small  intestine  has  the 
larger  calibre.  The  small  intestine, 
as  it  grows,  falls  into  folds,  and  the 
mesoblast  connecting  it  to  the  verte- 
bral column  forms  the  mesentery. 

The  large  intestine  is  at  first  a 
straight  tube  lying  to  the  left  of  the 
embryo ;  it  becomes  bent,  and  part 
of  the  tube  is  directed  toward  the 
right  side ;  this  develops  another 

flexure,  the  portion  of  intestine  below  which  grows  downward. 
That  part  remaining  on  the  left  side  forms  the  rectum,  the  sigmoid 
flexure,  and  the  descending  colon ;  while  that  between  the  flexures 
becomes  the  transverse  colon,  and  that  on  the  right  side  the 
ascending  colon. 


Diagram  of  the  alimentary  canal  of 
a  chick  at  the  fourth  day.  (Foster 
and  Balfour,  after  GOile.) 

Ig.  Diverticulum  of  one  lung.  St. 
Stomach.  I,  Liver,  p.  Pancreas. 


THE    ALIMENTARY   CANAL.  701 

The  caecum  is  developed  from  the  ascending  colon,  the  ileo- 
caecal  valve  shutting  off  one  part  of  the  intestinal  canal  from  the 
other.  The  vermiform  appendix  originates  from  the  inferior 
extremity  of  the  csecum*  which,  owing  to  its  feeble  growth,  is  of 
much  smaller  calibre  than  the  upper  part. 

The  epithelial  lining  of  the  intestines  is  derived  from  the 
hypoblast,  and  the  muscular,  vascular,  connective  tissue  and 
serous  coverings  are  mesoblastic  in  their  origin. 

The  liver  is  developed  from  two  diverticula  of  the  duodenum, 
in  connection  with  which  arise  cylinders  of  cells.  The  hypoblast 
develops  into  the  liver  cells  and  the  cells  lining  the  ducts,  the 
mesoblast  furnishing  the  vascular  and  connective  tissue  parts  of 
the  organ.  The  two  diverticula  are  connected  by  a  transverse 
piece,  and  form  the  right  and  left  lobes  of  the  liver. 

The  process  connecting  the  liver  to  the  duodenum  forms  the 
common  bile  duct,  and  from  this  the  gall  bladder  is  developed 
as  an  outgrowth. 

The  vessels  of  the  embryo,  which  are  in  relation  to  the  liver, 
will  be  described  under  the  vascular  system. 

The  pancreas  arises  as  an  outgrowth  from  the  duodenum,  its 
constituent  parts  originating  in  a  manner  similar  to  those  of  the 
liver. 

The  spleen  is  derived  from  the  mesoblast,  and  is  developed  in 
one  of  the  peritoneal  processes  of  the  stomach. 

The  lungs  are  developed  in  connection  with  the  oesophagus,  of 
which  they  are  early  outgrowths. 

The  canal  of  the  fore  gut  at  a  certain  point  becomes  laterally 
constricted,  its  transverse  section  presenting  an  hour-glass  shape, 
consisting  of  an  upper  and  lower  dilated  portion,  united  by  a 
central  constricted  neck.  The  lower  of  these  cavities  becomes 
subdivided  by  the  outgrowth  of  the  lateral  portions  and  the 
upgrowth  of  a  part  of  the  lower  wall  which  forms  a  central  sep- 
tum, so  that  the  fore  gut  is  composed  of  an  upper  undivided 
tube,  giving  off  two  appendages. 

These  appendages  consist  of  hypoblastic  tissue,  and  as  they 
grow  into  the  surrounding  mesoblast  they  divide  and  subdivide, 
until  at  last  they  consist  of  very  minute  tubules,  which  terminate 


702  MANUAL   OF   PHYSIOLOGY. 

in  dilated  extremities.  The  undivided  canal  forms  the  perma- 
nent trachea,  the  appendages  the  main  bronchi,  while  their 
minute  subdivisions  are  the  bronchioles,  which  terminate  in  the 
dilated  alveoli. 

The  hypoblast  forms  the  delicate  lining  membrane  of  the  air 
passages,  and  the  mesoblast  gives  rise  to  the  supporting  tissue 
holding  them  together,  to  the  blood  vessels,  the  muscular,  carti- 
laginous and  connective  tissue  of  the  bronchial  tubes. 

The  pleurae  surrounding  the  lungs  are,  like  the  other  serous 
membranes,  mesoblastic  in  their  origin. 

GENITO-URINARY  APPARATUS. 

In  the  interval  between  the  protovertebrse  and  the  cleavage  of 
the  .mesoblast  into  its  somatopleural  and  splanchnopleural  layers, 
a  mass  of  cells  arranges  itself  into  a  longitudinal  ridge.  This 
ridge,  which  lies  beneath  the  epiblast,  becomes  hollow,  and  thus 
a  tube  is  produced,  called  the  Wolffian  duct. 

From  this  tube  diverticula  arise,  which  extend  into  the  sur- 

FIG.  294. 


Transverse  section  through  the  embryo  of  a  chick  on  the  second  day,  where  the  medul- 
lary canal  is  closed.    (KOlliker.) 
mr.  Medullary  canal,    h.  Epiblast.   uivh..  Cavity  of  protovcrtebra  nw.    ««</.  Wolffian  duct. 

mp.  Mesoblast  dividing,  hpl.  Somatopleure.    df.  Splanchnopleure.  sp.  Pleuroperitoueal 

cavity,    dd.  Hypoblast.   ch.  Notochord. 

rounding  mesoblast;  they  are  tubular,  and  communicate  with  the 
central  duct.  The  processes  become  twisted,  and  at  their  extrem- 
ities the  neighboring  mesoblast  undergoes  differentiation,  and 
forms  vascular  capsules  corresponding  in  structure  to  the  Mal- 
pighian  corpuscles.  This  part  of  the  Wolffian  duct,  which  has 
acquired  a  glandular  structure,  is  the  Wolffian  body  or  primitive 
kidney  of  the  embryo,  while  the  Wolffian  duct  corresponds  to 
the  primitive  ureter. 


THE   WOLFFIAN   DUCT. 


703 


The  epithelium  lining  the  interval  between  the  somatopleure 
and  splanchnopleure  (pleuroperitoneal  cavity)  becomes  columnar 
in  character  close  to  their  origin  from  the  unclefo  mesoblast.  It 
receives  the  name  of  the  germinal  epithelium.  An  involution  of 
this  takes  place  into  the  mesoblast,  just  below  the  somatopleure, 
and  becomes  shut  off,  forming  a  hollow  cylinder. 

FlG. 295. 


Sestionof  the  inner  part  of  the  pleuroperitoneal  cavity  through  tho  origin  of  the  genito- 
urinary organs.    ( Waldeyer.) 
L.  Somatopleure.  m.  Splanchnopleure.   a.  Germinal  epithelium.  C,  o.  Primitive  ova.   E. 

Mesoblast  forming  the  ovary.    WK.  Wolffian  body.   y.  Wolffian  duct.   a'.  Epithelium 

giving  rise  to  the  duct  of  Mailer  z. 

By  this  means  a  second  duct  is  formed  in  close  relation  to  the 
first;  this  is  the  Mullerian  dud.  This  duct  is  developed  from 
before  backward. 

According  as  the  embryo  is  a  male  or  a  female,  so  one  or  other 
of  these  ducts  develops.  In  the  male  the  Wolffian  duct  remains 
as  the  vas  deferens,  and  the  Miillerian  duct  becomes  atrophied. 


704 


MANUAL   OF   PHYSIOLOGY. 


In  the  female,  on  the  other  hand,  the  Miillerian  duct  forms  the 
organs  for  the  conveyance  of  the  ova  out  of  the  body,  and  the 
Wolffian  duct  is  represented  by  a  rudimentary  structure  near  the 
ovary. 

FIG.  296. 


WR 


Trans  verse  section  through  the  lumbar  region  of  ?n  embryo  chick  at  the  end  of  the 
fourth  day.  (Foster  and  fialfour.) 

W.  P.  Wolffian  ridge,  g.  e.  Germinal  epithelium.  A.O.  Dorsal  aorta.  M.  Mesentery. 
SP.  Splanchnopleure.  d.  Alimentaiy  canal.  V.  Vessels,  m.p.  Commencing-  Miillerian 
duct.  Fo.  Somatopleure.  M.  b.  Wolffian  bcdy.  W.  d.  Wolffian  duct.  V.  c.  a.  Pos- 
leriorcaidinal  vein.  c.  h.  Notochord.  A.  W.  C.  Anterior  whii&column  of  spinal  cord. 
«.  r.  Anterior  root.  A.  G.  (\  Anterior  gray  column,  p.  r.  Posterior  root.  TO.  p.  Muscle 
]  late.  we.  Canal  of  spinal  cord. 

Part,  however,  of  the  Wolffian  duct  in  both  sexes  develops 
similarly :  this,  the  metanephros,  corresponds  to  that  part  of  the 
duct  nearest  to  the  tail  end  of  the  embryo.  It  forms  part  of  the 
urinary  organs,  and  develops  into  the  permanent  ureter  and  the 
kidney. 


THE   WOLFFIAN    DUCT. 


705 


FIG.  297. 


From  the  metanephros  a  projection  arises,  which  grows  quickly 
and  opens  into  the  cloaca  ;  this  remains  as  the  ureter.  From  the 
upper  part  of  the  ureter  arise  small  csecal  evolutions,  which 
become  convoluted  at  certain  points  and  surrounded  by  mesoblast; 
these  canals  are  the  urinary  tubules,  and  at  the  extremity  of 
each  is  developed  a  tuft  of 
vessels,  which  thus  forms  a 
Malpighian  corpuscle. 

The  straight  tubes  group 
themselves  together  at  the 
inner  part  of  the  gland, 
while  the  convoluted  tu- 
bules, with  the  Malpighian 
corpuscles,  are  aggregated 
at  the  periphery  of  the 
gland. 

At  the  junction  of  the 
ureter  to  the  glandular 
mass,  changes  take  place 
by  which  this  tube  is  split 
up  into  several  subdivi- 
sions, which  are  the  primi- 
tive calices  of  the  kidney, 
the  dilated  part  of  the 
ureter  forming  the  pelvis. 

The  testicle  arises  partly 
from  the  germinal  epithe- 
lium lining  the  inner  ex- 
tremity of  the  pleuroperi- 
toneal  cavity,  lying  close 
tothesplanchnopleure,  and 
partly  from  the  mesoblast  surrounding  the  Wolffian  body. 

The  germinal  epithelium,  the  cells  of  which  are  not  so  well 
developed  as  in  the  female,  sends  processes  into  the  mesoblast, 
and  these  are  said  to  form  the  spermatic  cells,  the  mesoblast 
becoming  differentiated  around  them  to  form  the  walls  of  the 
tubuli  seminiferi. 


Diagram  of  the  genital  organs  of  an  embryo 
previous  to  sexual  distinction.  (Allen  Thoni- 
son.) 

W.  Wolffian  body.  3.  Ureter.  4.  Bladder.  5. 
Urachus.  gc.  Genital  cord.  m.  Miillerian 
duct.  w.  Wolffian  duct.  vg.  Urogenital  sinus. 
cp  (  litoris,  or  penis.  /.  Intestine,  cl.  Cloaca. 
Is.  Part  from  which  the  scrotum  or  the  labia 
majora  are  developed,  ot.  Origin  of  the  ovary 
or  testicle  respectively,  x.  Part  of  Wolffian 
body  subsequently  developed  into  the  coui 
vasculosi. 


706 


MANUAL   OF   PHYSIOLOGY. 


The  Wolffian  duct,  which  persists  as  the  vas  deferens,  aids  in 
forming  the  testicle,  the  epididymis  being  merely  a  convoluted 
part  of  it,  and  the  vas  aberrans  one  of  the  csecal  tubes  in  con- 
nection with  the  duct.  The  coni  vasculosi  are  thought  to  be 
formed  from  some  of  the  tubules  of  the  Wolffian  body  ;  they  are 
connected  to  the  testicle  by  means  of  a  tube  which  is  split  up 
into  a  number  of  divisions  forming  the  vasa  efferentia. 


FIG.  298. 


Diagram  of  the  sexual  organs  of  the  male  embryo.  (Allen  Thomson.) 
3.  Ureter.  4.  Bladder.  5.  Urachus.  t.  Testicle,  m.  Atrophied  duct  of  Mailer  (hydatid 
ofMorgagni).  e.  Epididymis.  g.  Gubernaciilum  testis.  vs.  Vesicula  sewinalis.  i.  In- 
testine, pr.  Prostate,  IF.  Organ  of  Giralde*.  vh.  Vas  aberrans.  vd.  Vas  deferens. 
C.  Cowper's  gland,  cp.  Penis,  sp.  Spongy  part  of  the  Urethra,  t'.  Position  the  testicle 
ultimately  assumes.  *.  Scrotum. 

The  Wolffian  duct  forms,  beside  the  vas  deferens,  the  vesicula 
seminalis  (which  is  merely  a  blind  diverticulum  from  its  ex- 
tremity), and  terminates  in  the  ejaculatory  duct. 

The  two  Mullerian  ducts,  in  the  male,  join  and  form  a  single 
tube  ;  this  is  not  further  developed,  but  atrophies,  leaving  as  its 
representative  the  sinus  pocularis,  which  is  situated  in  the  floor 


THE   MULLERIAN   DUCT. 


707 


of  the  prostate.     The  upper  extremities  of  the  Mullerian  ducts 
form  the  hydatids  of  Morgagni. 

The  ovary,  like  the  testicle,  is  formed  from  the  germinal  epi- 
thelium, which  multiplies  and  forms  a  projection  close  to  the 
Wolffian  body.  The  cells  of  the  epithelium  become  involuted  and 
surrounded  by  the  uncleft  mesoblast,  to  form  ova  and  Graafian 
follicles.  The  glandular  part  of  the  ovary  thus  arises  from  the 
germinal  epithelium,  and  its  stroma  springs  from  the  mesoblast 
in  the  neighborhood  of  the  Wolffian  body. 

FIG.  299. 


Diagram  of  th3  sexual  organs  of  a  female  embryo.  (Allen  T  homson.) 
f.  Fimbriated  extremity  of  the  left  Fallopian  tube.  W.  Remains  of  the  Wolffian  tubes. 
ff.  Round  ligaments,  o.  Ovary,  po.  Parovarium.  it.  Uterus.  dG.  Remains  of  the 
Wolffian  duct,  or  duct  of  Gai'rtner.  m.  Right  Fallopian  tube  cut  short,  w.  Right 
obliterated  Wolffian  duct.  va.  Vagina.  3.  Ureter.  4.  Bladder.  5.  Urachus.  A.  In- 
ferior opening  of  vagina.  C.  Gland  of  Bartholin.  v.  Vulva,  sc.  Vascular  bulb.  cc. 
Clitoris.  ».  Nympha.  I.  Labium.  i.  Rectum. 

The  ducts  of  Mu'ller  are  the  precursors  of  the  female  genital 
passages.  They  approach  one  another  and  unite  along  a  certain 
distance  at  their  lower  extremities.  Of  this  united  part,  the 
upper  end  forms  the  uterus,  and  the  lower  the  vagina,  while  the 
ununited  parts  of  the  Miillerian  ducts  form  the  Fallopian  tubes, 
which  become  connected  with  the  ovaries,  while  their  cavities 
remain  continuous  with  the  pleuroperitoneal  space. 

In  the  female,  the  Wolffian  duct  and  body  atrophy,  the  paro- 


708  MANUAL   OF   PHYSIOLOGY. 

varium  being  in  the  adult  the  representative  of  the  Wolffian 
body. 

The  bladder  is  merely  a  dilated  portion  of  that  part  of  the 
allantois  which  is  in  immediate  connection  with  the  alimentary 
canal,  and  the  urachus  is  the  narrowed  part  of  the  allantois 
connecting  the  bladder  to  the  remainder  of  the  allantois  which 
is  without  the  body  walls  of  the  foetus. 

While  the  alimentary  canal  is  in  connection  with  the  allantois, 
the  intestinal  and  genito-urinary  passages  open  into  a  common 
cavity  at  their  termination  ;  this  is  the  cloaca,  and  it  is  in  the 
further  development  of  the  embryo  that  a  septum  arises,  dividing 
this  into  an  alimentary  or  anal  poition,  and  an  anterior  or  urinary 
portion.  The  septum,  dividing  the  urogenitary  from  the  alimen- 
tary portion  of  the  cloaca,  forms,  externally,  the  perinseum. 

At  the  aperture  of  the  cloaca  an  eminence  arises  which  de- 
velops into  the  penis  in  the  male,  the  clitoris  in  the  female. 
Around  this  eminence  is  a  fold  of  integuments,  which  forms  the 
labia  in  the  female,  the  scrotum  in  the  male. 

In  the  female  this  integumentary  covering  enlarges  much  more 
than  the  clitoris  and  covers  it  in,  the  urethral  orifice  opening 
just  below  the  clitoris. 

In  the  male,  the  urethral  orifice  at  first  opens  at  the  base  of  the 
penis,  but  eventually  a  groove  is  formed  on  the  under  surface  of 
this  organ,  which  becomes  converted  into  a  canal,  and  forms  the 
urethra. 

BLOOD-VASCULAR  SYSTEM. 

In  the  mammalian  embryo  this  may  be  appropriately  divided 
into  two  systems  of  different  dates  ;  the  first,  or  early  circulation, 
which  is  confined  to  the  yolk  sac ;  and  the  second,  or  later  cir- 
culation, which  passes  through  the  placenta. 

The  Primitive  Heart  arises  from  the  splanchnopleural  layer  of 
the  mesoblast,  just  at  the  point  where  this  forms  the  under  wall 
of  the  fore  part  of  the  alimentary  canal.  When  the  formation 
of  the  folds  of  the  embryo  was  described,  it  was  stated  that  the 
groove  of  the  cephalic  fold  tended  to  grow  backward  toward  the 
tail  end  of  the  embryo.  This  groove  is  limited  behind  by  the 
somatopleural  layer  of  the  mesoblast,  and  posteriorly  to  this  is 


FIG.  300. 


Transverse  section  through  the  region  of  the  heart  of  a  rabbit's  embryo  of  nine  days 

old.  (KOlliker.) 

jj-  Jugular  veins,  no.  Aorta,  ph.  Fore-gut,  bl.  Blastoderm,  hp.  Body  wall  reflected  in 
ect.  ent.  Hypoblast.  e'.  Prolongation  of  hypoblast  between  the  two  halves  of  the 
heart,  ah.  Outer  wall  of  the  haart.  p.  Cavity  of  the  pericardium,  ih.  Inner  lining  of 
the  heart,  ect.  Epiblust.  df.  Visceral  mesoblast. 


FIG.  301. 


Diagrammatic  views  of  the  under  surface  of  an  embryo  rabbit  of  nine  days  and  three 

hours  old,  showing  the  development  of  the  heart.     (Allen  Thomson.) 

A,  View  of  entire  embryo.    B,  an  enlarged  outline  of  the  heart  of  A.     C,  a  later  stage  of 

the  development  of  B.    hit.  Uuunited  heart,    aa.  Aortse.     VV.  Vitelline  veins. 


710 


MANUAL   OF   PHYSIOLOGY. 


a  cavity  formed  by  the  cleavage  of  the  mesoblast,  called  the 
pleuroperitoneal  cavity.  In  the  early  stages  of  development,  the 
posterior  wall  of  this  small  cavity  is  formed  by  the  splanchno- 
pleural  layer  of  the  mesoblast.  The  heart  arises  at  the  point  at 
which  the  splanchnopleure  tends  to  travel  forward  to  meet  the 
uncleft  mesoblast,  and  thus  completes  the  pleuroperitoneal 
cavity. 

The  heart  consists  at  first  of  a  single  cylinder,  which  in  the 


FIG.  302. 


FIG. 303. 


Human    embryo    of    about  three 
weeks.    (Allen  Thomson.) 

uv.  Yolk  sac. 

al.  Allantois. 

am.  A  inn  ion. 

ae.  Anterior  extremity. 

pe.  Posterior  extremity. 


Development  of  the  heart  in  the  human  embryo,  from  the  fourth  to  the  sixth  week. 

A.  Embryo  of  four  weeks.    (KMiker after  Cosle.) 

B.  Anterior,  and  (J.  posterior  views  of  the  heart  of  an  embryo  of  six  weeks.    (KOlliker 

after  Eeker.) 

a.  Upper  limit  of  buccal  cavity,  c.  Buccal  cavity.  I.  Lies  between  the  ventral  ends  of 
the  2d  and  3d  branchial  arches,  d.  Buds  of  upper  limbs,  e.  Liver.  /.Intestine.  1. 
Superior  vena  cava.  1'.  Left  superior  vena  cava  or  connection  between  the  left 
brachio-cephalic  vein  and  the  coronary  vein.  I".  Ojening  of  inferior  vena  cava. 
2.  2'.  Eight  and  left  auricles.  3.  3'.  Right  and  left  ventricles.  4.  Aortic  bulb. 

human  embryo  is  probably  formed  by  the  coalescence  of  two 
primary  tubes.  At  first  it  has  no  distinct  cavity,  but  soon  the 
cells  of  the  mesoblast  within  the  mass  forming  the  heart  become 
transformed  into  blood  corpuscles,  and  thus  it  is  hollowed  out. 
A  layer  of  endothelial  cells  line  the  cavity,  and  become  the 
endocardium. 

The  primitive  heart  is  connected  at  its  upper  end  with  the 


DEVELOPMENT   OF   THE   HEART. 


711 


two  aortse,  and  at  its  lower  end  with  the  oraphalo-mesenteric 
veins. 

After  a  time  the  tube  shows  signs  of  division  into  three  parts; 
the  upper  part  becomes  the  aortic  bulb,  next  to  which  is  formed 
the  cavity  of  the  ventricle,  continuous  with  which  is  the  auricu- 


1.0? 


Diagram  of  the  circulation  of  a  chick  at  the  end  of  the  third  day.    (Foster  and  Salfour.) 

//.Heart.  AA.  Aortic  arches  (2d,  3d  and  4lh).  A  o.  Dorsal  aorta.  L.of.A.,R  f>f.A.,  Right 
and  left  ornphalo-mesenteric  arteries.  S.T.  Sinus  terininalis.  R.of>,  and  Ljof.  Right  and 
left  oraphalo-mesenteric  veins.  S.  V.  Sinus  veuosus.  D.C.  Duct  of  Cuvier.  S.Ca.  and 
V.  Ca.  Superior  and  inferior  cardinal  veins. 

lar  space.  The  tube  also,  which  at  first  lies  in  a  straight  line, 
now  becomes  twisted  on  itself,  the  auricular  part  becoming 
posterior  and  superior,  while  the  ventricle,  with  the  aortic  bulb, 
remains  anterior  and  somewhat  below 


712  MANUAL   OF   PHYSIOLOGY. 

Each  primitive  cavity  of  the  heart  is  divided  into  two  by  the 
gradual  growth  of  partitions,  and  thus  the  four  permanent  heart 
cavities  are  developed. 

Externally  a  notch  shows  the  division  of  the  ventricle  into  right 
and  left  cavities,  while  from  the  inside  of  the  right  wall  there 
grows  a  projection  which  subdivides  the  ventricle  internally. 
This  septum  is,  however,  not  at  once  complete  at  its  upper  part, 
a  communication  between  the  right  and  left  sides  of  the  heart 
remaining  for  some  time  above  this  partition.  With  the  growth 
of  the  inter-ventricular  septum,  the  external  notch  becomes  less 
prominent,  but  is  permanently  recognizable  as  the  inter-ventri- 
cular groove. 

In  the  auricles  a  fold  develops  from  the  anterior  wall,  which 
ultimately  unites  with  a  process  of  later  development  from  the 
posterior  wall.  This  septum  is  not  complete  during  foetal  life,  but 
is  interrupted  by  an  opening  leading  from  one  auricle  to  the 
other,  called  the  foramen  ovale. 

Simultaneously  with  the  appearance  of  the  posterior  process 
of  the  septum,  another  fold  arises,  which  is  placed  at  the  mouth 
of  the  inferior  vena  cava,  and  forms  the  Eustachian  valve. 

The  aortic  bulb  likewise,  by  a  projection  from  the  inner  wall 
of  the  cavity,  becomes  divided  into  two  canals,  the  anterior  of 
which  remains  in  continuity  with  the  right  ventricle,  while  the 
posterior  is  continuous  with  the  left  ventricle.  The  anterior  thus 
becomes  the  pulmonary  artery,  and  the  posterior  the  permanent 
aorta. 

The  primitive  circulations  of  a  human  embryo  may  be  divided 
into  two,  which  differ  in  their  time  of  appearance  and  in  the 
accessory  organs  to  which  they  are  distributed.  Though  they 
may,  for  the  sake  of  clearness,  be  described  as  two  independent 
circulations,  they  are  not  strictly  so,  as  they  exist  for  a  short 
time  coincidently,  and  arise  in  connection  with  one  another  from 
the  same  heart, 

(a)  The  earlier  or  vitelline  circulation  is  that  which  is  directed 
to  the  yolk  sac,  the  embryo  obtaining  nourishment  from  the 
vitellus  or  yolk ;  this  is  an  organ  of  quite  secondary  importance 
in  the  mammalian  embryo,  and  hence  this  circulation  may  be 


VITELLINE   CIRCULATION. 


713 


better  studied  in  some  such  animal  as  the  chick,  which  depends, 
throughout  its  embryonic  life,  on  the  vitellus  for  nourishment. 
In  the  human  embryo  the  vitelline  circulation  is  chiefly  of  im- 
portance for  the  few  days  immediately  preceding  the  develop- 
ment of  the  placental  circulation. 

The  aortic  bulb  is  continuous  with  two  vessels  which  run  on 


FIG.  305. 


Diagram  of  the  vascular  system  of  a  human  foetus.    (Huxley.) 

II.  Heart.  T.  A.  Aortic  trunk,  c.  Common  carotid  artery,  c'.  External  carotid  artery. 
c''  Internal  carotid  arteiy.  s.  Subclavian  artery,  v.  Vertebral  artery.  1.2.3.  4.5,  Aortic 
arches.  A'.  Dorsal  aorta.  1.  Ornphalo-mesenteric  artery,  dv.  Vitelline  duct.  o'.  Om- 
phalo-mesenteric  vein.  v'.  Umbilical  vesicle,  vp.  Portal  vein.  L.  Liver.  MM.  Umbilical 
arteries.  u"u" .  Their  endings  in  the  pjacenta.  u'.  Umbilical  vein.  Dv.  Ductus  venosus. 
vh.  Hepatic  vein.  cv.  Inferior  vena  cava.  vil.  Iliac  veins,  az.  Venaazygos.  vc.  Posterior 
cardinal  vein.  DC.  Duct  of  Cuvier.  P.  Lungs. 

either  side  of  the  primitive  pharynx ;  these  are  the  aortse,  and 
from  each  of  them  a  large  branch  is  given  off.  These  omphalo- 
mesenteric  arteries  pass  to  the  yolk  sac,  and  there  become  split 
up  into  a  number  of  small  vessels,  the  blood  from  them  being 
returned  partly  by  corresponding  omphalo-mesenteric  veins, 
partly  by  a  large  vein  running  round  the  periphery  of  the  vas- 
60 


714  MANUAL   OF   PHYSIOLOGY. 

cular  area  known  as  the  sinus  terminalis.  The  sinus  terminalis 
opens  partly  into  the  right  and  partly  into  the  left  omphalo- 
mesenteric  veins,  which  subsequently  unite  into  a  common  venous 
trunk,  called  the  sinus  venosus,  which  is  continuous  with  the 
primitive  auricle. 

This  vitelline  circulation  in  the  human  embryo  persists  but  a 
short  time.  After  the  fifth  or  sixth  week  of  foetal  life  it  becomes 
obliterated,  the  yolk  then  being  atrophied,  and  the  placental  cir- 
culation well  developed. 

(&)  The  later  or  placenial  circulation  is  developed  in  the  meso- 
blastic  layer  of  the  allantois,  especially  in  that  part  which  is  in 
relation  with  the  decidua  serotina.  The  allantois,  when  fully 
developed,  extends  to  the  chorion,  over  which  it  spreads,  sending 
in  processes  to  occupy  the  villi.  These  chorionic  villi  are  em- 
bedded in  the  decidua  of  the  uterus,  and  are  especially  developed 
at  the  upper  part,  which  is  in  connection  with  the  decidua  sero- 
tina or  maternal  placenta. 

The  primitive  aortse,  which  were  at  first  two  separate  tubes, 
become  united  in  the  dorsal  region  of  the  embryo,  so  that  the 
two  aortic  arches  end  in  a  single  vessel,  which  extends  to  the 
middle  of  the  embryo,  and  there  divides  into  two  branches, 
each  of  which  gives  off  a  vessel  called  the  vitelline  or  omphalo- 
mesenteric  artery. 

From  the  branches  of  the  aortse  arise  two  large  vessels,  which, 
running  along  the  allantois,  spread  out  over  the  chorion,  being 
especially  directed  to  the  upper  part  of  this  membrane;  these  are 
the  umbilical  or  hypogastric  arteries,  which  carry  the  blood  from 
the  aortse  to  the  fcetal  placenta. 

Veins  arise  from  the  terminal  networks  of  these  arteries,  and 
combine  to  form  the  two  umbilical  veins.  The  umbilical  veins 
take  a  similar  course  to  the  arteries,  and  convey  the  blood  to  the 
venous  trunk  formed  by  the  junction  of  the  omphalo-mesenteric 
veins. 

After  a  time  the  right  umbilical  and  right  omphalo-mesenteric 
veins  disappear,  while  from  the  trunk  formed  by  the  junction  of 
the  left  umbilical  and  left  omphalo-mesenteric  veins,  branches 
are  given  off  to  the  liver  (vence  advehentes),  and  at  a  point 


THE   ARTERIAL   SYSTEM. 


715 


nearer  the  heart,  vessels  are  received  from  the  liver  (vena 
revehentes}. 

To  the  part  of  the  vessel  intervening  between  the  origin  of  the 
vena?  advehentes  and  the  entrance  of  the  vense  revehentes  is 
given  the  name  of  the  ductus  venosus. 

Thus  it  may  be  seen  that  in  the  placental  circulation  the 
blood  is  conveyed  from  the  aorta,  by  the  umbilical  arteries,  to 
the  foetal  placenta,  undergoes  changes,  owing  to  its  close  relation- 


fft 


Diagram  of  the  heart  and  principal  arteries  of  the  chick,    (Allen  Thomson.')    B.  and  C. 

are  later  than  A. 

1,1.  Omphalo-mesenteric  veins.    2.  Auricle.    3.  Ventricle.    4.  Aortic  bulb.   5,  5.  Primi- 
tive aortae.    6,  6.  Omphalo-nieseuteric  arteries.    A.  United  A.orta. 


ship  to  the  maternal  blood.  From  the  placenta  it  is  returned  by 
the  umbilical  vein,  which  sends  a  part  through  the  liver  and  a 
part  direct  to  the  heart.  The  more  minute  details  of  foetal  cir- 
culation will  be  described  later  on. 

The  Arterial  System. — Around  the  pharynx  are  developed  five 
pairs  of  aortic  arches.  These  commence  anteriorly  from-  the  two 
primitive  aortse,  and,  passing  along  the  side  of  the  pharynx,  end 
in  the  aortse  as  they  descend  to  become  united  in  the  dorsal 


716 


MANUAL   OF   PHYSIOLOGY. 


FIG.  307. 


region  of  the  embryo.  The  points  of  origin  of  the  arches  are 
termed  their  anterior  roots,  and  the  points  of  termination  their 
posterior  roots. 

Though  all  these  arches  do 
not  exist  at  the  same  time, 
still,  in  describing  the  vessels 
which  arise  from  them,  they 
may  be  conveniently  con- 
sidered together. 

On  the  right  side  the  fifth 
arch  disappears  completely. 
On  the  left  side  the  anterior 
root  and  neighboring  part  of 
the  fifth  arch  are  transformed 
into  the  pulmonary  artery ; 
the  remaining  part  of  this 
arch  continues  as  the  ductus 
arteriosus,  which  connects  the 
pulmonary  artery  with  the 
permanent  aorta. 

The  fourth  left  arch,  in 
mammalia,  becomes  the  per- 
manent aorta.  At  the  junc- 
tion of  the  fourth  and  fifth  left 
posterior  roots  the  left  sub- 
clavian  artery  is  given  off.  In 
birds  the  right  fourth  arch  is 
transformed  into  the  perma- 


Diagram of  the  aortic  arches;  the  permanent 
vessels  arising  from  them  are  shaded 
darkly.  (Allen  Thomson,  after  Rathke.) 

1,  2,  3,  4, 5,  Primitive  aortic  arches  of  right 
side. 

I,  ii.  in.  IV.  Pharyngeal  clefts  of  the  left 
side,  showing  the  lelationship  of  the  clefts 
to  the  aortic  arches. 

A.  Aorta.  P.  Pulmonary  artery,  d.  Ductus 
arteriosus,  a'.  Left  aortic  root.  a.  Right 
aortic  root.  A'.  Descending  aorta,  pn.pri. 
Right  and  left  vagi.  s.  s'.  Rieht  and  left 


g 
ub 


subclavinn  arteries,  v.  v'.  Right  and  left 
vertebral  arteries,  c.  Common  carotid  ar- 
teries, ce.  External  carotid,  ci.  ci'.  Right 
and  left  internal  carotid. 


nent  aorta  ;  and  in  examining 
the  development  of  the  aortic 
arch  of  the  chick,  it  must  be  borne  in  mind  that  it  is  on  the 
opposite  side  to  that  it  occupies  in  man. 

On  the  right  side  the  anterior  root  of  the  fourth  arch,  and  the 
part  of  the  aortic  trunk  leading  to  it,  persist  as  the  innominate 
artery,  the  fourth  arch  being  represented  by  the  right  subclavian 
artery. 

The  part  of  the  primitive  aortic  trunk  joining  the  fourth  and 


AORTIC   ARCHES. 


717 


third  right  anterior  roots 
becomes  the  common  caro- 
tid artery  of  the  same  side, 
while  arising  from  this  is 
the  internal  carotid, which, 
taking  the  position  of  the 
third  arch,  passes  to  the 
posterior  roots,  and  occu- 
pies the  trunk  of  the  primi- 
tive aorta  from  the  third 
to  the  first  arches. 

The  external  carotid, 
arising  from  the  common 
carotid  at  the  third  an- 
terior-root, occupies  the 
position  of  the  vessel  join- 
ing this  root  to  those  of 
the  second  and  first  arch. 

On  the  left  side  the 
common  carotid  and  its 
branches  are  developed 
similarly  to  those  on  the 
right,  the  only  difference 
being  that  the  common 
carotid  arises  from  the 
aorta  and  not  from  the 
innominate. 

The  iliac  arteries  are 
developed  from  the  hypo-  A 
gastric.  At  first  they  ap- 
pear as  branches,  but  with 
the  growth  of  the  limbs 
they  become  so  much 
larger  that  after  birth 
they  appear  to  be  the 
main  branches  from  the 
point  of  division  of  the 


Fic».  308. 


.  Plan  of  principal  veins  of  the  foetus  of  about  four 
weeks  old.  B.  Veins  of  the  liver  at  an  earlier 
period.  C.  Veins  after  the  establishment  of  the 
placenta]  circulation.  D.  Veins  of  the  liver  at  the 
same  period. 

Primitive  jugular  veins,  dc.  Ducts  of  Cuvier. 
ca.  Caidinal  veins,  ci.  Interior  vena  cava.  I.  Duc- 
tus  venosus.  «.  Umbilical  vein.  p.  Portal  vein. 
o.  Vitellinevem.  cr.  External  iliac  veins,  o'.  R'ight 
vitclline  vein.  u'.  Right  umbilical  vein.  I'.  Hepa- 
tic veins  (venae  revehentes).  p'p'.  Venae  ad vehentes. 
m.  Me&enteric  veins,  az.  Azygos  vein.  ca'.  Remains 
of  left  cardinal  vein.  li.  Cross  branch  from  left 
jugular,  which  becomes  the  left  brachio-cephalic 
vein.  ri.  Right  innominate  vein.  s.s.  Subclavian 
veins,  h.  Hypogastric  veins,  il.  Division  of  in- 
ferior vena  cava  into  the  common  iliac  veins. 


718  MANUAL   OF    PHYSIOLOGY. 

aorta,  the  hypogastric  arteries  now  being  merely  small  branches 
of  the  iliac  vessels. 

With  the  development  of  the  organs  and  limbs,  vessels  in  con- 
nection with  those  above  described  arise  in  the  mesoblast.  It  is, 
however,  beyond  the  scope  of  this  work  to  describe  in  detail  the 
origin  of  the  lesser  vessels. 

Venous  System. — The  blood  is  returned  from  the  head  by  the 
two  primitive  jugulars,  which  unite  with  the  cardinal  veins  con- 
veying the  blood  from  the  trunk  and  lower  extremities  to  form  a 
vessel  on  each  side,  called  the  duct  of  Cuvier. 

From  the  lower  extremity  of  the  embryo  the  inferior  vena 
cava  commences  by  the  union  of  the  external  iliac  veins ;  this 
passes  up  and  opens  into  the  venous  trunk  common  to  the  left 
vitelline  and  left  umbilical  veins. 

The  left  vitelline  becomes  continuous  with  the  vessels  from  the 
common  trunk  going  to  the  right  side  of  the  liver  (the  right  vena 
advehens),  and  forms  the  main  trunk  of  the  portal  vein  (Fig. 
308,  B.  &  D.). 

At  this  stage  of  the  formation  of  the  veins  there  are  three 
trunks  opening  into  the  auricle,  the  right  and  left  ducts  of  Cuvier 
and  the  inferior  vena  cava. 

As  development  proceeds,  the  lower  parts  of  the  cardinal  veins 
join  the  external  iliac  veins,  forming  the  common  iliacs,  and  so 
return  their  blood  into  the  inferior  vena  cava. 

The  upper  parts  of  the  cardinal  veins  become  continuous  with 
the  posterior  vertebral  veins  which  convey  the  blood  from  the 
parietes  of  the  embryo.  Between  the  latter  a  communicating 
branch  is  established,  which  helps  in  the  formation  of  the  azygos 
vein. 

The  ducts  of  Cuvier,  which  at  first  were  placed  almost  at  right 
angles  to  the  auricle,  become  more  oblique  in  their  direction  as 
the  heart  descends. 

Between  the  primitive  jugular  veins  a  cross  branch  is  developed, 
which  conveys  the  blood  from  the  left  side  of  the  head  and  upper 
extremity  of  the  duct  of  Cuvier  of  the  opposite  side. 

The  left  duct  of  Cuvier,  below  the  communicating  branch, 
atrophies  and  forms  part  of  the  coronary  veins  of  the  heart. 


VENOUS  SYSTEM. 


719 


The  connection  between  this  and  the  vein  above  the  cross  branch 
being,  in  the  adult,  represented  by  a  small  vein,  or  a  band  of 
fibrous  tissue,  called  the  vestigial  fold  of  the  pericardium. 

The  cross  branch  from  the  left  to  the  right  jugular  becomes 


FIG.  309. 


Diagram  illustrating  the  circulation  through  the  heart  and  the  principal  vessels  of  a 

foetus.    (Cleland.) 

a.    Umbilical  vein.     6.   Ductus  venosus.    /.    Portal  vein.     e.   Vessels  to  the  viscera. 
d.  Hypogastric  arteries,    c.  Ductus  arteriosus. 

the  left  innominate  vein.  The  right  duct  of  Cuvier  and  the 
right  jugular,  below  the  entrance  of  this  cross  branch,  form  the 
superior  vena  cava ;  while  the  part  of  the  right  primitive  jugular 


720  MANUAL   OF   PHYSIOLOGY. 

immediately  above  the  entry  of  the  left  innominate  vein  forms 
the  right  innominate  vein. 

The  posterior  vertebral  vein  of  the  right  side  forms  the  vena 
azygos  major  ;  the  corresponding  branch  of  the  opposite  side, 
together  with  the  part  of  the  left  primitive  jugular  below  the 
cross  branch,  forms  the  left  superior  intercostal  vein  and  the 
superior  vena  azygos  minor.  The  lower  part  of  the  left  posterior 
vertebral  vein,  together  with  the  connecting  branch  to  the  right 
vein,  remain  as  the  inferior  vena  azygos  minor. 

Foetal  Circulation. — The  course  taken  by  the  blood  through 
the  heart  and  vessels  of  the  embryo  differs  essentially  from  that 
which  persists  in  adult  life. 

Tracing  the  blood  from  the  placenta,  it  passes  along  the  um- 
bilical vein  toward  the  liver ;  here  it  may  take  either  of  two 
courses  to  reach  the  vena  cava,  one  which  follows  the  ductus 
venosus  and  avoids  the  liver,  the  other  which  passes  by  the  vena3 
advehentes  (portal  veins)  to  the  liver,  and  proceeds  by  the  venae 
revehentes  (hepatic  veins)  to  the  inferior  vena  cava,  which 
receives  all  the  blood  passing  by  both  of  these  channels.  From 
this  the  blood  is  emptied  into  the  right  auricle,  and  hence  is 
guided  by  the  Eustachian  valve  through  the  septum  by  the 
patent  foramen  ovale  to  the  left  auricle.  From  the  left  auricle 
it  passes  to  the  left  ventricle,  which  contracts  and  sends  the  blood 
into  the  aortic  arch,  where  it  is  split  up  into  two  streams,  one  of 
which  passes  into  the  vessels  of  the  head  and  neck,  the  other  by 
the  descending  aorta  to  the  trunk  and  lower  extremities. 

The  blood  from  the  head  and  neck  is  returned  to  the  right 
auricle  by  the  superior  vena  cava.  The  blood  from  this  vein 
passes  through  the  auricle  to  the  right  ventricle,  which  sends  it 
through  the  pulmonary  artery  toward  the  lungs. 

The  pulmonary  artery,  in  the  embryo,  has  one  very  large 
branch,  called  the  ductus  arteriosus,  which  joins  the  aorta  at  a 
point  just  below  the  origin  of  the  vessels  of  the  head  and  neck  ; 
hence  the  main  part  of  the  blood  passing  from  the  right  ventricle 
reaches  the  aorta  by  the  ductus  arteriosus,  and  only  a  very  small 
part  goes  to  the  lungs,  to  be  returned  from  them  by  the  pulmo- 
nary veins  to  the  left  auricle. 


DEVELOPMENT    OF    THE    EYE.  721 

The  blood  from  the  ductus  arteriosus  blends,  therefore,  with 
that  in  the  aorta  which  is  passing  to  the  viscera  and  lower 
extremities.  The  main  part  of  this  blood  travels  by  two  large 
branches  of  the  aorta  (the  hypogastric  arteries)  to  the  placenta, 
where  it  is  aerated  and  purified,  etc. 

It  is  evident,  if  the  placenta  is  the  great  renovating  organ  of 
the  blood  of  the  foetus,  that  the  blood  in  the  umbilical  vein  is 
the  most  arterial  in  the  foetal  circulation.  The  blood  in  the 
ascending  vena  cava  and  first  part  of  the  aorta  is  likewise  fairly 
arterial,  but  the  blood  in  the  descending  aorta  is  of  a  mixed 
character,  as  it  contains  blood  which  has  nourished  the  head  and 
neck,  besides  that  which  has  come  from  the  placenta  by  the 
inferior  vena  cava  through  the  right  auricle,  foramen  ovale,  left 
auricle,  and  left  ventricle. 

As  the  foetal  lungs  are  not  called  into  play  until  after  birth, 
but  little  blood  passes  to  them  in  the  foetus  ;  this  state  of  things 
is,  however,  completely  altered  at  birth,  when  the  lungs  of  the 
child  expand,  the  pulmonary  arteries  increase  in  size,  and  the 
ductus  arteriosus  dwindles  in  a  corresponding  degree. 

The  liver,  which  in  the  foetus  is  of  relatively  greater  size  than 
in  the  adult,  receives  much  blood  coming  from  the  placenta  to 
the  heart,  and  is  thought  to  contribute  to  it  several  essential 
constituents. 

The  head  and  brain,  which  are  largely  developed  in  the  foetus, 
receive  well  aerated  blood  ;  namely,  the  placental  blood  which 
has  passed  through  the  liver,  and,  in  the  inferior  vena  cava,  is 
mixed  with  blood  coming  from  the  lower  limbs.  The  rest  of 
the  foetus  receives  blood  that  is  less  aerated,  as  it  is  mixed  with 
that  which  is  returned  from  the  head  and  neck  to  the  right  side 
of  the  heart,  and  which  is  sent  through  the  ductus  arteriosus  to 
join  the  general  blood  current  in  the  aorta  going  to  the  viscera 
and  lower  extremities. 

DEVELOPMENT  OF  THE  EYE. 

The  optic  vesicles  arise  from  the  anterior  cerebral  vesicle  at  a 
very  early  period,  and  their  cavities  are  continuous  with  that  of 
the  fore-brain.     With  the  development  of  the  rudimentary  cere- 
61 


722 


MANUAL   OF   PHYSIOLOGY. 


FIG.  310. 


bral  hemispheres  the  optic  vesicles  become  displaced  downward, 
and  their  cavities  open  into  the  junction  of  the  cavities  of  the 
cerebral  hemispheres,  and  that  of  the  thalamencephalon,  which 
becomes  the  third  ventricle.  Later,  the  optic  vesicles  open 
directly  into  the  third  ventricle,  and  finally  are  displaced  back- 
ward, and  come  into  connection  with  the  mid-brain. 

The  optic  vesicles  are  at  first  hollow  prolongations,  which  con- 
sist of  an  anterior  dilated  portion,  forming  the  primary  optic 
vesicle,  and  a  posterior  tubular  portion  or  stalk  joining  the 
vesicle  to  the  fore-brain.  This  stalk  forms  the  optic  nerve. 

As  each  vesicle  grows  forward  toward  the  epiblast  covering  the 
head  of  the  embryo,  the  epiblastic  cells  at  the  spot  overlying  the 
vesicle  become  thickened,  an  involution  of  the  epiblast  takes  place 
toward  the  optic  vesicle,  and  indents  the 
latter,  approximating  its  anterior  to  its 
posterior  wall. 

By  this  means  the  anterior  and  pos- 
terior walls  of  the  primary  optic  vesicle 
come  into  close  contact,  and  the  cavity  of 
the  vesicle  is  obliterated.  The  two  layers 
of  the  vesicle  are  now  cup-shaped,  and 
receive  the  name  of  the  secondary  optic 
vesicle  or  the  optic  cup.  This  ultimately 
becomes  the  retina,  and  the  optic  stalk, 
losing  its  cavity,  is  transformed  into  the 
optic  nerve. 

Meanwhile,  the  local  involution  of  the 
epiblast  over  the  optic  cup,  which  is  the 
rudiment  of  the  crystalline  lens,  becomes 
gradually  separated  from  the  general 
epiblast,  and  is  finally  detached  from  its 
point  of  origin.  It  now  lies  as  a  some- 
what spherical  body  in  the  cavity  of  the 
optic  cup  within  the  superficial  mesoblast, 
which  has  closed  over  it. 

The  secondary  optic  vesicle  grows  (except  at  its  lower  part, 
just  at  the  junction  of  the  optic  stalk)  so  as  to  deepen  the  optic 


Section  through  the  head  of 
a  chick  at  ihe  third  day, 
showing  the  origin  of  the 
lens. 

a.  Epiblast  thickened  at  c, 
which  is  the  point  of  origin 
of  the  lens.  o.  Optic  vesicle. 
VI.  Anterior  cerebral  vesi- 
cle, V2.  Posterior  cerebral 
vesicle. 


DEVELOPMENT   OF   THE   EYE. 


723 


cup,  which  contains  the  rudimentary  lens.  At  the  lower  part 
an  interval  is  left,  which  receives  the  name  of  the  choroid  fissure. 
Through  this  gap  in  the  secondary  optic  vesicle  the  raesoblast 


Diagrammatic  sections  of  the  primitive  eye,  showing  the  choroidal  fissure.     (Foster  and 

Balfour.) 
D.  Horizontal  section.    E.  Vertical  transverse  section  just  striking  the  posterior  part  of 

the  lens.    F.  Vertical  longitudinal  section  through  the  optic  stalk,  and  the  fissure 

through  which  the  mesoblast  passes  to  form  the  vitreous  humor. 
h.  Superficial  epihlast.  x.  Point  of  origin  of  the  lens.    v.  h.  Vitreous  humor,    r.  Anterior 

layer  of  the  optic  vesicle,    u.  Posterior  layer  of  the  optic  vesicle,    c.  Cavity  of  the  optic 

vesicle,   f.  Choroidal  fissure,    s.  Optic  stalk,    s'.  Cavity  of  the  optic  stalk.    /.  Lens.  I'. 

Cavity  of  the  lens. 


enters  and  separates  the  lens  from  the  optic  cup,  forming  the 
vitreous  humor. 

The  mesoblast  surrounding  the  optic  cup  develops  two  cover- 


FIG.  312. 


Later  stages  in  the  development  of  the  lens.    (Cadiat.) 
a.  Epiblast.  c.  Rudimentary  lens.  o.  Optic  vesicle. 

ings  of  the  eye,  an  outer  fibrous  capsule  called  the  sclerotic  coat, 
and  a  vascular  coat,  the  choroid. 

In  front  of  the  lens,  beneath  the  epiblast,  the  mesoblast  forms 


724 


MANUAL   OF   PHYSIOLOGY. 


the  corneal  tissue  proper.     The  epiblast  forms  the  epithelial  or 
conjunctival  covering  of  the  eyeball. 

The  involution  of  mesoblast  through  the  choroidal  fissure, 
which  forms  the  vitreous  humor,  indents  the  optic  stalk,  and 
forms  the  central  artery  of  the  retina.  The  choroidal  fissure  is 

Fro.  313. 


A  further  stage  of  the  development  of  the  lens.    (Cadiat.) 

a.  Elongating  epithelial  cells  forming  lens;  b.  Capsule;  c.  Cutaneous  tissue  becoming 
conjunctiva;  d,  e.  Two  layers  of  optic  cup  forming  retina;/.  Cell  of  mucous  tissue  of 
the  vitreous  humor  ;  g.  Intercellular  substance  ;  h.  Developing  optic  nerve;  i.  Nerve 
fibres  passing  to  retina. 


gradually  obliterated,  and  its  position  may  sometimes  be  marked 
by  a  permanent  fissure  in  the  iris  (coloboma  iridis).  The  rudi- 
mentary lens  is  a  spherical  body,  hollow  in  the  centre,  made  up 
of  an  anterior  and  posterior  wall,  each  of  which  is  formed  of 
columnar  cells.  The  posterior  wall  of  the  lens  increases  greatly 


DEVELOPMENT    OF    THE    EYE.  725 

in  thickness,  and  approaching  the  anterior  obliterates  the  original 
cavity  of  the  lens. 

The  cells  forming  this  wall  become  very  much  elongated,  and 
develop  into  long  fibre-like  columnar  cells.  Those  of  the  ante- 
rior walls  from  being  a  columnar,  are  modified  to  a  flattened 
epithelium,  and  finally  become  the  layer  lining  the  anterior  sur- 
face of  the  capsule  of  the  lens.  The  capsule  of  the  lens  has 
been  variously  considered  as  arising  from  the  cells  of  the  lens 
substance,  or  as  originating  from  a  thin  layer  of  mesoblast,  which 
forms  not  only  the  lens  capsule,  but  also  the  hyaloid  membrane, 
which  is  continuous  with  it. 

The  optic  cup  gives  origin  to  the  retina.  The  inner  or  anterior 
layer  of  the  cup  becomes  thickened,  and  from  it  are  differentiated 
the  various  layers  of  the  retina,  except  that  layer  of  pigment 
cells  which  lies  next  to  the  choroid.  The  posterior  layer  develops 
this  layer  of  pigment  cells,  which,  from  their  intimate  connec- 
tion to  the  choroid,  were  formerly  considered  as  part  of  that 
membrane. 

The  thickening  of  the  inner  or  anterior  layer  of  the  optic  cup 
ceases  at  the  ora  serrata.  The  outer  layer  with  its  contiguous 
choroid  is  thrown  into  a  number  of  folds — the  ciliary  processes — 
and  passing  in  front  of  the  lens,  helps  to  form  the  iris. 

In  front  of  the  ora  serrata  the  anterior  layer  of  the  cup  is  no 
longer  differentiated  into  the  special  retinal  elements,  but  joins 
with  the  posterior  to  form  a  layer  of  columnar  cells, — the  pars 
ciliaris  retince.  In  front  of  this  the  anterior  rim  of  the  optic  cup 
passes  forward  and  lines  the  posterior  surface  of  the  iris,  forming 
the  uvea  of  that  organ,  and  terminating  at  the  margin  of  the 
pupil. 

The  rest  of  the  substance  of  the  iris  is  developed  from  the 
mesoblast,  from  which  also  arise  the  choroid,  the  cornea  and  the 
sclerotic. 

The  development  of  the  eye  may  be  thus  briefly  described. 
An  offshoot  of  nervous  matter  from  the  fore-brain  forms  the 
retina  and  the  uvea,  and  its  stalk,  or  connection  with  the  brain, 
develops  into  the  optic  nerve. 

An  involution  of  epiblast  which  grows  into  the  nervous  cup 


726 


MANUAL   OF   PHYSIOLOGY. 


forms  the  lens,  while  from  the  adjacent  mesoblast  arise  the  sur- 
rounding parts  of  the  eye.  The  vitreous  is  produced  by  the 
mesoblast  growing  through  a  fissure  in  the  lower  part  of  the 
optic  cup  to  fill  its  cavity. 

DEVELOPMENT  OF  THE  EAR. 

The  ear  is  developed  chiefly  from  the  epiblast,  a  special  and 
independent  involution  of  which  forms  both  its  essential  nervous 
structures  and  the  general  epithelium  lining  the  membranous 

FIG.  314. 


Transverse  section  through  the  head  of  a  fcetal  sheep  in  the  region  of  the  hind-brain. 

(Boettcher.) 
H.B.    Hind-brain,    cc.    Canal  of  the  cochlea.    RV.   Recessus  vestibuli.    VB.    Vertical 

semicircular  canal.    G.C.  Auditory  ganglion.     G'.  Auditory  nerve.    N.  Connection  of 

auditory  nerve  to  the  hind-brain. 

labyrinth.  The  mesoblast  supplies  the  surrounding  firmer  struc- 
tures, such  as  the  fibrous  substance  of  the  inner  ear,  and  the  bony 
parts  in  which  the  organ  lies.  The  auditory  nerve  grows  as  a 
bud  from  the  neural  tissue  forming  the  hind-brain,  and  connects 
it  with  the  delicate  specialized  auditory  cells. 


DEVELOPMENT   OF   THE    EAR.  727 

The  process  begins  by  the  appearance  of  a  depression  of  the 
general  epiblast  covering  the  head,  which  forms  a  tubular  diver- 
ticulum,  lying  in  the  mesoblast  at  the  side  of  the  hind-brain. 

This  diverticulum  becomes  separated  from  the  epiblast  by  the 
obliteration  of  its  outer  extremity,  which  united  it  to  the  super- 
ficial epiblast,  and  is  converted  into  a  cavity  receiving  the  name 
of  the  otic  vesicle.  It  soon  becomes  somewhat  triangular  in  shape, 
the  base  of  the  triangle  lying  upward. 

From  the  lower  angle  arises  a  projection,  which  is  the  rudi- 
mentary canal  of  the  cochlea.  The  angle  lying  next  to  the 
neural  epiblast  similarly  gives  off  a  tubular  process,  which  forms 
the  recessus  vestibuli. 

Elevations  in  the  primitive  vesicle  indicate  the  origin  of  the 
semicircular  canals,  which  become  tubular,  opening  at  their  ends 
into  the  general  cavity  of.  the  vesicle.  The  two  superior  canals 
are  the  first  to  appear,  the  horizontal  arising  somewhat  later. 

The  part  of  the  otic  vesicle  in  connection  with  the  canal  of 
the  cochlea  becomes  separated  from  the  latter  by  a  narrow  con- 
striction which  forms  the  canalis  reuniens,  the  part  of  the  vesicle 
beyond  this  developing  into  the  saccule. 

The  utricle  arises  from  that  part  of  the  vesicle  which  is  in 
connection  with  the  semicircular  canals.  It  is  at  first  in  direct 
connection  with  the  saccule,  but  after  a  time  it  only  communi- 
cates by  means  of  a  narrow  canal  with  a  similar  one  from  the 
saccule ;  these  two  canals  are  connected  with  a  third,  which  lies 
in  the  aqueductus  vestibuli. 

The  canal  of  the  cochlea  is  at  first  a  straight  tube,  but  as  it 
develops  it  becomes  coiled  upon  itself. 

The  walls  of  the  primitive  otic  vesicle,  formed  from  the  epi- 
blast, become  developed  into  the  epithelium  lining  the  internal  ear. 
The  mesoblast  immediately  surrounding  the  vesicle  forms  a  sup- 
porting capsule  of  fibrous  tissue,  which  completes  the  membran- 
ous parts  of  the  internal  ear. 

Part  of  the  mesoblast  around  the  optic  vesicle  becomes  lique- 
fied, and  gives  origin  to  the  canals  and  spaces  in  which  the 
membranous  labyrinth  lies ;  the  neighboring  mesoblast  is  changed 
into  cartilage  which  ossifies  and  forms  the  bony  parts  of  the  ear. 


728 


MANUAL   OF   PHYSIOLOGY. 


The  auditory  nerve  is  developed  from  the  hind-brain,  and  grows 
through  the  mesoblast  toward  the  otic  vesicle.  It  is  recognizable 
from  its  having  some  ganglion  cells  in  its  growing  extremity  from 
a  very  early  period  of  its  development. 

The  Eustachian  tube  and  the  tympanum  are  formed  in  con- 
nection with  the  inner  part  of  the  first  visceral  cleft,  and  the 


FIG. 315. 


CJC 


Section  through  the  head  of  a  foetal  sheep.    (Boetfcher.) 

R.V.  Recessus  vestibuli.     V.B.  Vertical  semicircular  canal.      H.B.  Horizontal  semicircu- 
lar canal.    G.  Auditory  ganglion,    c.c.  Canal  of  the  cochlea. 

ossicles  are  developed  from  the  corresponding  visceral  arch — hyo- 
mandibular. 

The  membrana  tympani  is  formed  at  the  surface  of  the  embryo, 
the  adjacent  parts  grow  outward  and  give  rise  to  the  external 
auditory  meatus. 


DEVELOPMENT    OF   THE   SKULL   AND    FACE. 


729 


DEVELOPMENT  OF  THE  SKULL  AND  FACE. 

The  bones  of  the  roof  of  the  skull  and  of  the  face  are  chiefly 
derived  from  membrane,  those  of  the  base  of  the  skull  being  laid 
down  in  cartilage. 

At  the  cephalic  extremity  of  the  notochord  is  a  mass  of  uncleft 
mesoblast,  called  the  investing  mass,  corresponding  to  that  from 
which  the  vertebrae  are  developed. 

From  this  arises  two  prolongations,  which  diverge  and  then 
unite  again,  leaving  an  interval ;  and  the  united  portion  becomes 
once  more  divided  into  two  processes,  the  trabeculce  cranii. 


FIG.  316. 


FIG.  317. 


Basis  cranii  of  a  chick,  sixth  day. 
(Hvxley.) 

1.  Chorda  dorsalis. 

2.  Basal  cartilage. 

3.  Trabecula?. 

4.  Pituitary  space. 

5.  Internal  ear. 


Longitudinal  section  through  the  hend 
of  an  embryo  of  four  weeks.  (KOl- 
liker.) 

v.  CaTity  of  cerebral  hemisphere. 

a. no.  Optic  vesicle. 

z.  Cavity  of  third  ventricle. 

m.  Cavity  of  mid-brain. 

h.  Cerebellum. 

n.  Medulla. 

o.  Auditory  depression. 

t.  Basis  cranii. 

I'.  Tentorium, 

p.  Pituitary  body. 

The  mesoblast  behind  the  interval  receives  the  name  of  the 
occipito-sphenoid  portion ;  the  interval  is  the  rudiment  of  the  sella 
tunica,  which  is  occupied  by  the  pituitary  body.  The  part  of  the 
mesoblast  in  front  of  this  is  called  the  spheno-ethmoidal  portion. 

From  the  occipito-sphenoidal  portion  are  developed  the  basi- 
occipital  and  the  posterior  part  of  the  sphenoid.  At  the  sides  of 
the  medulla  oblongata  processes  are  sent  up,  which  unite  round 
it  [and  form  the  foramen  magnum.  Laterally  the  mesoblast 
envelops  the  auditory  vesicles  and  forms  the  side  portions  of  the 
occipital  bone. 


730 


MANUAL   OF   PHYSIOLOGY. 


In  the  cartilaginous  antecedent  of  the  temporal  bone  there  are 
three  centres  of  ossification — the  epiotic,  which  develops  the  mas- 
toid  process ;  the  prootic,  which  is  in  the  region  of  the  superior 
semicircular  canal ;  and  the  opisthotie,  which  is  at  the  cochlea. 

The  spheno-ethmoidal  portion  develops  the  anterior  part  of 
the  sphenoid  together  with  the  ethmoid  bones  and  the  cartilage 
of  the  septum  of  the  nose,  the  first,  arising  from  the  back  part, 
is  developed  from  membrane.  The  trabeculse  are  carried  for- 
ward, and  bending  down  at  the  nasal  depression  form  the  lateral 
nasal  cartilages  and  the  anterior  part  of  the  septal  cartilage. 


FIG.  318. 


Different  stages  of  the  development  of  the  head  and  face  of  a  human  embryo. 
A.  Embryo  of  four  weeks.      (Allen   Thomson.)     B.  Embryo  of  six  weeks.      (Ecker:) 

C.  Embryo  of  nine  weeks.     (Allen  Thomson.} 
a.  Auditory  vesicle.  1.  Lower  jaw.  I7.  First  pharyngeal  cleft. 

The  face  is  developed  in  connection  with  ridges  known  as  the 
visceral  folds  or  arches,  between  which  are  a  number  of  clefts,  the 
visceral  clefts. 

The  eyes  and  the  openings  of  the  nose  are  in  the  face ;  while 
the  ear  arises  at  the  side  of  the  face,  in  connection  with  one  of 
the  visceral  clefts. 

The  nasal  depressions  or  pits  appear  in  the  wall  of  the  head, 
covering  the  anterior  part  of  the  brain. 

Just  above  the  first  visceral  arch  or  fold  is  the  depression 
which  ultimately  becomes  the  buccal  cavity,  and  unites  with  the 
alimentary  tract  to  form  the  mouth. 


DEVELOPMENT  OF  THE  SKULL  AND  FACE. 


731 


The  first  fold  is  called  the  mandibular  ;  this  gives  off  at  either 
end  a  process  which  grows  upward  and  inward,  forming  the 
rudiment  of  the  superior  maxillary  bone  and  side  of  the  face. 

Between  these  is  a  median  process,  the  fronto-nasal,  which 
gives  off,  on  the  inner  sides  of  the  nasal  grooves,  projections 
which  form  the  inner  nasal  processes;  these  unite  with  the  supe- 
rior maxillary  processes  to  close  in  the  nostril  and  form  the  lip. 

The  outer  nasal  process  is  a  thickening  on  the  outer  side  of 
the  nasal  depression,  which  running  down  toward  the  superior 
maxillary  process,  forms  eventually  the  lachrymal  duct. 

FIG.  319. 


Vertical  section  of  the  head  of  an  embryo  of  a  rabbit.     (Mihalkovics.) 
In  A.  there  is  no  connection  between  the  buccal  cavity  and  the  fore-gut.   In  B.  the  con- 

nection is  established. 

m.  Epiblast  of  neural  canal,    h.  Heart,    c.  Cavity  of  fore-brain,    me.  Cavity  of  mid- 
brain.    mo.  Cavity  of  medulla,    sp.o.  Spheno-occipital  parts  of  the  basis  cranii.    sp.e. 
Spheno-ethmoidal  part  of  the  basis  cranii.    be.  Part  of  basis  cranii  which  receives  the 
ituitary  body.    am.  Amnion.    py.  Part  of  heart  cavity  going  to  form  the  pituitary 
i.f.  Fore-gut,    sh.  Notochord.    if.  Infundibulum. 


pituit 
body. 


The  mandibular  arch  forms  the  lower  jaw,  and  between  this 
and  the  superior  maxillary  process  the  buccal  cavity  is  developed 
chiefly  by  the  outgrowth  of  the  surrounding  tissues  ;  the  epiblast 
lining  this  becomes  thinned  away,  and  the  subjacent  mesoblast 
and  hypoblast  disappear  ;  the  buccal  cavity  is  made  continuous 
with  that  of  the  alimentary  canal. 

The  cavities  of  the  nasal  depressions  at  first  open  freely  into 
the  buccal  cavity  by  means  of  the  nasal  grooves  ;  after  a  time 


732  MANUAL   OF   PHYSIOLOGY. 

processes  arise  from  the  superior  maxillae,  and,  growing  inward, 
finally  meet  one  another  in  the  middle  line,  to  form  a  broad 
plate  of  tissue  intervening  between  the  nasal  cavity  above  and 
the  buccal  cavity  below.  This  plate  is  first  completed  in  front 
and  then  gradually  closes  toward  the  back  of  the  buccal  cavity, 
where  the  communication  between  the  nose  and  the  pharynx  is 
left. 

Imperfect  development  of  these  parts  gives  rise  to  the  common 
congenital  deformities,  cleft-palate  and  hare-lip. 

The  first  cleft  is  the  hyo-mandibular ;  it  forms  the  tympano- 
Eustachian  cavity,  which  becomes  separated  from  the  surface  by 
the  closure  of  its  outer  end  by  the  growth  of  the  membrana 
tympani,  the  external  auditory  meatus  and  ear  being  formed  by 
an  outgrowth  of  the  tissue  surrounding  the  tympanic  membrane. 

The  mandibular  arch  contains,  close  to  its  connection  with  the 
superior  maxillary  process,  a  rod  of  cartilage,  called  Meckel's 
cartilage.  This  becomes  partly  converted  into  the  malleus, 
partly  into  the  internal  lateral  ligaments  of  the  temporo-maxil- 
lary  articulation. 

The  second,  or  hyoid  arch,  gives  origin  to  the  incus,  the  stylo- 
hyoid  process  and  ligament,  and  the  lesser  wings  of  the  hyoid 
bone. 

From  the  third  arch  arise  the  body  and  greater  wings  of  the 
hyoid  bone  and  the  thyroid  cartilage. 


GLOSSARY. 


Abscissa.  The  line  forming  the  basis  of  measurement  of  graphic  records,  along 
which  the  time  measurement  is  usually  made. 

Accommodation.  Focusing  the  eye  for  different  distances  ;  it  is  effected  by  the 
lens  becoming  more  convex  for  near  objects,  owing  to  the  ciliary  muscle 
drawing  forward  its  choroidal  attachment,  and  relaxing  the  suspensory 
ligament. 

Acinous  glands.  Secreting  organs  composed  of  small  saccules  filled  with  gland- 
ular epithelium  connected  with  the  twigs  of  a  branched  duct. 

Adenoid  tissue.  A  delicate  feltwork  of  reticular  tissue  containing  lymph  cor- 
puscles (Lymphoid  tissue). 

Adequate  stimulus.  The  particular  form  of  stimulus  which  excites  the  nerve 
endings  of  a  special  sense  organ. 

Afferent  nerves.  Those  bearing  impulses  to  the  nervous  centres  from  the 
periphery  to  excite  reflex  actions  or  stimulate  the  sensorium. 

Agminate  glands.  A  name  applied  to  the  lymph  follicles  occurring  in  groups  in 
the  lower  part  of  the  small  intestine. 

Albumins.  A  term  derived  from  the  Latin  for  white  of  egg  (Albumen),  denoting 
a  group  of  complex  chemical  substances  obtained  from  ova,  blood  plasma  and 
many  tissues  of  animals  and  plants. 

Albuminoids.  A  class  of  nitrogenous  substances  allied  to  the  albumins  in  com- 
position, but  differing  from  them  in  many  important  respects. 

Allantois.  A  vascular  outgrowth  from  the  embryo,-  in  mammals  it  helps  to 
form  the  placenta,  and  in  the  chick  forms  the  respiratory  organ. 

Alveoli.  The  term  used  to  denote  small  cavities  found  in  many  parts,  such  as 
the  air  spaces  of  the  lungs. 

Amnion.  The  membranous  sac  which  grows  around  the  embryo  and  encloses  the 
foetus,  etc.,  during  its  development. 

Amoeba.     A  unicellular  organism  consisting  of  a  nucleated  mass  of  protoplasm. 

Amorphous.     Without  definite  or  regular  form  ;  the  opposite  of  crystalline. 

Ampulla.     A  dilatation  on  the  semicircular  canals  of  the  ear. 

Amylolytic.     Relating  to  the  conversion  of  starch  into  dextrine  and  grape  sugar. 

Amylopsin.     A  ferment  in  the  pancreatic  juice,  which  converts  starch  into  sugar. 

Anabolic.  An  exciting  influence  exerted  by  nerves  increasing  the  metabolism  of 
tissues. 

Analgesia.  A  condition  of  the  nervous  centres  in  which  pain  cannot  be  felt,  but 
tactile  and  other  sensations  remain  unimpaired. 

Analysis.  A  separation  into  component  parts;  the  splitting  up  of  a  chemical 
compound  into- its  constituents. 

Anastomoses.  The  direct  union  of  blood  vessels  without  the  intervention  of  a 
capillary  network. 

Anelectrotonus.  An  electric  condition  of  a  nerve,  resulting  from  the  passage 
of  a  current  through  a  part  of  it  ;  it  is  confined  to  the  regions  of  the  positive 
pole. 

Anode.  The  positive  pole  or  electrode — i.  e.,  the  pole  by  which  the  electric  cur- 
rent enters. 

Apnoea.  A  state  of  cessation  of  the  breathing  movements  from  non-excitation  of 
the  respiratory  nerve  centre. 

733 


734  GLOSSARY. 

Area  opaca.     The  outer  zone  of  the  part  of  the  blastoderm  from  which  the  foetal 

membranes  are  developed. 
Area  pellucida.     The  central  spot  of  the  part  of  the  blastoderm  from  which  the 

embryo  chick  is  developed. 
Arteriole.     A  small  artery ;  usually  applied  to  those  vessels  the  walls  of  which 

are  largely  composed  of  muscle  tissue. 

Arthroses.     Movable  joints  having  a  synovial  membrane. 
Asphyxia.     Literally,  cessation  of  the  pulse,  such  as  occurs  from  interruption  of 

respiration,  now  used  as  synonymous  with  suffocation. 
Assimilation.       The  chemical   combination  of  new  material  (nutriment)  with 

living  tissues.     Power  to  assimilate  forms  the  most  characteristic  property  of 

living  matter. 
Astigmatism.     Unevenness  of  the  refracting  surfaces  of  the  eye;  when  engaging 

the  entire  cornea  it  is  called  "regular,"  and  affecting  a  local  part,  "  irregu- 
lar," astigmatism. 

Atoms.     The  ultimate  indivisible  particles  of  matter. 
Atrophy.     A  wasting  from  insufficient  nutrition. 
Automatic.     Self-moving — i.  e.,  acting  without  extrinsic  aid  ;  a  term  applied  to 

the  independent  activity  of  certain  tissues  (such  as  the  nerve  centres),  the 

exciting  energies  of  which  are  not  readily  determined. 
Axis  cylinder.     The  essential  conducting  part  of  a  nerve  fibre,  composed  of  fine 

strands  of  protoplasm. 

Bacteria.     A  class  of  minute  fungi  occurring  in  decomposing  animal  or  vegetable 

substances. 

Bilirubin.     The  red  coloring  matter  of  the  bile  of  man  and  carnivora. 
Biliverdin.     The  greenish  coloring  matter  of  the  bile  of  herbivorous  animals. 
Binocular.     Pertaining  to  vision  with  two  eyes.     A  combination  of  the  effect  of 

two  retinal  impressions  by  means  of  which  the  appearances  of  distance  and 

solidity  are  arrived  at. 

Biology.     The  science  of  life,  including  morphology  and  physiology. 
Blastoderm.     The  primitive  cellular  membrane  formed  by  the  segmentation  of 

the  ovum,  in  a  part  of  which  the  embryo  is  developed. 
Blood  pressure.     The  force  exercised   by  the  blood   against  the  walls  of   the 

vessels.      It  is  very  great  in  the    arteries,  and  therefore  causes  a  constant 

stream  through  the  capillaries  to  the  veins. 

Canaliculi.  Minute  channels  connecting  the  small  cell  spaces  or  lacunae  of  bone, 
and  containing  protoplasmic  filaments  uniting  the  neighboring  cells. 

Carbohydrates.  Compounds  of  carbon,  hydrogen  and  oxygen,  in  which  the 
oxygen  and  hydrogen  exist  in  the  proportions  requisite  to  form  water. 

Cardiograph.  An  instrument  by  means  of  which  the  heart's  impulse  is  trans- 
mitted, through  an  air  tube,  from  a  tambour  on  the  chest  wall  to  another 
which  makes  a  record  on  a  moving  surface  by  means  of  a  lever. 

Catelectrotonus.  An  electric  state  of  nerve  in  the  region  where  the  current 
leaves  the  nerve,  i.  e.,  near  the  negative  pole. 

Cathode.  The  negative  pole  or  electrode — i.  e.,  the  pole  by  which  the  electric 
current  leaves. 

Cellulose.     The  substance  of  which  vegetable  cell  walls  are  formed. 

Centrifugal.     Efferent. 

Centripetal.     Afferent. 

Cerebral  vesicles.  Primitive  swellings  on  the  primary  neural  tube  of  the  early 
embryo  which  develop  into  the  brain. 

Chemical  elements.  Substances  which  cannot  be  split  up  into  components,  and 
therefore  are  regarded  as  sjmple. 

Chlorophyll.  The  green  coloring  matter  of  the  cells  of  plants.  It  is  supposed 
to  be  the  agent  which,  under  the  influence  of  light,  decomposes  carbon 
dioxide  and  water  to  form  the  cellulose  and  starch  of  the  plant. 


GLOSSARY.  735 

Ch.olesteri.il.     A  substance  occurring  in  the  bile,  white  matter  of  the  brain  and 

spinal  cord,  and  in  small  quantities  in  many  other  tissues.     Chemically  it  is 

a  monatomic  alcohol. 

Chorda  dorsalis.     The  precursor  of  the  vertebral  column  of  the  embryo. 
Chorion.     The  outer  layer  of  the  membranes  of  the  ovum,  part  of  which  becomes 

vascular,  and  helps  to  form  the  placenta. 
Choroid.     The  vascular  coat  of  the  eyeball. 
Chromatic  aberration.     The  alteration  of  white  light  into  prismatic  colors  during 

its  passage  through  an  ordinary  lens. 

Chyle.     The  fluid  absorbed  from  the  small  intestines  by  the  lacteals. 
Chyme.     The  fluid  absorbed  by  gastric  digestion. 

Cilia.     Minute  vibratile  processes  which  occur  on  the  surface  cells  of  the  respira- 
tory and  many  other  epithelial  membranes. 
Circumvallate.      Large  papillae  situated  at  the  back  of  the  tongue.      They  are 

surrounded  by  a  fossa  in  the  walls  of  which  lie  taste  buds. 
Cloaca.      The  opening  common  to  the  genito-urinary  organs  in  the  primitive 

hind  gut  of  the  embryo.     The  cloaca  persists  in  birds. 
Colloid.     That  condition  of  quasi-dissolved  matter  in  which  it  will  not  diffuse 

through  a  membrane  such  as  parchment.     The  opposite  of  crystalloid. 
Colostrum.     The  first  milk  secreted  after  delivery. 
Coordination.     The  adjustment  of  separate  actions  for  a  definite  result,  as  when 

the  nerve  centres  cause  various  distinct  muscles  to  act  together  and  produce 

complex  movements. 
Curara.     A  poison  causing  motor  paralysis  by  impairing  the  function  of  the  nerve 

terminals. 
Cytod.     A  living  protoplasmic  unit  which  has  no  nucleus. 

Decidua  reflexa.     The  outgrowth  of  the  uterine  mucous  membrane  which  sur- 
rounds the  ovum. 
Decidua  serotina.     That  part  of  the  modified  mucous  membrane  of  the  uterus  in 

which  the  fecundated  ovum  is  lodged. 
Decidua  vera.     The  altered  mucous  membrane  of  the  uterus,  which  lines  that 

organ  during  gestation. 
Deglutition.     The  act  of  swallowing. 
Desquamation.     The  term  used  to  denote  the  casting  off  of  the  outer  layer  of  the 

skin. 
Dialysis.      The  diffusion  of  soluble  crystalloid   substances   through  membranes 

such  as  parchment. 

Diastole.     The  period  of  relaxation  and  rest  of  the  heart's  muscle. 
Dicrotic.     The  double  wave  of  the  arterial  pulse.     The  dicrotic  wave  is  seen  on 

the  descending  part  of  the  pulse  curve. 
Dioptric  media.     Transparent  bodies,  such  as  those  parts  of  the  eye  which  so 

refract  the  light  that  images  come  to  a  focus  on  the  retina. 

Discus  proligerus.     Pait  of  the  granular  layer  of  the  Graafian  follicle  surround- 
ing the  ovum. 

Distal.     A  term  used  to  denote  a  part  relatively  far  from  the  centre. 
Ductus  arteriosus.     A  short  bond  of  union  between  the  pulmonary  artery  and 

the  aorta,  which  in  the  foetus  carries  blood  from  the  right  side  of  the  heait 

into  the  aorta. 
Ductus  venosus.     A  vessel  which,  in  the  foetus,  carries  blood  from  the  umbilical 

vein  to  the  vena  cava.     After  birth  it  becomes  a  fibrous  cord. 
Ductus  vitello-intestinalis.     The  union  between  the  yolk  sac  and  the  intestine 

of  the  embryo. 
Dyspnoea.     Difficulty  in  breathing;  a  condition  in  which  inordinate  respiratory 

movements  are  excited  by  an   unusually   venous  state  of  the  blood  in   the 

respiratory  nerve  centre. 

Ectoderm.     The  outer  layer  of  simple  organisms. 

Ectosarc.     The  outer  layer  or  covering  of  certain  unicellular  organisms. 


736  GLOSSARY. 

Electrodes.  The  terminals  which  are  applied  to  a  substance  in  order  to  complete 
the  circuit  in  passing  a  current  through  it. 

Electretonus.  A  peculiar  electric  state  of  nerves  resulting  from  the  passage  of 
a  continuous  current  through  them. 

Embryo.     The  name  given  to  the  animal  at  the  earliest  period  of  its  development. 

Emmetropic.  A  term  applied  to  the  normal  eye,  in  which  parallel  rays  of  light 
are  brought  to  a  focus  at  the  retina  without  accommodation. 

Emulsification.  Tffe  suspension  of  very  fine  particles  in  a  liquid  unable  to  dis- 
solve them. 

Endoderm.     The  inner  layer  of  simple  organisms. 

Endogenous  reproduction.  The  formation  of  new  cells  or  organisms  within  the 
body  of  the  parent  individual. 

Endolymph.     The  liquid  contained  within  the  membranous  labyrinth  of  the  ear. 

Endosarc.     The  inner  layer  of  certain  unicellular  organisms. 

Endosmosis.     The  diffusion  of  a  fluid  into  a  texture. 

Endothelium.  The  single  layer  of  thin  cells  lining  the  serous  cavities,  lym- 
phatic and  blood  vessels,  and  all  spaces  in  the  connective  tissues  (mesoblastic 
lining  cells). 

Epiblast.  The  uppermost  of  the  three  layers  of  the  blastoderm,  from  which  are 
developed  the  epidermis  and  the  nerve  centres. 

Epithelium.  The  non-vascular  cellular  tissue  developed  from  the  epi-  and 
hypoblast  of  the  blastoderm. 

Eupnoea.  A  term  used  to  denote  the  normal  rhythm  of  respiratory  movements 
in  contradistinction  to  dyspnoea  and  apncea. 

Excito-motor.     Impulses  which,  reflexly,  call  forth  motion. 

Excito-secretory.  Impulses  calling  forth  the  activity  of  gland  cells,  commonly 
applied  to  afferent  influences  which  act  reflexly. 

Fibrinogen.  A  form  of  globulin  obtained  from  serous  fluids,  which,  on  being 
added  to  a  liquid  containing  serum-globulin,  gives  rise  to  the  formation  of 
fibrin. 

Fibrinoplastin.     A  term  sometimes  applied  to  paraglobulin  or  serum-globulin. 

Filiform.  A  name  given  to  a  class  of  papillae  of  the  tongue,  the  points  of  which 
taper  off  to  a  thread. 

Foetus.     The  fully-formed  embryo  while  in  the  uterus  or  egg. 

Fovea  centralis.     The  depression  in  the  centre  of  the  macula  lutea. 

Fungiform.     A  class  of  papillae  of  the  tongue,  shaped  like  a  toadstool. 

Galvanometer.      An   instrument  for  measuring  the  direction  and  strength    of 

electric  currents  by  means  of  the  deflection  of  a  magnetic  needle. 
Ganglion.     A  swelling.     Chiefly  used  to  denote  swellings  on  nerves  containing 

nerve  corpuscles.     Hence,  any  group  or  mass  of  nerve  cells. 
Gastrula.     A  stage  in  the  development  of  animals  in  which  they  consist  of  a 

small  sac  composed  of  two  layers  of  cells. 
Gemmation.     Budding — a  process  of  reproduction  in  which  a  bud  forms  on  the 

parent  organism,  and  finally  separates  as  a  distinct  being. 
Globulin.      A  form  of  albumin   insoluble   in   pure  water   but   soluble  in  weak 

solutions  of  common  salt. 
Glomerulus.     A  bundle  of  capillary  loops  which  form  part  of  the  Malpighian 

body  of  the  kidney. 
Glycocholic  acid.     An  acid  existing  in  large  quantities  combined  with  soda  in 

the  bile  of  man. 
Glycogen.     Animal  starch  ;  a  substance  belonging  to  the  carbohydrates,  which  is 

made  in  the  liver.      It  may  be  readily  converted  into  grape  sugar — from 

which  fact  it  derives  its  name. 
Gustatory.     Pertaining  to  the  sense  of  taste. 

Haematin.  A  dark-red  amorphous  body  containing  iron ;  obtained  from  the 
decomposition  of  the  coloring  matter  of  the  blood  (haemoglobin). 


GLOSSARY.  737 

Haematoin,  or  Haematoporphyrin.     Iron-free  haematin  prepared  with  strong 

acetic  acid. 
Haematoidin.     A  substance  found  in  old  blood  clots,  as  crystals,  which  "cannot 

be  artificially  prepared. 
Haemin.     Hydrochlorate  of  haematin;  easily  obtained,  as  small,  dark  crystals,  by 

boiling  blood  to  which  some  common  salt  and  glacial  acetic  acid  have  been  added. 
Haemochromogen.     Unoxidized  haematin,  the  first  outcome  Q£  the  decomposition 

of  hasmoglobin. 

Haemoglobin,     Reduced  oxyhcemoglobin. 
Holoblastic.     The  form  of  ova  the  entire  yolk  of  which  enters  into  the  process 

of  segmentation. 
Homceothermic.     Even  temperatured — a  term  applied  to  those  animals  that  keep 

up  a  regular  temperature,  independent  of  their  surroundings — warm-blooded 

animals. 
Hyaloid.     Glass-like ;  a   name   given   to   the  delicate   membrane   enclosing  the 

vitreous  humor. 
Hydrocarbons.     Compounds  of  carbons  and  hydrogen.     Fats,  though  containing 

oxygen  in  addition,  have  been  called  hydrocarbons. 
Hypermetropia.     The  condition  in  which  the  focus  of  parallel  rays  of  light  lies 

beyond  the  retina;  also  called  long  siyht. 
Hypertrophy.     Increased  growth  from  excessive  nutrition. 
Hypoblast.     The  undermost  of  the  layers  of  the  blastoderm,   from  which  the 

pulmonary  and  alimentary  tracts  and  their  glands  are  formed. 

Infusoria.  A  name  given  to  a  large  class  of  simple  organisms  which  are  found 
in  dirty  water. 

Inhibition.  A  checking  or  preventive  action  exercised  by  some  nervous  mechan- 
isms over  nerve  corpuscles  and  other  active  tissues. 

Inosit.     A  sugar  peculiar  to  muscle. 

Irradiation.  The  phenomenon  that  bright  objects  appear  larger  than  they  really 
are.  It  is  due  to  the  extension  of  the  effect  to  those  parts  of  the  retina 
immediately  adjacent  to  where  the  light  rays  impinge. 

Karyokinesis.  A  series  of  changes  occurring  in  the  arrangement  of  the  nuclear 
network  prior  to  the  division  of  the  protoplasm  of  cells. 

Katabolic.  A  lowering  influence  exerted  by  certain  nerves,  decreasing  metabolism. 
Inhibitory  action. 

Keratine.  The  characteristic  chemical  constituent  of  the  horny  layer  of  the  skin 
and  epidermal  appendages. 

Kymograph.  An  instrument  used  for  recording  graphically  the  undulations  of 
blood  pressure,  measured  directly  from  a  blood  vessel  by  means  of  a  mano- 
meter. 

Lachrymal.     Pertaining  to  the  secretion  of  tears. 

Lacunas.     Small  spaces   in  the  substance  of  bone  tissue,  occupied  during  life  by 

the  bone  cells.     They  appear  black  in  sections  of  dry  bone,  owing  to  their 

containing  air,  which  replaces  the  shriveled  cells. 
Latency,  or  Latent  period.     The  time  that  elapses  between  the   moment   of 

stimulation  and  the  response  given  by  an  active  tissue. 
Leucin.      This  is  a  common  product  of  the  decomposition  of  proteids.      It  is 

formed  in  the  later  stages  of  pancreatic  digestion. 

Leucocytes.     A  term  applied  to  the  white  blood  corpuscles  and  lymph  cells. 
Lumen.     The  open  space  seen  on  section  of  a  tube,  vessel,  or  glandular  saccule  ; 

the  cavity  surrounded  by  the  gland  cells,  in  which  the  secretion  collects. 
Luscitas.     Fixation  of  the  eyeball  in  the  outer  canthus,  owing  to  the  unopposed 

action  of  the  external  rectus  muscle. 
Lymph.      The  liquid  collected  by  the  absorbent  vessels  from,  the  tissues ;  the 

return  flow  of  the  irrigation  stream  escaping  from  the  blood  vessels  to  nourish 

the  tissues. 


62 


738  GLOSSARY. 

Macula  Lutea.     That  part  of  the  retina  near  the  axis  of  the  eyeball,  in  which 

vision  is  most  acute. 
Manometer.     An  instrument  for  measuring  pressure ;  made  of  a  U-shaped  tube 

containing  liquid,  commonly  mercury. 
Medullary  sheath.     A  soft,  clear  sheath  around  the  axis  cylinder  of  medullated 

nerves,  which,  owing  to  its  refracting  power,  gives  them  a  white  appearance. 
Menstruation.     The  monthly  change  in  the  mucous  membrane  of  the  uterus, 

which  accompanies  the  discharge  of  the  ovum. 
Meroblastio.     The  form  of  ova  in  which  the  yolk  does  not  undergo  complete 

segmentation,  as  that  of  birds. 
Mesenoephalon.     Those  parts  of  the  brain  developed  from  the  middle  cerebral 

vesicle,  viz.,  crura  cerebri  and  corpora  quadrigemina. 
Mesoblast.     The  middle  of  the  three  layers  of  the  blastoderm  from  which  the 

connective  tissues  and  vascular  apparatus  of  the  embryo  are  formed. 
Metabolism.     The  intimate  chemical  changes  occurring  in  the  various  organs  and 

tissues  upon  which  their  nutrition  and  functions  depend. 
Metanephros.     The  hinder  portion  of  the  Wolffian  body  which  develops  into  the 

kidney  and  ureter. 
Metazoa.     A  term  used  to  denote  all  those  animals  whose  ova  undergo  division, 

in  contradistinction  to  Protozoa. 
Methaemoglobin.     A  compound  formed  by  oxy haemoglobin  combining  more  firmly 

with  additional  oxygen. 
Micrococcus.     An  extremely  minute  fungus  of  a  round  sh*ape.     Micrococci  occur 

in  many  solutions  of  decomposing  organic  matter. 
Micturition.     The  act  of  voiding  urine. 
Molecules.     The  smallest  physical  particles  of  matter  that  can  exist  in  a  separate 

state.     They  are  probably  always  constituted  of  two  or  more  atoms. 
Morphology.     The  science  which  treats  of  the  forms  and  structures  of  living 

beings. 
Morula.     The  stage  of  development  of  the  ovum  after  segmentation,  in  which  all 

the  young  cells  are  alike,  before  the  blastoderm  is  formed. 
Mucin.     The  characteristic  constituent  of  mucus. 
Miillerian  duct.     An   embryonic  structure  from  which  are  formed  the  genital 

passages  in  the  female,  viz.,  Fallopian  tube,  uterus  and  vagina. 
Mydriasis.     A  dilated  state  of  the  pupil. 

Myograph.     An  instrument  for  graphically  recording  muscle  contraction. 
Myopia.     The  condition  in  which  the  focus  of  parallel  rays  of  light  falls  short  of 

the  retina;  short  sight. 
Myosin.     The  substance  formed  by  the  coagulation  of  muscle  plasma.     It  is  one 

of  the  globulins. 

Natural  nerve  currents.    The  electrical  currents  passing  through  an  exposed 

muscle  or  nerve  while  in  a  state  of  rest. 
Neuroglia.     The  reticular  connective  tissue  which  binds  together  the  elements  of 

the  nerve  centres. 
Non-polarizable  electrodes.     Electric  terminals  specially  constructed  so  as  not 

to  set  up  secondary  currents  on  application  to  moist  living  tissues. 
Notochord.     The  primitive  vertebral  axis  of  the  embryo. 
Nucleolus.     A  small  spot  observable  in  some  nuclei. 
Nucleus.      A  central  part  of  a  cell  differentiated  from  the  main   protoplasm, 

commonly  round,  but  sometimes  elongated,  as  in  muscle. 

Odontoblasts.     Living  cells  lining  the  pulp  cavity  of  the  interior  of  a  tooth,  and 

presiding  over  the  growth  and  nutrition  of  the  dentine. 
Olfactory.     Pertaining  to  the  sense  of  smell. 
Omphalo-mesenteric.     The  vessels  connecting  the  embryonic  circulation  with  the 

yolk  sac,  which  are  early  obliterated  in  the  mammalian  foetus. 
Ophthalmoscope.      An  instrument  consisting  of  a  small  mirror,  by  which  the 

interior  of  the  eye  can  be  illuminated  so  that  the  fundus  may  be  viewed. 


GLOSSARY.  739 

Optic  cup.     The  involuted  optic  vesicle  which  is  developed  into  the  retina,  etc. 
Osteoblast.     The  active  cells  in  forming  bone. 
Osteoliths.     Calcareous  particles  lying  in  the  endolymph. 
Oxyhaemoglobin.  .  The  coloring  matter  of  the  blood  corpuscles. 

Paraglobulin.      One  of   the   more   abundant  albumins  of  the  blood  —  serum 

globulin. 
Paramaecium.     A  unicellular  organism  composed  of  a  soft  mass  of  protoplasm 

enclosed  in  a  firmer  case,  and  covered  with  motile  cilia. 
Parapeptone.     A   body  produced  in  gastric  digestion  during  the  formation  of 

peptone. 
Pepsin.     A  ferment  existing  in  the  gastric  juice  which  converts  proteids  into 

peptones. 
Peptone.     A  form  of  albumin  which  is  produced  during  the  digestion  of  proteids  j 

it  is  very  soluble,  and  diffuses  readily  through  a  membrane. 
Perilymph.     The  liquid  surrounding  the  membranous  labyrinth  of  the  ear. 
Peristalsis.     The  mo  le  of  contraction  of  the  muscular  walls  of  certain  tubes,  as 

the  oesophagus  an  I  intestine,   the  effeat  of  which  is  to  cause  a  progressive 

constriction,  and  so  force  the  contents  of  the  tube  onward. 
Phakoscope.     An  instrument  for  estimating  the  changes  in  the  shape  of  the  lens 

during  accommodation,  by  doubling  the  reflected  images  with  a  prism. 
Placenta.     The  intra-uterine  organ  by  means  of  which  the  foetil  blool  is  brought, 

into  close  relationship  to  that  of  the  mother,  so  as  to  gain  nutriment  and 

oxygen,  and  get  rid  of  effete  matters. 
Plasma.      A  term  meaning  anything  formed  or  moulded  ;    used  in  physiology  to 

indicate  chemically  complex  kinds  of  matter  which  subserve  to  the  formation 

of  the  living  tissues. 
Poikilothermlc.     Varying  in  temperature.     A  term  applied  to   those   animals 

whose   temperature   varies  with  that  of  the  surrounding  medium  —  "cold- 
blooded animals." 

Presbyopia.     A  loss  of  power  of  accommodation  for  near  vision  which  accom- 
panies old  age. 
Prosencephalon.      That  part  of  the  developing  anterior  cerebral  vesicle  from 

which  are  formed  the  olfactory  and  optic  lobes,  the  hemispheres,  and  corpora 

striata  and  optic  thalami. 
Protista.     A  large  group  of  organisms  which  remain  in  the  primitive  state  of  a 

single  cell  during  their  lifetime. 
Protococcus.     A  unicellular  vegetable  organism,  the  protoplasm  of  which  contains 

chlorophyll. 
Protoplasm.     The  substance  which  gives  rise  to  the  primitive  vital  phenomena, 

seen  in  unicellular  organisms,  and  which  is  the  chief  agent  in  executing  the 

functions  of  all  the  active  tissues. 
Protovertebrae.     The  primitive  segments  of  the  mesoblast  in  the  site  of  the  future 

vertebral  column. 
Protozoa.     That  division  of  the  protista  which  has  been  assigned  to  the  animal 

kingdom. 

Proximal.     A  term  used  to  denote  a  part  relatively  nearer  to  the  centre. 
Pseudopodia.     Projections  thrown  out  by  moving  protoplasm,  by  means  of  which 

cells,  such  as  amoebae,  move. 

Ptosis.     Drooping  of  the  eyelid  accompanying  paralysis  of  the  third  nerve. 
Ptyalin.     The  ferment  of  the  saliva.     In  a  weak  alkaline  solution  it  converts 

starch  into  dextrine  and  sugar. 

Reflex  action.  The  activity  caused  by  a  ganglion  cell  reflecting  an  afferent 
impulse  along  an  efferent  nerve  to  the  neighborhood  of  original  stimula- 
tion. 

Reflexion.     The  return  of  rays  of  light  from  a  surface. 

Refraction.  The  bending  which  rays  of  light  undergo  when  passing  obliquely 
from  one  medium  to  another  of  different  density. 


740  GLOSSARY. 

Reticulum.     A  network;  a  term  applied  to  the  interlacement  of  fibres,  seen  in 

reticulated  connective  tissue,  etc. 
Kheoscopic  frog.     An  arrangement  by  which  the  change  in  the  electric  current 

of  one   muscle  of    a   frog   is   made   to   act   as   a   stimulus   to   the   nerve  of 

another. 

Saponification.      The  formation  of  soap;  the  decomposition  of  oils  or  fats  by 

means  of  alkalies  into  salts  of  the  fatty  acids  and  glycerine. 
Sarcolactic  acid.     The  principal  acid  in  dead  muscle.     It  has  a  dextro-rotatory 

power  on  polarized  light,  which  ordinary  lactic  acid  does  not  possess. 
Sareolemma.     The  delicate  sheath  surrounding  the  fibres  of  skeletal  muscles. 
Sclerotic.     The  fibrous  coat  of  the  eyeball. 

Sensorium.     That  part   of    the  nerve  centres   supposed  to  receive   sensory    im- 
pressions. 
Somatopleure.     The  subdivision  of    the  mesoblast  which,  with  the    attached 

epiblast,  forms  the  body  walls  of  the  embryo. 
Specific  gravity.    The  relation  of  the  weight  of  a  given  volume  of  any  substance 

to  the  weight  of  an  equal  vdlume  of  distilled  water  at  4°  C. 
Spherical  aberration.     An  indistinctness  of  the  image  caused  by  the  difference 

in  refraction  at  the  centre  and  margin  of  a  lens  giving  rise  to  different  focal 

lengths. 
Sphygmograph.     An  instrument  for  obtaining  a  graphic  representation  of  the 

pulse  wave  by  means  of  a  lever  applied  to  the  radial  artery  at  the  wrist. 
Splanchnopleure.     The  subdivision  of  the  mesoblast  which,  with  the  attached 

hypoblast,  forms  the  chief  visceral  cavities  of  the  embryo. 
Sporadic  ganglia.     Swellings  occurring  in  the  course  of  the  peripheral  nerves 

caused  by  a  group  of  nerve  corpuscles. 
Steapsin.     A  ferment  existing  in  the  pancreatic  juice  which  causes  or  aids  the 

saponification  of  the  fats. 
Sudoriferous    glands.      The    small    tubular  glands   of  the  skin  which  secrete 

perspiration. 
Summation.     The  fusion  of  several  single  contractions  of  muscle  to  form  a  tetanic 

contraction ;  the  accumulation  of  stimuli. 
Sutures.     Unions  formed  by  the  direct  apposition  of  bones  without  intervening 

cartilage.     They  do  not  permit  of  motion. 
Sympathetic  nerve.     The  ganglionic  nervous  cord  on  either  side  of  the  vertebral 

column.      It   transmits   most  of  the  vasomotor   impulses   coming   from  the 

cerebro-spinal  centres. 
Symphysis.     A  form  of  joint  without  synovial  membrane  in  which  the  bones  are 

fixed  together  by  fibro-cartilage. 
Synthesis.     The  artificial  construction  of  a  chemical   compound    from   simpler 

materials. 
Systole.     The  period  of  contraction  of  the  heart's  muscle. 

Taurocholic  acid.     An  acid  existing  in  combination  with  soda  in  the  bile. 
Tetanus.     In   physiology  is   used   to   denote   the   prolonged   contraction  of  the 

skeletal   muscles   which   follows   rapidly   repeated    stimulations    or   nervous 

impulse. 
Thalamencephalon.     That  part  of  the  anterior  cerebral  vesicle  which  is  left  after 

the  differentiation  of  the  optic  thalami,  cerebral  hemispheres,  etc. 
Thrombosis.     The  occlusion  of  a  vessel  by  a  local  coagulation  of  the  blood. 
Trabeculae.     Supporting   bars  of  tissue   passing  through   some  organs,  such  as 

those  proceeding  from  the  capsule  to  the  interior  of  the  spleen  or  lymphatic 

glands. 

Trophic.     Relating  to  nutrition. 
Trypsin.     A  ferment  in  the  pancreatic  juice  which  in  alkaline  solutions  converts 

proteids  into  peptones. 
Tyrosin.     A  substance  formed  together  with  leucin  during  pancreatic  digestion: 

it  is  also  produced  by  putrefaction  of  proteids. 


GLOSSARY.  741 

Urachus.  The  bond  of  union  which  at  an  early  period  connects  the  urinary 
bladder  with  the  allantois  in  the  embryo;  it  is  subsequently  obliterated  in 
the  foetus. 

Vacuoles.  Small  cavities  occurring  in  cells.  They  are  supposed  to  have  important 
functions  in  the  unicellular  organisms. 

Vagus.  The  part  of  the  eighth  pair  of  nerves  distributed  to  the  viscera  of  the 
throat,  thorax  and  abdomen;  the  great  regulating  nerve  of  the  vegetative 
functions. 

Vaso-constrictor.  Those  impulses  which  excite  contraction  of  the  vascular 
muscles. 

Vaso-dilator.     Those  impulses  which  inhibit  the  action  of  the  vascular  muscles. 

Vasomotor.  Those  nervous  mechanisms  controlling  the  movements  of  the  blood 
vessels. 

Villus.  A  hair-like  process.  A  term  applied  to  the  small  projections  character- 
istic of  the  small  intestine.  They  contain  bloodvessels  and  lacteals,  and  are 
important  in  absorption. 

Vitellus.  The  yolk  of  the  ovum,  which  in  mammals  divides  completely  to  form 
the  embryo.  In  birds  only  a  part  divides,  and  the  rest  serves  to  nourish  the 
chick. 

Vorticella.  Bell  animalcule,  a  bell-shaped  unicellular  organism  with  a  rudi- 
mentary, ciliated  mouth  cavity  and  rapidly  contractile  stalk. 

Wolffian  body.  An  embryonic  structure,  the  forerunner  of  certain  parts  of  the 
genito-urinary  apparatus. 

Zymogen.     A  peculiar  substance  supposed  to  give  rise  to  the  pancreatic  ferments. 


INDEX. 


A  BDOMINAL  respiration,  332. 
/\    Abductor  nerve  of  the  eye, 

523. 
Abnormal  constituents  in  the  urine, 

406. 

Absorbents  of  stomach,  149. 
Absorption,  190. 

intestinal,  199. 
interstitial,  192. 
mechanism  of,  203. 
Accommodation,  570. 
Acid  albumin,  69. 
Acinous  glands,  131. 
Active  tissues,  43. 
Adipose  diarrhoea,  206. 
Afferent  nerve,  48,  498. 
Agminate  glands,  202. 
Air  passages  of  lung,  326. 
Albumin,  acid,  69. 
alkali,  70. 
coagulated,  70. 
conversion  into  peptone, 

155. 

egg,  68.  ^ 
in  the  urine,  406. 
serum,  68. 
syntonin,  69. 
tests  for,  67. 
Albuminates,  69. 
Albuminoids,  composition  of,  72. 
Albuminous  bodies,  66. 

as  food,  97,  100. 
Albumins,  classification  of,  68. 

chemical    composition 

of,  67.  m 

characteristics  of,  71. 
Alcoholic  fermentation,  78,  90. 
Alimentary  canal,  111. 

development  of,  697. 
Alkali  albumin,  70. 
Allantoin,  76. 
Allantois,  679. 
Alveoli,  339. 
Amnion,  675. 


Amoeba,  39. 

change  in  form,  83. 

effect  of  stimulation,  83. 

life  of,  91. 

locomotion  of,  83. 

pseudopodia  of,  83. 
Amoeboid  movement  of  white  blood 

corpuscles,  226. 

Amount  of  blood  in  the  body,  215. 
Amphioxus,  blood  of.  228. 
Ampullae   of  semicircular   canals, 

608. 

Amylopsin,  167. 
Analgesia,  623. 
Anelectrotonus,  508. 
Animal  heat,  428. 

production  of,  431. 
Anode,  503. 

Anterior  roots  of  spinal  cord,  617. 
Anus,  111. 
Apncea,  347. 
Aortic  arches,  716. 
valves,  263. 
Area  opaca,  674. 

pellucida,  674. 
Arterial  blood,  355. 

pulse,  the,  307. 

system,    development  of, 
716. 

tone,  317. 
Arteries,  small,  284. 

walls  of,  283. 
Arterioles,  286. 
Arthrosis,  479. 
Arytenoid  cartilages,  487. 
Asphyxia,  362. 
Assimilation,  31. 
Astigmatism,  574. 
Atmospheric  air,  351. 
Atoms,  30. 
Auditory  nerve,  stimulation  of,  610. 

terminals  of,  607. 
Auerbach's  plexus,  127. 
Augmentation,  517. 


743 


744 


INDEX. 


Auricles  of  the  heart,  259. 
Automatic  centres  in  spinal  cord, 

635. 

Automatism,  517,  634. 
Axis  cylinder,  49,  498. 


gACTERIUM,  88,  89. 
Basilar  membrane,  608. 
ttery,  Daniell's,  500. 
Bell  animalcule,  94. 
Bertin,  columns  of,  391. 
Bile,  168. 

acids    and    pigments  in   the 

urine,  407. 
cholesterine     contained     in, 

177. 

composition  of,  175. 
ducts,  174. 

functions  of  the,  180. 
method  of  obtaining,  174. 
pigments,   Gmelin's  test  for, 

176. 
salts,  Pettenkofer's  test  for, 

73,  177. 

secretion  of,  178. 
Biliary  fistula,  174. 
Bilirubin,  177. 
Biliverdin,  177. 
Binocular  vision,  596. 
Bladder,  evacuation  of  the,  413. 
nervous    mechanism    of, 

632. 
passage  of  the  urine  to 

the,  411. 

structure  of  the,  412. 
Blastoderm,  38,  42,  671,  673. 
Blind  spot,  586. 

Blood,  action'  of  reagents  on,  230. 
amount  of,  21 7. 
arterial,;354. 
buffy  coat  of  clot,  246. 
carbon  dioxide  in  the,  244. 
changes  during  respiration, 

354. 

changes  in  the,  230. 
changes  in  the  spleen,  372. 
chemistry   of   the   coloring 

matter,  235. 
chemistry   of    the    stroma, 

241. 

circulation  of  the,  256. 
circulation    in    frog's   web, 
315. 


Blood,  circumstances  influencing 
coagulation,  248. 

clot,  247. 

coagulation  of  the,  245. 

coagulation  within  the  ves- 
sels, 249. 

colorless  corpuscles,  225. 

colorless  corpuscles,  origin 
of,  227. 

composition  of,  215. 

constitution  of  the,  214. 

corpuscles,  217,  224. 

corpuscles,  increase  in  num- 
ber of  white,  225. 

corpuscles,  number  of,  224. 

corpuscles,  separation  from 
plasma,  218. 

crenated  red  corpuscles, 
230. 

current,  velocity  of  the,  313. 

defibrinated,  246. 

density  of  the,  231. 

development  of  red  discs, 
241. 

enumeration  of  blood  cor- 
puscles, 233. 

fibrin  formation  in,  252 

fibrin  of,  219,  222,  246. 

fibrinoplastin  of,  222. 

fibrin  in,  of,  222. 

gases  in  the,  243,  357. 

haematin  of,  240. 

haematoidin  of,  240. 

hasmin  of,  240. 

ha3mochromogen  of,  240. 

lakey,  232. 

methaemoglobin  of,  239. 

of  amphioxus,  228.    s 

of  cold-blooded  animals, 
252. 

origin  of  colorless  corpus- 
cles of,  227. 

oxygen  in  the,  245. 

oxyhaemoglobin  of,  235. 

plasma,  coagulation  of,  220. 

plasma,  chemical  composi- 
tion of,  220. 

preparation  of  oxyhasmo- 
globin,  236. 

red  corpuscles  of,  alteration 
in  shape,  230. 

red  corpuscles,  228. 

red  corpuscles  forming  rou- 
leaux, 231. 


INDEX. 


745 


Blood,  red  corpuscles   of,  relative 

size  in  different  animals, 

229. 
red     corpuscles,   shape    in 

different  animals,  '229. 
•serum,  composition  of,  223. 
specific  gravity  of  the,  215. 
spectra  of,  357. 
spectra  of  haemoglobin,  238. 
stroma  of  the,  232. 
thrombosis,  251. 
venous,  355. 
Blood  pressure,  291. 

influence  of  respiration  on, 

301. 

in   the   pulmonary   circula- 
tion, 307. 

in  the  capillaries,  306. 
in  the  veins,  306. 
measurement      by      Fick's 

spring  manometer,  302. 
measurement  by  mercurial 

manometer,  295. 
record  of,  299. 
tracing  of,  294,  303. 
relative  height  of,  299. 
respiratory  wave  of,  301. 
variations  of,  300. 
Blood  vessels,  283. 

clotting  of  blood  in  the,  252. 
action  of  depression  nerve 

on,  319. 

nervous  control  of  the,  321. 
relative  capacity  of,  288. 
vasomotor  centres  of,  318. 
vasomotor  nerves  of,  317. 
Bone,  58. 

composition  of,  58. 
growth  of,  59. 
Brain,  development  of,  646,  693. 

effect     of    stimulation     of, 

657. 

fibres  and  cells  in  the,  655. 
functions  of  the,  660. 
recovery    after    injury    of, 

661. 
result  of  removal  of  parts 

of,  659. 

structure  of,  645. 
ventricles  of,  638,  653 
volition,  634. 
Breaking  shock,  503. 
Bronchial  tubes,  328. 
Brownian  movement,  82. 
63 


Brunner's  glands,  182. 
Butter  as  food,  104. 


,  572. 

V_y     Calculi,  urinary,  407. 
Camera  obscura,  565. 
Canaliculi  of  bone,  58. 
Capillaries,  285. 

lymphatic,  197. 
Carbohydrates,  77. 

as  food,  97,  99. 
Carbon,  30. 

Carbon  dioxide  in  the  blood,  244. 
Carbonates  in  the  tissues,  81. 
Carbonic  acid,  351. 
Carbonic    acid    gas     C02    in    the 

tissues,  80. 
Cardiac  centre,  643. 

cycle,  265. 

impulse,  270. 

movements,  268. 

nerve  mechanism,  277. 
Cardiograph,  271. 
Cartilage,  57. 
Casein,  70,  383. 
Catelectrotonus,  507. 
Cathode,  504. 
Cell  contents,  34,  36. 
Cell  sap,  34. 
Cell  wall,  34,  36. 
Cells,  animal,  33. 

budding  of,  86. 

development  of,  86. 

differentiated,  38. 

division  of,  86. 

fat,  37. 

ganglion,  49,  521. 

germination  of,  85. 

goblet,  200. 

history  of,  33. 

indifferent,  38. 

liver,  34. 

of  mucous  tissue,  53. 

nerve,  49,  521. 

nucleus    and  nucleolus  of, 
34. 

pigment,  37. 

reproduction  of,  85,  87. 

varieties  of,  38. 

vegetable,  33. 
Cellular  theory,  33. 
Cellulose,  91. 
Centrifugal  nerves,  496. 


746 


INDEX. 


Centripetal  nerves,  496. 
Cereals  as  food,  107. 
Cerebellum,  649. 
Cerebral  hemispheres,  656. 

functions,    localization    of, 

660. 

Cerebro- spinal  axis,  614. 
Cerebrum,  646. 
Chalaza,  671. 
Cheese  as  food,  105. 
Chemical    stimulation   of  muscle, 

452. 

of  nerve,  501. 
Chemistry  of  the  body,  62. 
Chloride  of  sodium  in   the  urine, 

406. 

Chlorophyll,  91. 
Cholesterin,  74,  177. 
Cholin,  74. 
Chondrin,  72. 
Chorion,  681. 
Choroid,  557. 

Chromatic  aberration,  574. 
Chyle,  194. 

characters  of,  208. 

movement  of,  213. 

quantity  of,  209. 
Cicatricula,  671. 
Ciliary  ganglion,  528. 

processes,  557. 
Ciliated  epithelium,  46. 
Circulation  of  the  blood,  255. 

physical  forces  of  the  289. 
Circumvallate  papilla?,  131,  552. 
Cleft  palate,  732. 
Clot  of  blood,  219. 
Coagulation  of  the  blood,  219,245. 
Cochlea,  607. 

canal  of  the,  608. 

'organ  of  Corti  in,  610. 
Cold-blooded  animals,  428. 
Colostrum,  384. 
Colorless  blood  corpuscles,  origin 

of,  227. 

Colorless  corpuscles  of  blood,  225. 
Color  blindness,  594. 

perceptions,  591. 
Columnar  epithelium,  46. 
Common  salt  in  the  urine,  405. 
Composition  of  lymph,  209. 

of  the  blood,  215. 
Connective  tissue,  37. 

tissues,  classification  of,  52. 
Constitution  of  the  blood,  214. 


Constrictors  of  pharynx,  115. 
Continuous  current,  454. 
Contractile  tissues,  441. 
Convulsions,  363. 
Coordination,  517,  CS'S. 
Cornea,  557. 

nitrate   of   silver   staining, 

196. 
Corpora  quadrigemina,  638. 

striatum,  638,  653. 
Corpus  callosum,  638. 
Corpuscles,  blood,  217,  224. 

enumeration  of  blood,  233. 

lymph,  210. 
Corti,  organ  of,  610. 
Costal  respiration,  332. 
Coughing,  349. 
Cranial  nerves,  631. 
Cricoid  cartilage,  487. 
Crusta  petrosa,  113. 
Crying,  350. 
Crystallin,  69. 

Crystalline  lens  at  different  periods, 
561. 

fibres  of,  563. 

Crystals  of  haemoglobin,  236. 
Curara,  452. 

Cutaneous  desquamation,  389. 
Cytode,  35. 


BANIELL'S  battery,  502. 
Decidua  reflexa,  682. 
cidua  serotina,  682. 

vera,  682. 

Defecation,  mechanism  of,  125. 
Deglutition,  113. 

nervous  mechanism  of,  118. 
Dentine,  112. 
Depressor  nerve,  282. 

action  of,  on  blood  vessels, 

320. 

Derived  albumins,  69. 
Development,  685. 

of  alimentary  canal,  697. 
arterial  system,  716. 
blood  vascular     system , 

708. 

brain,  646,  693. 
cells,  86. 
ear,  726. 
endothelium,  59. 
eye,  562,  721. 


INDEX. 


747 


Development  of  genito-urinary  ap- 
paratus, 702. 
heart,  710. 
kidneys,  704. 
liver,  701. 
lungs,  701. 
nose  and  mouth,  730, 

731. 

oesophagus,  701. 
pancreas,  700. 
red  blood  corpuscles, 

241. 

sexual  organs,  708. 
skull  and  face.  729. 
spinal  cord,  691. 
spleen,  701. 
venous  system,  718. 
Dextrose,  77. 
Diaphragm,  333. 
Diastole  of  heart,  267. 
Diet  table,  427. 
Differentiated  cells,  38. 
Digestion,  alimentary  tract,  111. 
defecation,  125. 
deglutition,  113. 
glands  of  stomach,  149. 
mastication,  112. 
mechanism  of,  110. 
movements    of   the   in- 
testines, 123. 
small  intestine,  182. 
stomach,  120. 
Discus  proligerus,  669. 
Division  of  egg  cell,  87. 
Drum  of  ear,  603. 
Du  Bois  Reymond's  induction  coil, 

453,504;   myograph,  461. 
Ductless  glands,  366. 
Ductus  arteriosus,  719. 

venosus,  719. 
Ducts  of  Cuvier,  718. 
Duodenum,  111,  182. 
Dyspnoea,  346,  362. 


EAR,  cochlea,  607. 
development  of,  726. 
Eustachian  tube  of,  606. 
external,  602. 
labyrinth  of,  607. 
membrani  tympani  of,  603. 
organ  of  Corti,  610, 
ossicles  of  the,  605. 
semicircular  canals  of,  608. 


Ectoderm,  41. 

Ectomeres,  673. 

Ectosarc,  93. 

Efferent  nerve,  47,  498. 

Egg  albumin,  68. 

Eggs  as  food,  106. 

Elastic  fibro-cartilage,  56. 

Elastin,  73. 

Electric  stimulation  of  muscle,  453. 

nerve,  502. 

Electrodes,  non-polarizable,  448. 
Electrotonus,  507. 
Elements,  29. 
Emmetropic  eye,  570. 
Emulsion  of  fat,  206. 
Enamel,  112. 
Endoderm,  41. 

Endogenous  reproduction,  86. 
Endolymph,  607. 
Endosarc,  94. 

Endothelium,  development  of,  59. 
with  nitrate  of  silver, 

197. 

Entomeres,  673. 
Entoptic  images,  575. 
Epiblast,  38,  42,  673. 
Epiglottis,  487. 
Epithelial  tissues,  43. 
Epithelium,  46. 

of  the  tubules  of  the 

kidney,  392. 
Eupncea,  345. 
Eustachian  tube,  606. 
Excreting  glands,  378. 
Excretions,  387. 
Exhaustion,  362. 
Expiration,  337. 
External  auditory  meatus,  602. 
Eye,  abductor  nerve  of  the,  523. 

accommodation,  570. 

blind  spot  of,  586. 

convergent  rays,  567 

development  of,  562,  721. 

dioptrics  of  the,  564. 

inversion  of  image  by  the,  567. 

motor  oculi  nerve,  622. 

pigment  cells  of  the   retina, 
590. 

refraction  of  the,  565. 

tunics  of  the,  657. 

yellow  spot,  587. 
Eyeball,    dioptric    media    of   the, 

560. 
Eyeballs,  movement  of  the,  595. 


748 


INDEX. 


"CAGE,  development  of,  730. 
\?     Faeces,  composition  of,  188. 
Fallopian  tube,  682. 
Fat,  absorption  of,  205. 
Fats  as  food,  97,  99. 
Fat  cells,  37. 

emulsion,  206. 

Fat  of  adipose  tissue  in  man,  78. 
Fats,  78. 

Fehling's  solution,  146. 
Fenestra  ovalis.  607. 

rotunda,  607. 
Fibrin,  219,  222,  246. 

chemistry  of,  70. 
ferment,  222. 
Fibrinogen,  69. 
Fibrinoplastin,  222. 
Fick's  spring  manometer,  802. 
Filiform  papillae,  131,  551. 
Fistula,  gastric,  151. 

intestinal,  184. 
pancreatic,  161. 
Fretal  circulation,  720. 
Follicles,  lymphatic,  194. 

of  Lieberkuhn,  201. 
Food,   action  of  gastric  juice  on, 

154. 

of  saliva  on,  144. 
albuminous  bodies,  97,  99. 
carbohydrates,  97,  99. 
chemical     composition     of, 

100. 

diet  table,  427. 
digestibility  of,  100. 
excessive    consumption    of, 

424. 

fats,  97,  99. 
mixed  diet,  422. 
nitrogenous  diet,  421. 
non-nitrogenous  diet,  422. 
requirements,  100,  421. 
tables,  101. 
Food  stuffs,  96. 

classification  of,  98. 
ultimate  uses  of,  425. 
Fovea  centralis,  566. 
Fungiform  papillae,  131,  551. 
Fungus  cells  dividing,  86. 


GALL  BLADDER,  176,  180. 
Galvanometer,  449. 
Ganglion  cells  of  heart,  49. 


Gases  in  the  blood,  243,  358. 

urine,  406. 
Gastric  digestion,  156. 

fistula,  151. 

glands,  149. 

juice,  action  on  food,  154. 

juice,  artificial,  151. 

juice,  composition  of,  150. 

juice,  method  of  obtaining, 

151. 

Gastrula  stage,  41. 
Gelatin,  72. 
Gemmation,  85. 

Genito-urinary  apparatus,  develop- 
ment of,  702. 
Germinal  vesicle,  670. 
Giddiness,  549. 
Gills,  325. 
Gland  cells,  132. 
Glands,  acinous,  131. 

blood  elaborating,  365. 

ductless,  366. 

excreting,  378. 

lachrymal,  378. 

lymphatic,   194. 

mammary,  382. 

Meibomian,  381. 

mucous,  133,  379. 

of  stomach,  149. 

salivary,  133. 

sebaceous,  381. 

secreting,  378. 

sudoriferous,  387. 

sweat,  387. 

thymus,  368. 

thyroid,  367. 
Glandular  epithelium,  46. 
Glisson,  capsule  of,  169. 
Globin,  241. 
Globulins,  68. 
Gloeocapsa,  86. 
Glomerulus  of  kidney,  394. 
Glosso-pharyngeal  nerve,  529. 
Glottis,  shape  of  the  opening  of  the, 

488. 

Gluten,  107. 
Glycin,  76. 
Glycocine,  76. 
Glycocholic  acid,  74,  176. 
Glycocoll,  76. 
Glycogen,  78. 

preparation  of,  376. 
Glycogenic  function  of  liver,  373. 
Gmelin's  test  for  bile  pigments,  178. 


INDEX. 


749 


Goblet  cells,  47,  200. 
Graafian  follicle,  667. 
Grape  sugar,  77. 

in  the  urine,  406. 

starch  converted  into, 

145. 

Graphic  method.  460. 
Gravid  uterus,  684. 
Green  vegetables  as  food,  107. 


,  240. 

fl      Ha3matoidin,  240. 
Hasmin.  240. 
Hsemochromogen,  240. 
Hemoglobin  crystals,  236. 

decomposition  of,  239. 
spectra  of,  237. 
Hare  lip,  732. 
Haversian  system,  57. 
Hearing,  598. 

external  ear,  602. 
organ  of  Corti,  610. 
Heart,  255. 

accelerator  nerves  of,  281. 

action  of  the  valves  of,  263. 

afferent  cardiac  nerves,  282. 

anabolic  action  of,  282. 

anatomy  of,  258. 

atropine,    curara,    digitalis, 
nicotin,  action  of,  281. 

Bidder's  ganglion,  278. 

cardiac  centre,  643. 

cardiac  movements,  268. 

cardiograph,  271. 

capacity  of,  258. 

cause  of  sounds  of,  273. 

cycle  of,  267. 

diastole  of,  267. 

depressor  nerve,  282. 

development  of,  710. 

extrinsic  nerves,  279. 

first  sound  of,  272. 

inhibitory  nerves  of,  280. 

innervation  of,  275. 

intrinsic  nerves,  276. 

katabolic  action,  282. 

lymph,  211. 

muscle  of,  259,  443. 

nerve  cells  of,  49. 

passive  interval,  269. 

second  sound  of,  272. 

semilunar  valves,  263. 

Stannius's  experiment,  279. 


Heart,  systole  of,  267. 

vagus,  action  of,  280. 
valves  of,  262. 
work  done  by  the  316. 
Heart  beat,  nerve  mechanism  con- 
trolling, 277. 
Heart's  impulse,  270. 
Heat,  balance  of,  434. 

compensation    for    external 

variations,  438. 
compensation    for    internal 

variations,  437. 
expenditure  of,  434. 
income,  432. 
regulation.  437. 
Henle's  tubules,  392. 
Hepatic  artery  and  vein,  171. 
Hiccough,  350. 
Hippuric  acid,  77. 

in  the  urine,  404. 
Histology,  26. 
Holoblastic  ovum,  673. 
Homceothermic  animals,  428. 
Hunger  and  thirst,  548. 
Hyaline  cartilage,  56. 
Hydra,  neuro-muscular  cells  of,  46. 
Hydrocele  fluid,  220. 
Hydrochloric  acid  in  the  tissues, 

80. 

Hypermetropia,  573. 
Hypoblast,  38,  42,  673. 
Hypoglossal  nerve,  532. 


JDIOSYNCRASY,  100. 
Ileo-cecal  valves,  125. 
lium,  111. 
Incus,  605. 
Indican,  77.  405. 
Indifferent  cells,  38. 
Indol,  77,  189. 
Induced  current,  454,  504. 
Induction  coil,  454,  501. 
Infusoria,  93. 
Inhibition,  517. 
Innervation  of  the  heart,  275. 
Inorganic  bodies,  79. 

substances,  30. 
Inosit,  78. 
Inspiration,  333. 
Intercellular  substance.  36. 
Intercostal  muscles.  335. 
Interlobular  vein,  170. 
Internal  respiration,  359. 


750 


INDEX. 


Interrupted  current,  454. 
Interstitial  absorption,  192. 
Intestinal  absorption,  199. 
digestion,  180. 
fistula,  184. 
juice,  function  of,  185. 
secretion  of,  184. 
motion,  nervous  mechan- 
ism of,  128. 

secretion,  method  of  ob- 
taining, 18t. 

Intestine,  histology  of  small,  123. 
large,  187. 
lymph  follicles,  201. 
small,  182. 

Intestines,  movements  of  the,  123. 
Intralobular  vein,  171. 
Inversion  of  image,  567. 
Iris,  558,  575. 

functions  of  the,  575. 
nervous   mechanism   control- 
ling the,  576. 


JEJUNUM,  111. 

Joints,  478. 

1/ARYOKINESIS,  36. 
j\     Keratin,  73,  390. 
Kidney,  391. 

blood  vessels  of,  393. 

development  of,  704. 

relation  of  blood  vessels  to 
tubules.  395. 

tubules  of,  392. 
Krause's  end  bulbs,  539. 
Kreatin,  75. 
Kreatinin,  76. 

in  the  urine,  403. 
Kymograph,  298. 


T  ABYRINTH  of  ear,  607. 
jj,     Lachrymal  glands,  378. 
Lactation,  386. 
Lacteals,  191,  194,  199,  212. 
Lactic  fermentation,  78. 
Lactose,  78. 
Lacunae  of  bone,  58. 
Lakey  blood,  232. 
Larynx,  486. 

muscles  of,  326. 
Latent  period,  462. 


Laughing,  350. 
Lecithin,  30,  74,  177. 
Leucin,  76. 

and  ty rosin  in  the  urine.  407. 
in  pancreatic  digestion,  166. 
Leucocytes,  225. 
Leucocythemia,  225. 
Levatores  costarum,  335. 
Levers,  the  three  orders  of,  478. 
Lieberkiihn's  follicles,  183,  2J1. 
Light,  591. 
Liquor  folliculi,  669. 

sanguinis,  217. 
Listing's  measurements,  507. 
Liver,  168. 

bile  ducts,  174. 
bile  salts,  73,  175. 
capillaries  of,  285. 
capsule  of  Giisson,  169. 
cells,  34,  171. 
development,  701. 
glycogenic  function  of,  373. 
hepatic  artery,  171. 
interlobular  vein,  170. 
lobules  of,  169. 
method   of   obtaining    bile, 

174. 

portal  vein,  374. 
Ludwig's  kymograph,  298. 
Lumen  of  cells,  163. 
Lung,  bronchial  tubes,  328. 
development  of,  702. 
structure  of,  326. 
Lymph  and   chyle,   characters  of, 

208. 

channels.  193. 
composition  of,  209. 
corpuscles,  210. 
follicles  in  intestine,  201. 
hearts,  211. 
movement  of  the,  211. 
spaces,  192. 
vascular  system,  192. 
vessels,  valves  in,  213. 
Lymphatic  gland,  194. 
system,  192. 
vessels,  197.  L 
Lymphatics,  191. 


M 


AKING  shock,  503. 

Malassez'  apparatus  for  the 
enumeration  of  blood  cor- 
puscles, 233. 


INDEX. 


751 


Malleus,  605. 

Malpighian  body  of  spleen,  373. 

capsule,  391. 
Mammary  glands,  382. 

during  lactation,  385. 
Marey's  sphygmograph,  309. 

tambour,  270. 
Mastication,  112. 

nervous  mechanism  of,  118. 
Meat  as  food,  105. 
Mechanical  stimulation  of  muscle, 

452. 

of  nerve,  501. 

Mechanism  of  absorption,  203. 
Medulla  oblongata,  as  central  or- 
gan, 639. 
as  conductor,  638. 
cardiac  centre,  643. 
decussation  of  fibres 

in  638. 

pyramids  in.  638. 
respiratory    centre 

in,  640. 
roots  of  nerves  in, 

639. 
vasomotor     centre 

in,  641. 

Medullated  nerve  fibres,  47,  497. 
Meibomian  glands,  381. 
Meissners  plexus,  129. 
Membrana  tympani,  603. 
Membrane  of  Reissner,  608. 
Membranous  spiral  lamina,  608. 
Memory,  658. 
Menstruation,  669. 
Meroblastic  ovum,  673. 
Mesoblast,  38,  42,  673. 
Methaemoglobin,  239. 
Metabolism,  365. 
Metazoa,  40. 
Micrococcus,  88. 
Micturition,  nervous  mechanism  of, 

413. 
Milk,    chemical     composition    of, 

102,  103,  383. 
as  food,  102. 
sugar,  78. 
tests,  103. 

Millon's  re-agent,  67. 
Mitral  valve  of  the  heart,  260. 
Moist  chamber,  461. 
Molecules,  30. 
Morphology,  25. 
Morula  stage,  40,  673. 


Motion,     application    of    skeletal 

muscles,  477. 
rate  "of  running,  505. 
standing,  481. 
walking  and  running,  484. 
Motor  nerves,  631. 

oculi  nerve,  522. 
Mouth,  development  of,  730. 

digestion,  131. 

Movement  of  the  chyle,  213. 
of  the  heart,  268. 
of  the  lymph,  211. 
of  white   blood  corpuscles, 

226. 

Mucin,  72. 
Mucous  glands,  133,  379. 

tissue,  54. 

Mlillerian  duct,  703. 
Murexide  test  for  uric  acid,  76. 
Muscle,  active  state  of,  451. 

altitude  of  curve,  466. 
changes    during     contrac- 
tion, 454. 

chemical  change  in,  446. 
composition  of,  445. 
consistence  of,  445. 
contraction  of  unstriated, 

475. 

curves,  461. 
death  rigor,  473. 
Du  Bois  Reymond's  coil, 

453,  504. 
duration     of    contraction, 

464. 
effect  of  temperature    on 

contraction  of,  465. 
elasticity  of,  447. 
electric  phenomena,  448. 
fatigue,  470. 
graphic  method,  460. 
irritability  of,  451,  470. 
latent  period,  462. 
maximum  contraction, 466. 
natural  currents,  448. 
negative  variation,  457. 
non-polarizable  electrodes, 

448. 

non-striated,  50,  442. 
of  heart,  443. 
passive  state  of,  445. 
plasma,  66. 
properties  of,  445. 
rate  of  contractions  in  in- 
sects, 464. 


752 


INDEX. 


Muscle,  rheoscopic  frog,  457. 

rigor  mortis,  473. 

sarcolemma,  61,  444. 

single  contraction,  461. 

stimulation  of,  452. 

striated,  50,  443. 

summation,  467. 

tetanus  468. 

tone,  470. 

tracings,  462. 

wave  of  contraction,  463. 
Muscles  of  deglutition,  115. 

of  the  eyeballs,  595. 

of  the  iris,  577. 

of  the  larynx,  326,  489. 

of  mastication,  113. 

origin  and  insertion  of,477. 

of  respiration,  333. 

the  three  orders  of  levers, 

478. 

Muscular  fibres  of  heart,  259. 
Muscularis  mucosaa,  183,  201. 
Myograph,  pendulum,  461. 

spring,  461. 
Myopia,  572. 
Myosin,  69. 


NASAL  ganglion,  628. 
Native  albumins,  68. 
Nausea.  548. 
Nerve,  496,  519. 

III.  motor  oculi  nerve,  522. 

IV.  trochlear  nerve,  523. 

V.  trigeminus,  or   trifacial 
nerve,  525. 

VI.  abductor  nerve  of  the 
eye,  523. 

VII.  (portio  dura)    motor 
nerve  of  the  face,  524. 

VIII.  vagus,  530. 
VIII.  glosso-pharyngeal 

nerve,  529. 

VIII.  spinal    accessory 
nerve,  529. 

IX.  hypoglossal  nerve,  532. 
active  state  of,  501. 
anode,  503. 

ascending  current,  512. 
axis  cylinder,  49,  498 
Auerbach's  plexus,  127. 
breaking  shock,  502. 
breaking   tetanus    (Hitters' 
tetanus),  502. 


Nerve,  cathode,  503. 

ciliary  ganglion,  528. 
descending  current,  512. 
electric  change  in,  506. 
electrical  properties  of,  500. 
electrotonus,  507. 
ganglion  cells,  49,  521. 
irritability  of  nerve   fibres, 

508. 

law  of  contraction,  511. 
making  shock,  503. 
mechanism  of  heart,  277. 
medullary  sheath,  49,  498. 
medullated  fibres,  47,  497. 
Meissner's  plexus,  129. 
multipolar  cells,  48. 
natural  current  in,  500. 
negative  variation,  506. 
nerve-muscle    preparation, 

501. 
non-medullated  fibres,   47, 

497. 

nodes  of  Ranvier,  47,  497. 
otic  ganglion,  528. 
polarizing  current,  507. 
primary  coil,  504. 
primitive  sheath,  49,  498. 
proximal   and    distal    end, 

499. 

secondary  coil,  504. 
sensory  cells,  46. 
spinal  cord,  615. 
spinal,  619. 
stained  with  osmic  acid,  47, 

497. 

stimulation  of,  501. 
submaxillary  ganglion,  529. 
tissue,  46. 
to  spheno-palatine  ganglion, 

528. 

velocity  of  nerve  force,  504. 
Nerves,  afferent,  47,  498. 
efferent,  47,  498. 
anterior  roots  of,  519. 
posterior  roots  of,  519. 
cranial,  522. 
functional  classification  of, 

498. 
Nerve  cells,  49.  (< 

functions  of,  515. 
in  spinal  cord,  616. 
Nerve  endings,  47,  513. 

in  the  ear,  608. 
in  the  eye,  585. 


INDEX. 


753 


Nerve  endings,  in  nose,  553. 

in  the  skin,  538. 
in  tongue,  552. 

Nerve  fibre,  chemistry  of,  500. 
fibres  in  spinal  cord,  616. 
roots  in  medulla  oblongata, 

639. 

Nervous  control  of  the  blood  ves- 
sels, 321. 
Nervous  mechanism  of  respiration, 

347. 

of  salivary  se- 
cretion, 136. 
of  urinary  se- 
cretion, 410. 
of  voice,  493. 

Nervous  organs,  central,  614. 
Nerve-muscle  preparation,  501. 
Neurin,  74. 
Neuroglia,  497,  614. 
Neuro-muscular  cells  of  hydra,  47. 
Nitrogen  in  the  blood,  244. 
in  the  tissues,  81. 
Nitrogenous  diet,  421. 

ingredients  in  the  tis- 
sues, 64. 
Non-medullated   nerve   fibres,   47, 

497. 
Non-nitrogenous  diet,  422. 

ingredients  of  the 

body,  77. 

Non-striated  muscle,  50,  442. 
Nose,  development  of,  730. 

nerve  endings  in  the,  553. 
Notochord,  673. 
Nuclear  matrix,  36. 
Nucleus,  method  of  staining,  35. 
Nucleolus,  36. 
Nutrition,  416. 
Nutritive  equilibrium.  420. 

materials     in     vegetable 

food,  108. 
value  of  food,  100. 


ODONTOBLASTS,  114. 
(Esophagus,  development  of, 

701. 

histology  of,  117. 
Olein,  78. 

Ophthalmic  ganglion,  528. 
Ophthalmometer,  571. 
Ophthalmoscope,  578. 


I   Optic  nerve,  560. 

thalami,  638,  654. 
Organic  substances,  30. 

substance,  instability  of,  31. 
Organisms,   chemical   composition 

of,  30. 

death  of,  32. 
general  characters  of, 

28. 

reproduction  of,  32. 
vital  characters  of,  82. 
Origin  of  white  blood  corpuscles, 

227. 

Osseous  spiral  lamina,  608. 
Ossicles  of  ear,  605. 
Ossifying  cartilage,  59. 
Osteoblasts,  58. 
Otic  ganglion,  528. 
Otoliths,  608. 
Ovary,  668. 

Ovoid  cells  of  stomach  glands,  149. 
Ovulation,  669. 
Ovum,  668. 

changes  in  the,  671. 
division  of,  40. 
Oxalic  acid  in  the  urine,  404. 
Oxygen,  353. 

in  the  blood,  243. 
in  the  tissues,  81. 

Oxyhasmoglobin,  66,  235,  237,  357. 
composition  of,  235. 
preparation  of,  236. 


"QACTNIAN  corpuscles,  540. 
1       Pain,  547. 
Palmitin,  78. 
Pancreas,  160. 

changes  in  the  cells  dur- 
ing secretion,  162. 
nervous  influence  on  the, 

162. 

development  of,  700. 
Pancreatic  fistula,  161. 

juice,  action  on  fat,  166. 
action  on  proteids, 

165. 
action   on    starch, 

167. 

artificial,  161. 
characters  of  the 

secretion,  161. 
Papilla?  of  tongue,  131,  551. 


754 


INDEX. 


Paraglobulin    (fibrinoplastin),    69, 

221. 

Paramoecium,  42,  93. 
Parapetone,  155. 
Pavy's  solution,  146. 
Pendulum  rayograph,  Fick's,  461. 
Peduncles  of  the  cerebrum,  638. 
Pepsin,  151. 
Peptic  cells,  153. 
Peptone,  71. 

absorption  of,  205. 
tests  for,  71. 
Perilymph,  607. 
Peristaltic  contraction  of  intestine, 

124. 

Perspiration,  388. 
Pettenkofer's  test  for  bile  salts,  73, 

177. 

Peyer's  patches,  202. 
Phakoscope,  571. 
Pharynx,  muscles  of,  115. 
Phosphates  in  the  tissues,  81. 
in  the  urine,  405. 
Physical  forces  of  the  circulation, 

289. 

Physiology,  objects  of,  25. 
Picric  acid  test  for  albumen,  68. 
Pigment  cells,  37. 

of  the  eye,  590. 
Placenta,  681. 

uses  of  the,  685. 
Plant  cell,  33. 
Plasma,  218. 

chemical  composition  of, 

220. 
of  the  blood,  coagulation 

of.  219. 
method  of  obtaining  from 

blood,  219. 
Plasmata,  64. 
Pleura,  327. 

functions  of  the,  338. 
Pneumogastric  nerve,  348. 
Poikilothermic  animals,  428. 
Poisonous  gases,  360. 
Polarizing  current,  507. 
Pons  varolii,  638,  652. 
Portal    system,    development    of, 

714 

vein,  169,  374. 
Portio  dura,  seventh  nerve,  524. 

mollis,  598. 
Porus  opticus,  585. 
Posterior  roots  of  spinal  cord,  616. 


Potassium  chloride  in  the  tissues, 

81. 

Potatoes  as  food,  107. 
Presbyopia,  573. 
Primitive  groove,  42. 
Products  of  tissue  change,  71. 
Protagon,  74. 
Protamreba,  91. 
Protista,  39. 
Protococcus,  91. 
Protoplasm,  29. 

assimilation  of,  85. 
change  in  form,  83. 
of  cell,  34. 
in  the  tissues,  65. 
structure  of,  35. 

Protoplasmic  movements,  83,  84. 
Protozoa,  40. 
Pseudopodia  of  amoeba,  83. 

of   white  blood    cor- 
puscles, 226. 
Ptyalin,  145. 
Puerile  breathing,  343. 
Pulmonary  circulation  of  the  blood, 

258. 

Pulp  cavity  of  the  teeth,  112. 
Pupil  of  the  eye,  558. 
Pulse,  307. 

Marey'ssphygmograph,  309. 
tracings,  309. 
variations  in  the,  312. 
Purkinje's  cells  in  the  cerebellum, 

649. 

figures,  587. 

Putrefactive  fermentation  in  the  in- 
testine, 188. 
Pylorus,  120. 


UADRATUS  lumborum,  334. 


Q 


RANVIER'S  nodes,  47,  497. 
Receptaculum  chyli,  194. 
Recording  apparatus,  297. 
Rectum,  111. 

Red  blood  corpuscles,  217. 
Reduced  haemoglobin,  237. 
Reflex  action,  48,  516,  627. 

inhibition  of,  629. 
Reflex  centres,  special,  632. 
Reflexion,  516,  634. 
Refraction,  564. 


INDEX. 


755 


Reproduction,  666. 

of  cells,  85,  87. 
Respiration,  323. 

asphyxia,  362. 
changes    in    the   air, 

862. 
changes  in  the  blood 

during,  354. 
chemistry  of,  351. 
complemental  air, 342. 
construction    of    tho- 
rax, 329. 
diffusion,  342. 
expiration,  337. 
expired  air,  352. 
frequency  of,  331. 
functions  of  the  pleu- 
ra, 338.  ^ 

in  lower  animals,  324. 
inspiration,  333. 
internal  or  tissue,  359. 
mechanism  of,  323. 
muscles  of,  333. 
nervous      mechanism 

of,  343. 

of  abnormal  air,  360. 
poisonous  gases,  360. 
pressure  differences 

in  the  air,  340. 
reserve  air,  341. 
residual  air,  342. 
sounds  of,  343. 
tidal  air,  341. 
ventilation,  361. 
vital  capacity,  342. 
vital  point,  344. 
volume  of  air,  341. 
Respiratory  centre,  640. 

movements,  330. 
sounds,  343. 
wave,  301. 
Retention  of  urine  in  the  bladder, 

412. 

Retina,  function  of  the,  583. 
pigment  cells  of,  590. 
ophthalmic    view    of   the, 

581. 

structure  of,  583. 
Rheoscopic  frog,  457. 
Ribs,  336. 
Rigor  mortis,  473. 
Ritter's  tetanus,  502. 
Rods  and  cones  of  the  retina,  585. 
Rods  of  Corti,  610. 


PACCHAROMYCES    cerevisiaj, 

O     77,  85. 

Sacculated  glands,  131. 

Sacule  of  semicircular  canals,  608. 

Saliva,  action  on  food,  144. 

chemistry  of,  135. 

effect  of  drugs  on  secretion 
of,  138. 

effect  of  nervous  influence, 
138. 

method  of  secretion,  136. 
Salivary  gland,  J33. 

histology  of,  142. 
Salts  as  food,  109. 
Salts  in  the  tissues,  80. 
Saponification,  166. 
Sarcolemma  of  muscle,  51,  444. 
Scaleni  muscles,  335. 
Scaly  epithelium,  45. 
Scheiner's  experiment,  569. 
Sclerotic,  557. 
Sebaceous  glands,  381. 
Secreting  glands,  378. 
Secretion,  378. 
Secretion  of  urine,  395. 
Segmentation  sphere,  672. 
Semicircular  canals,  608. 
Semilunar  valves,  263. 
Sensations,  general,  547. 
Sensory  nerve  cells,  46. 
Sensory  nerves,  631. 
Serum  albumin,  68. 

composition  of,  223. 

globulin,  221. 

separation  of,  219. 
Sexual    organs,    development    of, 

708. 

Sexual  reproduction,  666. 
Shivering,  549. 
Sighing,  350. 
Sight,  556. 
Skatol,  189. 

Skin,  cutaneous  desquamation,  389. 
general  sensations,  537,  547. 
glands  of,  381. 
Krause's  end  bulbs,  539. 
nerve  endings  in,  538. 
sense  of  temperature,  545. 
Skull,  development  of,  729. 
Smell,  553. 
Sneezing,  350. 
Sobbing,  350. 

Sodium  chloride  in  the  tissues,  80. 
Solitary  glands,  202. 


756 


INDEX. 


Somatopleure,  674. 
Sound,  598. 

direction  of,  613. 
tuning  forks,  599. 
vibrations  through  the  tym- 
panum, 603. 
Special  senses,  534. 
hearing,  598. 
smell,  553. 
taste,  550. 
touch,  537. 
vision,  656. 
Spectra  of  blood,  357. 

of  hemoglobin,  237. 
Spectrum,  analysis  of  the,  592. 
Speech,  486. 
Spermatozoa,  667. 
Sphenopalatine  ganglia,  528. 
Spherical  aberration,  574. 
Spheroidal  cells  of  stomach  glands, 

149. 

Sphygmograph,  309. 
Spinal  accessory  nerve,  529. 
Spinal  cord,  615. 

automatic  centres   in 

the,  635. 

automatism,  634. 
cellular    columns     in 

the.  625,  626. 
cervical  region,  619. 
collection     of    nerve 

cells  in  the,  625. 
coordination,  634. 
course  of  fibres  in, 

624. 

development  of,  688. 
dorsal  region,  618. 
experiments   on    the, 

622. 

lumbar  region,  618. 
motor  channels,  631. 
of  embryo,  620. 
pyramidal    tracts    of, 

619. 

reflex  action,  627. 
roots    of    nerves    of, 

616,  617. 

sensory  channels,  631. 
white  tracts  on,  619. 
Splanchnopleure,  674. 
Spleen,  changes  of  blood   in  the, 

371. 

development  of,  701. 
extirpation  of  the,  372. 


Spleen,  function  of,  372. 
structure  of,  369. 
Spring  myograph,  461. 
Standing,  481. 
Stapes,  605. 
Starch  as  food,  107. 

converted  into  grape  sugar, 

145. 
microscopic  appearance  of, 

109. 

tests  for,  146. 
Starvation,  417. 
Steapsin,  186. 
Stearin,  78. 
Steno's  duct,  134. 
Stomach,  digestion,  148. 

histology  of,  148. 

motion  of  the,  120. 

nervous  influence  on, 121. 

walls  of  the,  119. 
Stomata.  197. 
Stratified  epithelium,  44. 
Striated  muscle,  50,  443. 
Stroma  of  the  blood,  232. 

chemistry  of  the,  241. 
Submaxillary  ganglion,  529. 
Sudoriferous  glands,  387. 
Sulphates  in  the  tissues,  81. 

in  the  urine,  405. 
Summation,  467. 
Supporting  tissues,  43. 
Supra-renal  capsule,  367. 
Sweat  glands,  387. 
Symphyses,  479. 
Syntonin,  69. 
Systemic  circulation  of  the  blood, 

258. 
Systole  of  heart,  267. 


TACTILE  impressions,  542. 
sensations,  537. 
Tambour  (Marey's),  271. 
Taste,  550. 
Taste  buds,  552. 
Taurin,  76. 

Taurocholic  acid,  73,  176. 
Tegmentum,  652. 
Teeth,  development  of,  114. 
structure  of  the,  112. 
Temperature,  maintenance  of  uni- 
form, 435. 
measurement  of,  429. 


INDEX. 


757 


Temperature,  normal  variations  in, 

429. 

Tendon  cells  stained  with  gold,  54. 
Testicle,  667. 
Tests  for  albumin,  67. 
for  peptone,  71. 
Tetanus  of  muscle,  468. 
Thermic  stimulation  of  muscle,  452. 

nerve,  502. 

Thermometer,  clinical,  429. 
Thirst  and  hunger,  548. 
Thoracic  duct,  191,  212. 

lymph  of,  208. 

Thorax,  construction  of,  329. 
Thrombosis,  251. 
Thymus  gland,  368. 
Thyroid  body,  367. 

cartilage,  487. 
Timbre  of  a  note,  600. 
Tissue  changes  in  starvation,  417. 

change,  products  of,  71. 

connective,  52. 

differentiation,  39. 

epithelial,  43. 

lymphoid,  195. 

mucous,  54. 

muscle,  50,  442. 

nerve,  46,  496. 

white  fibrous,  54. 

yellow  elastic,  56. 
Tissues,  classification  of,  43. 
Titillation,  549. 
Tones,  600. 
Tongue,  550. 

taste  buds  of,  552. 
Torula  cerevisia,  90. 
Touch,  537. 

cells,  539. 

corpuscles(Meissner's),538. 

general  sensation,  547. 

sense  of  locality,  541. 

sense  of  pressure,  543. 
Tradescantia  Virginica,  82. 
Tricuspid  valve  of  the  heart,  259. 
Trigerainus  nerve,  525. 
Trochlear  nerve,  523. 
Trommer's  test,  146. 
Trypsin,  164,   186. 
Tubules  of  kidney,  392. 
Tubuli  seminiferi,  667. 
Tunica  adventitia,  283. 

intima,  285. 
granulosa,  669. 

media,  283. 


Tuning  forks,  599. 
Tympanum  of  ear,  603. 
Tyrosin,  76. 


/D. 

in    pancreatic    digestion, 
166. 


UMBILICAL  cord,  635. 
Unicellular  organism,  39. 
Unstriated  muscle,  contraction  of, 

475. 

Urachus,  681. 
Urea,  75,  400. 

artificial  preparation  of,  75. 
estimation  of,  402. 
source  of,  408. 
Ureters,  411. 
Uric  acid,  76. 

in  the  urine,  403. 
murexide  test  for,  76. 
Urinary  calculi,  407. 
Urine,  395. 

abnormal     constituents    in 

the,  406. 
acid    fermentation   of   the, 

407. 
chemical     composition    of, 

400. 

coloring  matter  of,  404. 
inorganic  salts  in  the,  405. 
quantity  secreted,  396. 
retention  of,  412. 
secretion  of,  397. 
source  of  urea,  408. 
specific  gravity  of,  396. 
Urinary  excretion,  391. 

secretion,   nervous  mech- 
anism of,  410. 
Urobilin,  404. 
Urochrome,  404. 
Uroerythrin,  405. 
Uterus,  666. 


VACUOLES,  34,  94. 
Vagus  nerve,  280,  530. 
Vagus,  stimulation  of,  280. 
Valentine's     method,     estimating 

amount  of  blood,  216. 
Valsalva's  experiment,  606. 
Valves  in  lymph  vessels,  213. 
of  heart,  262. 
of  veins,  287. 
of  the  heart,  action  of,  263. 


758 


INDEX. 


Yalvulae  conniventes,  160,  182. 
Vasa  deferentia,  667. 

motor  centre,  641. 
motor  nerves,  317. 
Vascular   system,  development  of, 

60,  708. 
Vegetable  cells,  33. 

food,  107. 
Veins,  coats  of,  287. 

valves  of,  287. 

Velocity  of  the  blood  current,  313. 
Venae  advehentes,  714. 
Vena  cava,  259. 
porta,  374. 
Venous    system,    development  of, 

718. 

Ventilation,  361. 
Ventricles  of  brain,  638. 
of  heart,  259. 
Vesiculaa  seminalis,  667. 
Vessels,  lymphatic,  197. 
Vibration,  599. 
Villi,  183. 

lacteals  in,  199. 
Vision,  556. 

accommodation,  570. 

astigmatism,  574. 

binocular,  596. 

chromatic  aberration,  574. 

color  blindness,  594. 

color  perceptions,  591. 

complementary  colors,  592. 

conditions  affecting,  589. 

convergent  rays,  567. 

entoptic  images,  575. 

function  of  the  retina,  583. 

hypermetropia,  573. 

irradiation,  589. 

judgment  of  distance,  597. 

judgment  of  size,  597. 

light  impressions,  582. 

mental  operations  in,  694. 

myopia,  572. 

negative  after  image,  589. 

ophthalmoscope,  578. 

positive  after  image,  589. 

presbyopia,  573. 

Purkinje's  figures,  587. 

refraction,  564. 

retinal  stimulation,  587. 

Scheiner's  experiment, 
568. 


Vision,  spherical  aberration,  574. 
Vital  capacity,  342. 

characters  of  organisms,  82. 

phenomena,  26,  32. 

point,  344. 
Vitellin,  69. 

Vitelline  membrane,  670. 
Vitreous  humor,  560. 
Vocal  cords,  487. 
Voice  and  speech,  486. 

nervous  mechanism  of,  493. 
properties    of    the    human, 

491. 

Volition,  634. 
Vomiting,  121. 
Vorticella,  42,  94. 


TT  BALKING  and  running,  484. 
W      Warm-blooded  animals,  428. 
Waste   products   of  animal  body, 

75. 

Water  H20,  30,  79. 
Water  as  food,  108. 
Weber's  method, estimating  amount 

of  blood,  215. 
Welcker's   method,  estimating 

amount  of  blood.  216. 
Wharton's  duct,  134. 
White  blood  corpuscles,  225.  . 
fibro-cartilage,  57. 
fibrous  tissue,  54. 
substance  of  Schwann,  498. 
Wolffian  bodies,  702. 


yANTHIN  in  the  urine,  403. 
y\    Xanthoproteic  test,  67. 


YAWNING,  350. 
1      Yeast  plant,  77,  85,  90. 
Yellow  elastic  tissue,  56. 

spot,  587. 
Yolk  of  egg,  670. 
sac,  678. 


ZONA  pellucida,  670. 
tendinosa,  260. 
z^ymogen,  164. 


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